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The job of the river
Abstract
Earth scientists have traditionally conceptualized rivers and streams as geomorphic machines, whose role is to transfer sediment and to sculpt the landscape. Steady-state relationships between sediment supply and transport capacity have traditionally been considered normative in fluvial systems. Rivers are hydrological entities, however, whose function is to redistribute excess moisture on land. The geomorphic work of the river – erosion, transport, deposition, etc. – is a byproduct of the hydrological job of the river. There is therefore no reason to expect any particular relationship between sediment supply and transport capacity to develop as a normative condition in fluvial systems. The apparent steady-state equilibrium slope adjustments of rivers are a byproduct of four basic phenomena: (1) hydraulic selection, which favors channels and branching networks over other flux patterns; (2) water flows along the available path of least resistance; (3) energy dissipation; and (4) finite relaxation times. Recognizing converging trends of stream power or slope and sediment supply as common (but far from inevitable) side effects rather than self-regulation has important implications for interpreting and predicting fluvial systems, and for river management and restoration. Such trends are variable, transient, contingent, and far from universal. Where they occur, they are an emergent byproduct of fundamental physical mechanisms, not a goal function or attractor state. Copyright © 2009 John Wiley & Sons, Ltd.
... If we consider a flood through a stream corridor whose peak discharge and slope achieve the critical stream power required to maintain sediment transport continuity through a reach, Ω cr , then this flood and its sediment supply would result in no net change in sediment within a reach. The channel slope associated with the critical stream power, S cr , can be thought of as a slope leading to sufficient transport capacity under the imposed sediment load and discharge rate (Phillips, 2010). Note, however, that stream corridor adjustment (and hence fluvial hazards) may still occur even if the above factors leading to sufficient transport capacity are met. ...
... The concept of meta-stability states that stream form at any given moment is an emergent property resulting from four basic principles and processes (Phillips, 2010): ...
... These tend to maintain a dynamic equilibrium over the medium term (decades) as channels adjust their slope, cross sections, and planform to maintain a balance between sediment supply, frictional resistance within a stream corridor, and the sediment transport capacity of flow (Leopold, 1994;Phillips, 2010). A disturbance, such as a flood, may alter this balance as a stream adjusts towards pre-disturbance conditions over time, or alternatively it may shift the stream to a different meta-stable state as dictated by the geomorphic context (landscape, valley, and reach scale), sediment supply (size and quantity), and biotic drivers (vegetation, beavers, and wood) (Phillips, 2010;Thorne and Wellford, 1994). ...
This document, produced with funding from the State of Colorado Water Conservation Board, provides recommendations and guidance for Fluvial Hazard Zone mapping teams and also provides background and contextual information that may be useful for planners, community officials, and NGOs. The FHZ Protocol is intended to be a tool for a team of Qualified Professionals to use in conjunction with their observation, training and experience, judgement, and critical thinking to delineate Fluvial Hazard Zone boundaries. Further information and appendices can be found at: www.ColoradoFHZ.com
... These criteria also include key aspects of differing ideas regarding fluvial equilibrium, which is a special case of the more general disequilibrium condition (e.g. Phillips, 2010). ...
... In an alternative class of studies we define as "Steady-State Equilibrium" (see Phillips, 2010Phillips, , 2011, fluvial equilibrium occurs for comparable values of Q ss and Q sf and which corresponds to steady downstream elevation profiles and bank positions. Blom et al., (2016Blom et al., ( , 2017 focus on the downstream profile aspect of "Steady-State Equilibrium" within a theoretical framework to predict the graded alluvial river profile (Mackin, 1948), defined as "the mean profile that the river approaches when flow, sediment supply, and base level vary around stable values for a long time in the absence of subsidence or uplift" (Blom et al. 2016 p. 6285). ...
... The previous paragraphs raise an important question: Is fluvial landscape evolution best understood on the basis of Process or Steady-State Equilibrium (cf. Abrahams, 1968;Chorley and Kennedy, 1971;Ahnert, 1994;Thorn and Welford, 1994;Phillips, 2010;2011;Pittaluga et al. 2014)? The definitions for Process and Steady-State Equilibrium apply over different spatial scales, therefore the question of appropriateness is difficult to answer. ...
A river at equilibrium is described by a statistically‐stationary mean bed elevation profile that arises in response to steady supplies of relief, water and sediment. Outside of the profile shape, how is the equilibrium state of a river most reliably identified and rigorously defined? Motivated by a proposed link between equilibrium and physical processes, we use scaling theory to develop the dimensionless channel response number ξ=KUb/Up. ξ is a metric for the local disequilibrium state of gravel‐bed mountain streams, which reflects a balance between the rate of topographic adjustment Ub, and the rate of bed sediment texture adjustment Up. The coefficient K can take one of two forms depending on choice of length scale for topographic adjustment. We hypothesize that equilibrium occurs where and when ξ≈O(1), and consequently, disequilibrium is the more general state captured by conditions of ξ≉O(1). The rates Ub and Up are controlled by the mechanics of sediment deposition and entrainment at the local scale of the channel width. The extent to which either process regulates disequilibrium depends on the bed strength, which is set by the time‐varying grain size distribution and packing. We use flume experiments to understand ξ and find that in the limit ξ>>1, the time‐varying response of an experimental channel depends sensitively on the spatially‐averaged bed shear stress ratio τ/τref. When τ/τref≈1.5, Ub was the dominant control on disequilibrium. However, when τ/τref≈2.0, Up contributed more significantly to disequilibrium. These results suggest that after an upstream supply perturbation, the equilibrium timescale is governed by Up, which we show is consistent with expectations from linear damping theory. Our experimental test of ξ is promising, but inconclusive with respect to our hypothesis. This uncertainty can be readily addressed with numerical or additional physical experiments. This article is protected by copyright. All rights reserved.
... The principle of efficiency selection (ES) in geomorphology includes principles of hydraulic, gradient, and resistance selection. As water is redistributed at Earth's surface', according to the principle of hydraulic selection the most efficient pathways tend to persist and prevail, whereas the less efficient options are abandoned (Phillips, 2010). Channel development initiates through chance concentrations of water and shear stress; and the selection favors steeper, smoother, and/or (if conveyance capacity is limited) larger channels, which grow and persist via positive feedbacks. ...
... Channel development initiates through chance concentrations of water and shear stress; and the selection favors steeper, smoother, and/or (if conveyance capacity is limited) larger channels, which grow and persist via positive feedbacks. The principle of gradient selection refers to the persistence and growth of gradient-associated geomorphic features that result from the selection of the steepest flow path (Phillips, 2010(Phillips, , 2011. As mass and energy fluxes tend to follow the steepest gradients of potentials and concentrations, this leads to the development of the geomorphic features associated with these gradients via positive feedback. ...
... Nanson and Huang (2008) showed that the extremal hypotheses for fluvial hydraulic geometry are special cases of the LAP. The LAP applied to fluvial systems is, in turn, equivalent to a tendency to favor configurations of maximum efficiency Nanson, 2000, 2007;Huang, 2008, 2016;Phillips, 2010Phillips, , 2011Huang et al., 2014). Leopold (1994) and Twidale (2004) also described fluvial system evolution as a process of preferential selection of the most efficient flow routes, and Smith (2010) incorporated this principle in his model for the development of channelized drainage. ...
... Rivers and their floodplains are process-response systems whose geomorphic state is the sum result of geologic setting and hydrologic drivers (Nanson and Croke, 1992;Phillips, 2010). Significant changes to hydraulic gradient, discharge, or sediment load result in adjustment toward a new geomorphic condition (e.g., Knox, 2006;Hupp et al., 2009aHupp et al., , 2015. ...
... Significant changes to hydraulic gradient, discharge, or sediment load result in adjustment toward a new geomorphic condition (e.g., Knox, 2006;Hupp et al., 2009aHupp et al., , 2015. The impacts of localized geomorphic change are often insignificant to the function of entire large river systems (Phillips, 2010), but geomorphic adjustment at the reach and regional scale can trigger substantial change to river-floodplain interactions and feedback mechanisms between geomorphic, hydrologic, and ecological processes (e.g., Hupp, 1992;Ligon et al., 1995;Knox, 2006;Hupp et al., 2009b;Osterkamp and Hupp, 2010). ...
... The dominant channel adjustment in the lower portion of the study area has been channel incision with coupled bank widening. Incision and bank widening processes are vehicles for dissipating excess energy following perturbation to existing slope-sediment transport regimes (Simon, 1992;Nanson and Huang, 2008;Phillips, 2010). Geomorphic adjustment of river channels can be framed as a consequence of slope (S) surplus or deficit relative to the critical slope (S cr ), where for a given discharge, stream power is just enough to transport sediment load (Phillips, 2010). ...
Responses of large regulated rivers to contemporary changes in base level are not well understood. We used field measurements and historical analysis of air photos and topographic maps to identify geomorphic trends of the lower White River, Arkansas, USA, in the 70 years following base-level lowering at its confluence with the Mississippi River and concurrent with flood control by dams. Incision was identified below a knickpoint area upstream of St. Charles, AR, and increases over the lowermost ~ 90 km of the study site to ~ 2 m near the confluence with the Mississippi River. Mean bankfull width increased by 30 m (21%) from 1930 to 2010. Bank widening appears to be the result of flow regulation above the incision knickpoint and concomitant with incision below the knickpoint. Hydraulic modeling indicated that geomorphic adjustments likely reduced flooding by 58% during frequent floods in the incised, lowermost floodplain affected by backwater flooding from the Mississippi River and by 22% above the knickpoint area. Dominance of backwater flooding in the incised reach indicates that incision is more important than flood control on the lower White River in altering flooding and also suggests that the Mississippi River may be the dominant control in shaping the lower floodplain. Overall, results aght the complex geomorphic adjustment in large river-floodplain systems in response to anthropogenic modifications and their implications, including reduced river-floodplain connectivity.
... Rivers are important elements in the ecological balanced system of a city and precious resources of the landscape system. Urban rivers are significant for the construction of ecological cities, such as flood control, drainage, ecological landscape, etc. 1 Due to the development and utilization of water storage, linking river system for surrounding flood control and other factors, urban landscape water, ecological river and creeks in the city become powerless and lifeless especially after adding gates, which leads to its self-purification capacity degradation and environmental deterioration. All these consequences cannot satisfy the ecological requirements and need to take some measures to alleviate the deterioration of water quality. ...
... [5][6][7] Physical methods had high efficiency and the process was simple, but the equipment could be costly, and the treatment results were not thorough and the sustainability was poor. The second approach was to 1 apply chemicals such as algicide or adsorbent in the polluted water, so that the pollutants could be transferred from water phase to another phase by the action of these chemicals. 8,9 Even though this approach could be effective, it only changed the state and could not fundamentally eliminate the pollutants. ...
... where n 11 and Q 11 respectively stand for the pump's actual speed and actual discharge when D 1 ¼ 1 m and H ¼ 1 m , and n, Q, D 1 , H, and are rotating speed, unit's discharge, runner diameter, pump lift, and unit efficiency, respectively. Assuming that H is the same on the prototype and the model units, the calculated model rotational speed was n M ¼ 720 r/min when the prototype rotational speed was n P ¼ 480 r/min using equation (1). The main working parameters of model pump are shown in Table 1. ...
To make water in urban rivers flowing and improve water quality, an eco-gate pump unit which uses plate gate as a carrier was designed and developed based on computational fluid dynamics method and the model test in this paper. Firstly, the bidirectional operation energy performance of eco-gate pump was verified by the model test. The results showed that the numerical simulation results were consistent with the model test results with an error of ±2% at the optimum operating condition both in the pumping mode and the powering mode, which verified the reliability and accuracy of numerical stimulation. The major sources of error associated with the power losses were due to the fact that the blade tip leakage and the blade deflection were not considered in computational fluid dynamics simulation. The thickened blade edges also contributed to the differences. On this basis, different schemes of runner blade’s setting angles, blade number, blade lean angle, bulb body’s length, and shoulder profile line were designed, and their influences on eco-gate pump’s performance were analyzed to choose the best runner type and bulb body’s profile line through the computational fluid dynamics numerical simulations. The numerical results showed that the eco-gate pump had the best hydraulic performance when the blade’s setting angle was ϕ = 23°, blade number z = 5, blade’s lean angle a = 0° and bulb body’s shoulder profile line was convex type with length L = 100 mm. The developed pump showed the highest efficiency of 70.95% under the condition of design discharge of 500 L/s and design lift of 1.0 m. Furthermore, the whole flow pattern of the optimized pump was analyzed. Overall, the eco-gate pump presented in this paper can make use of its low-lift pumping function to replenish the target water body more efficient than other similar products. At the same time, it also can utilize the micro-head water energy fully to generate power.
... Others are either directly based on MEP, or propose maximum Chapter 6 Attractors and goal functions in landscape evolution 179 energy throughput, which also implies MEP (Fath et al., 2001;Ozawa et al., 2003;Dewar, 2005;Kleidon et al., 2010). A third group is based on preferential utilization, preservation, or replication of the most efficient flux gradients, and is thus based on a principle of gradient selection (GS; Phillips, 2010a;2011). These are summarized in Tables 6.1 and 6.2. ...
... Self-organizing mechanisms promote maximum energy efficiency Levchenko (1999), Levchenko et al. Focuses on mutual aid in evolution rather than competition Kropotkin (1902) Minimum stream power Fluvial channels Channels adjust so as to transport sediment with minimum possible expenditure of work Phrased in various forms e.g., Brebner and Wilson (1967), Yang (1971), reviews: Griffiths (1984), Paik and Kumar (2010) Maximum flow efficiency Fluvial channels Channels adjust to maximize flow efficiency and minimize energy expenditure Phrased in various forms e.g., Davies and Sutherland (1980), Yang et al. (1981), Jia (1990) reviews: Molnar and Ramirez (1998), Paik and Kumar (2010) Increasing ascendency Ecosystems Ecosystem development characterized by increasing ascendency Ascendency ¼ f(total mass/ energy flux, specificity of each flow) Ulanowicz (1980Ulanowicz ( , 1997 Minimize empower/ exergy ratio Ecological systems Efficiency enhanced by maximizing empower relative to exergy Empower ¼ rate of emergy acquisition Bastianoni and Marchettini (1997) Least action Fluvial channels Channels tend to adjust so as to transport sediment with the minimum possible work Consistent with maximum flow efficiency Huang and Nanson (2000), Nanson and Huang (2008) Gradient selection Geomorphic systems Steeper, more efficient flux paths tend to persist and grow Also: resistance selectiondpreferential preservation of more resistant features Phillips (2010aPhillips ( , 2011 attractive when they can be framed in terms of emergent behavior rather than purported goals of environmental systems. Emergence is independent of any teleological implications and is simpler than postulating goal functions. ...
This chapter focuses on the (not necessarily final) destination of landscape evolution—the attractors that landscapes may move toward and the goal functions that govern these trajectories. Single-outcome concepts posit that landscape systems move toward a single self-perpetuating state. These include notions of progression toward climax or mature forms, stable equilibrium conditions, or self-organized critical states. Multi-outcome models include notions of alternative stable states, nonequilibrium systems, and unstable attractors. As they evolve, landscapes have plasticity defined by their degrees of freedom, and constraints imposed by limits on energy, matter, and geographical space. These can be described using concepts of a multidimensional resource or landscape evolution space. Goal functions for landscape evolution are generally based on increasing fitness, often assessed in terms of optimality hypotheses, which systems strive toward maximizing or minimizing some aspect of energy and/or mass flux. Many of these are directly or indirectly related to the least action principle and maximum entropy production. Apparent goal functions can generally be explained on the basis of emergent phenomena. Landscape systems cannot aspire to anything in a literal sense, and there exist no laws that dictate trends toward the optimal states. However, if these optimal states are associated with advantages in the formation, survival, and replication of landscape components, then trends toward the optima will frequently be observed. Emergence and general principles of selection can tie together the majority of the concepts of attractors and goal functions in landscape evolution.
... It is unclear over which temporal and spatial scales this condition of bed equilibrium holds true. Another criticism of the concept of bed equilibrium is the very existence of a goal function that would push the stream to reach equilibrium: admittedly, for humans, the river's role is to transport water and sediment, but there is no specific reason why a river would maximize its transport capacity and seek to maintain its bed at equilibrium (Phillips, 2010). ...
... Although geomorphologists have mostly regarded bedload transport as a mechanism of bed equilibrium (Phillips, 2010), some have begun to consider rivers to be closer to punctuated equilibrium systems than to stable systems satisfying Lane's balance in Eqs (8). Indeed, on sufficiently large spatial and temporal scales, rivers experience bursts of bedload transport with rapid changes in bed morphology over short time intervals interrupted by long periods of weak activity (stasis). ...
By the late nineteenth century, the scientific study of bedload transport had emerged as an offshoot of hydraulics and geomorphology. Since then, computing bedload transport rates has attracted considerable attention, but whereas other environmental sciences have seen their predictive capacities grow over time, particularly thanks to increased computing power, engineers and scientists are unable to predict bedload transport rates to within better than one order of magnitude. Why have we failed to improve our predictive capacity to any significant degree? A commonly shared view is that the study of bedload transport has more in common with the earth sciences than hydraulics: bedload transport rates depend on many processes that vary nonlinearly, involve various time and space scales, and are interrelated to each other. All this makes it difficult to view bedload as merely particle transport in a turbulent flow – something which can be studied in the laboratory in isolation from the natural environment. Over the last two decades, more emphasis has been put on the noisy dynamics characterizing bedload transport. This Vision Paper makes a strong case for recognizing noise (e.g. bedload transport rate fluctuations) as an intrinsic feature of bedload transport. Improving our predictive capacities requires a better understanding of the origins and nature of noise in bedload transport. This paper also reviews some of the challenges that need to be addressed in current research and teaching.
... What is an appropriate measure of fitness in Earth surface systems (ESS)? A number of hypotheses have been proposed for optimal structures and configurations of, e.g., ecosystems, channel networks, topography, etc. Most, if not all, of these principles can be reduced to (or subsumed under) the idea of efficiency or resistance selection--more efficient and resistant forms and structures are more likely to persist and grow, or to occur repeatedly, than less efficient ones (see Jorgensen, 1997;Fath et al., 2001;Ulanowicz et al., 2006 on ecological systems; Huang and Nanson, 2000;Nanson and Huang, 2008;2018;Paik and Kumar, 2010;Phillips, 2010;2011;Smith, 2010;and Huang et al., 2014 on geomorphic systems;andOzawa et al., 2003 andKleidon et al., 2013 on geophysical phenomena more generally). To the extent that "fitter" (more efficient and/or resistant) geomorphic features and structures are subject to not just preferential preservation, but also enhancement (via positive feedbacks) and/or propagation, then algorithmic evolution models may apply to geomorphology, too--implying the possibility of geomorphic creativity. ...
... Thus creativity and innovation in biological evolution was accompanied by creativity and innovation in fluvial systems. However, selection for maximum efficiency structures and configurations has been shown to operate for a number of abiotic phenomena, including atmospheric fluid flows (Ozawa, et al., 2003); development of subsurface flow networks (Hunt, 2016), and evolution of stream channel networks, planforms, and cross-sections (Huang and Nanson, 2000;Nanson and Huang, 2008;2018;Kleidon et al., 2013;Paik and Kumar, 2010;Phillips, 2010;2011;Smith, 2010;Huang et al., 2014). ...
In biological evolution, creativity occurs in the appearance of new entities by evolutionary dynamics. This is linked to mutations and genetic drift, which cannot occur in geophysical phenomena. Biota can exhibit evolutionary creativity that influences landforms, but how does creativity (defined here as the capacity for emergence of new entities that increase the adjustedness of the landscape to environmental conditions) occur in landforms and landscapes as entities independent of biota? Creativity in geomorphic evolution does not require any sort of goal functions or purposeful innovation‐‐just that geomorphic development is capable of producing novelties that may be better adapted (more efficient or durable) than predecessors. Independently of biota, evidence exists that landforms may develop to become more or less "fit" in terms of efficiency and/or durability. Thus emergence of novel features may lead to their persistence. Emergence of novel forms is illustrated for the case of karst sinkholes (dolines), which indicates increasing geomorphic diversity over Ma and Ga timescales. A case study of fluviokarst chronosequences in Kentucky demonstrates emergence and elimination of landforms as landscapes evolve. Some of these may represent generally (as opposed to locally) novel landforms. While this paper is more suggestive than demonstrative, results strongly suggest evolutionary creativity in geomorphology both tied to, and independent of, biological evolution. This occurs due to emergence of geomorphic entities that are subject to selection that tends to increase efficiency and durability.
... Rodriguez-Iturbe and Rinaldo, 1997) and the proposals to adopt a variety of extremal or optimisation hypotheses in river hydraulics (discussed below). Phillips (2006aPhillips ( , 2006bPhillips ( , 2010Phillips ( , 2011 has provided a number of important assessments on how geomorphologists might approach their research, focussing on issues such as the effect of deterministic chaos, non-linearity in landform responses, evaluating the primary role or task of rivers and the place of emergence and pseudo-equilibrium in geomorphology. With the exception of ideas relating to equilibrium these proposals have not been widely adopted as general theories as yet, possibly because of a lack of clear evidence. ...
... The level to which an acceptance of the need occurs to involve equilibrium theory in fluvial geomorphology will undoubtedly be influenced by a comprehensive understanding of the details of the function or purpose of a river. Phillips (2010) strips function down to the redistribution of excess water via channels and channel networks that follow the paths of least resistance, usually along the steepest gradients, with sediment erosion, deposition and transport being essentially a by-product of this process. In alluvial channels, however, this overlooks the interaction of the three basic flow equations of continuity, flow resistance and, importantly, sediment transport, all three playing an integral role in determining the equilibrium form of the channel (Huang and Nanson, 2000). ...
Until recently no universal agreement as to a philosophical or scientific methodological framework has been proposed to guide the study of fluvial geomorphology. An understanding of river form and process requires an understanding of the principles that govern the behaviour and evolution of alluvial rivers at the most fundamental level. To date, the investigations of such principles have followed four approaches: develop qualitative unifying theories that are usually untested; collect and examine data visually and statistically to define semi-quantitative relationships among variables; apply Newtonian theoretical and empirical mechanics in a reductionist manner; resolve the primary flow equations theoretically by assuming maximum or minimum outputs. Here we recommend not a fifth but an overarching philosophy to embrace all four: clarifying and formalising an understanding of the . evolution of river channels and iterative directional changes in the context of . least action principle (LAP), the theoretical basis of . variational mechanics. LAP is exemplified in rivers in the form of . maximum flow efficiency (MFE). A sophisticated understanding of evolution in its broadest sense is essential to understand how rivers adjust towards an optimum state rather than towards some other. Because rivers, as dynamic contemporary systems, flow in valleys that are commonly historical landforms and often tectonically determined, we propose that most of the world's alluvial rivers are over-powered for the work they must do. To remain stable they commonly evolve to expend surplus energy via a variety of . dynamic equilibrium forms that will further adjust, where possible, to maximise their stability as much less common MFE forms in . stationary equilibrium. This paper: . 1. Shows that the theory of evolution is derived from, and applicable to, both the physical and biological sciences; . 2. Focusses the development of theory in geomorphology on the development of equilibrium theory; . 3. Proposes that river channels, like organisms, evolve . teleomatically (progression towards an end-state by following natural laws) and iteratively (one stage forming the basis for the next) towards an optimal end-state; . 4. Describes LAP as the methodological basis for understanding the self-adjustment alluvial channels towards MFE. . 5. Acknowledges that whereas river channels that form within their unmodified alluvium evolve into optimal minimum-energy systems, exogenic variables, such as riparian or aquatic vegetation, can cause significant variations in resultant river-styles. We specifically attempt to address Luna Leopold's lament in 1994 that no clearly expressed philosophy explains the remarkable self-adjustment of alluvial channels.
... In this context, the equilibrium condition performs as the attractor state in the evolution of fluvial systems, though it may not necessarily be the actual state that the river achieves once it has relaxed following disturbance Nanson, 2000, 2002;Nanson and Huang, 2008). Phillips (2010) argued, however, that the dynamic and variable responses in geomorphology of rivers were just emergent byproducts of fundamental physical mechanisms within them. The steady-state was just a possible state towards which a channel changes, rather than inevitable or normal trends; thus, it should not imply the goal function or attractor state of river evolution. ...
... The steady-state was just a possible state towards which a channel changes, rather than inevitable or normal trends; thus, it should not imply the goal function or attractor state of river evolution. In recognition of multiple potential characteristic or equilibrium forms in the evolution of fluvial systems, Phillips (2009Phillips ( , 2010 indicated that responses or adjustments to imposed changes were finite and therefore they were ultimately characterized by relaxation time equilibrium, the weakest form of equilibrium notions, implying that a stream has completed its response to a given disturbance or environmental change. Therefore, in practice, the equilibrium concept or relaxed state associated with the tendency for responses to disturbance that are dominated by negative feedback mechanisms may be taken as an asymptotic or reference state towards which rivers relax in modelling the evolution of fluvial systems. ...
A general framework for modelling morphological responses to perturbation is proposed, based on the underpinning principle that the rates of morphological response in alluvial channels are initially high and then decrease through time as the system relaxes following disturbance. The framework includes three morphological response models, each developed from the fundamental rate law, which has the form of an exponential decay function. These models consider the possibility that characteristic behaviours of the fluvial system, such as delayed response and/or cumulative effects, may affect morphological responses, making them capable of representing relaxation paths and times for a range of morphological response variables, whatever their initial states. To test their utility, the models in the framework were applied to simulate the sequence of geomorphological responses to disruption observed in selected rivers with well-documented histories of morphological perturbation, adjustment and recovery. The results demonstrate that the models in the general framework can successfully simulate temporal and spatial patterns of morphological response in the fluvial system under a range of different circumstances, while also indicating how their reliability could be further improved.
... These processes are driven in part by the hydraulics of river energy transfer, and in part by the massenergy interactions that characterise bar development Zolezzi et al., 2012). Such interactions are complex; they may arise autogenically or be driven by external forcing (Nanson and Huang, 2008;Phillips, 2010), and disentangling the various drivers remains a key challenge in understanding how rivers 'come to be different', and how they are influenced by environmental change Brewer, 2001, 2003;Kleinhans, 2010;Ashworth and Lewin, 2012). ...
... Sinuosity is certainly a useful metric, as it relates directly to channel slope and stream power. However, the focus on sinuosity is severely restrictive in that it ignores other mechanisms by which energy may be mediated in alluvial channel environments, such as bar formation and dissection (Huang and Nanson, 2007;Phillips, 2010), or changes in channel width (Harmar and Clifford, 2006;Luchi et al., 2010). Coupled with neglect of the process of chute cutoff (except in the case of Constantine and Dunne, 2008), a focus on sinuosity essentially restricts the relevance of these analyses to a single class of planform pattern (this is acknowledged by Stølum, 1998); low energy meandering rivers that migrate slowly and are dominated by neck cutoff (see Kleinhans and van den Berg, 2011). ...
... With respect to hydrological and geomorphological phenomena, this often occurs due to gradient, resistance, and efficiency selection. Pathways with the steepest flux gradients, landscape elements with the highest resistance (to, e.g., weathering and erosion), and configurations with the greatest efficiency for work (consistent with the least action principle) preferentially occur, recur, grow, and survive (Hunt, 1998;2016;Huang and Nanson, 2000;Phillips, 2010;2011c;Smith, 2010;Huang, 2017, 2018). ...
In Earth surface systems (ESS), everything is connected to everything else, an aphorism often called the First Law of Ecology and of geography. Such linkages are not always direct and unmediated, but many ESS, represented as networks of interacting components, attain or approach full, direct connectivity among components. The question is how and why this happens at the system or network scale. The crowded landscape concept dictates that linkages and connections among ESS components are inevitable. The connection selection concept holds that the linkages among components are (often) advantageous to the network and are selected for, and thereby preserved and enhanced. These network advantages are illustrated via algebraic graph theory. For a given number of components in an ESS, as the number of links or connections increases, spectral radius, graph energy, and algebraic connectivity increase. While the advantages (if any) of increased complexity are unclear, higher spectral radii are directly correlated with higher graph energy. The greater graph energy is associated with more intense feedback in the system, and tighter coupling among components. This in turn reflects advantageous properties of more intense cycling of water, nutrients, and minerals, as well as multiple potential degrees of freedom for individual components to respond to changes. The increase of algebraic connectivity reflects a greater ability or tendency for the network to respond to changes in concert.
... In liquid form, going somewhere is driven by gravity. Fluvial systems, for instance, develop so as to do this, with selection processes favouring concentrated or channelized over diffuse flows and branching channel networks (Phillips, 2010). Associated processes, such as erosion, sediment and solute transport, and biogeochemical fluxes are byproducts of the fundamental hydrological job of accommodating excess water. ...
The question of whether the concept of adaptation can be applied to Earth surface systems (independently of biological adaptation) is addressed by examining hydrolog- ical flow systems. Hydrological systems are represented in terms of a partitioning of water inputs among various flux and storage components and outflows or outputs of the system. Partitioning is contingent on the flow system in question and the synoptic situation (i.e., drier, low-input vs. wetter, high-input conditions). The general allocation among inputs, flows through or within the system, storage and outputs is examined via analysis of 20 scenarios for soil hydrology, a fluvial channel-wetland complex and a fluviokarst landscape representing different combinations of positive, negative and zero (neutral) relationships among these elements, and positive self- reinforcing and negative self-limiting effects. Conditions for stability were deter- mined using the Routh–Hurwitz criteria and linked to the two fundamental roles or ‘jobs’ of hydrological flow systems. The ecological job is to support biota and biogeo- chemical fluxes and transformations necessary for ecosystem functions. The geophysical job is to remove excess water. Results show that low-input scenarios for the soil, fluvial wetland and fluviokarst scenarios are marked by dynamical instability. During drier periods the geophysical job is irrelevant and the ecological functions are suboptimal. Instability allows for rapid state changes when moisture inputs increase, to system states that support ecosystem functions. High-input, excess moisture and flood scenarios, by contrast, are generally dynamically stable. In wetter conditions, the ecological functions are not moisture-stressed, and the geophysical job becomes paramount. The high-input stability is associated with activation of ‘spillway’ mecha- nisms that allow the systems to maintain themselves by efficient export and aug- mented storage of excess water. Contingent partitioning indeed appears to be an adaptation mechanism in hydrological systems and suggests the possibility of adapta- tion in other Earth surface systems with important abiotic components.
... However, nonuniform and unsteady bed load transport was the prevailing outcome ( Figure 1 and associated discussion), indicating that transport fluctuations are just as important as averaged conditions, assuming transport rates converge to an average quantity for a given sampling interval and total observation time period. Notably and importantly, prior results raise an expectation of transport nonuniformity between riffle and pool during periods of flow unsteadiness (Jackson & Beschta, 1982;Milan, 2013), and for increasing discharge (Sear, 1996), but not necessarily for prolonged periods of flow and sediment supply steadiness, provided concepts such as "equilibrium" or transport "steady-state" are invoked (Chartrand et al., 2019;Phillips, 2010Phillips, , 2011Thorn & Welford, 1994). ...
This study examines channel dynamics and bed load transport through a riffle and pool sequence forced by downstream channel width variations within an experimental flume. The experiment consisted of four runs across, which we compare and contrast local and spatially averaged bed surface texture and topography, sediment transport rates, and sediment mobility at five locations across a pool‐riffle pair. Sediment transport was measured using mini Helley‐Smith samplers and particle tracers seeded in the monitored riffle and pool. In this study, “local” sediment transport rates were highly variable across the five monitoring locations. The lowest sediment transport rate was recorded at the riffle tail whereas the highest rates were measured at the riffle head and the pool center. The texture of the bed surface and transported load do not explain measured bed load transport trends and depending on how the measurements are aggregated differing interpretations are supported. In general, the bed texture in the pool was finer than the texture in the riffle, however, specific grain‐size percentile classes derived from pooled population analysis suggests little to no difference between pool and riffle texture. The combined results highlight the importance of acknowledging and applying analysis techniques to better understand the inherent variability of bed load transport within channel reaches where morphology differs, such as pools and riffles.
... With respect to hydrological and geomorphological phenomena, this often occurs due to gradient, resistance, and efficiency selection. Pathways with the steepest flux gradients, landscape elements with the highest resistance (to, e.g., weathering and erosion), and configurations with the greatest efficiency for work (consistent with the least action principle) preferentially occur, recur, grow, and survive (Hunt, 1998;2016;Huang and Nanson, 2000;Phillips, 2010;2011c;Smith, 2010;Huang, 2017, 2018). ...
In Earth surface systems (ESS), everything is connected to everything else, an aphorism often called the First Law of Geography and of ecology. Such linkages are not always direct and unmediated, but many ESS, represented as networks of interacting components, attain or approach full, direct connectivity among components. The question is how and why this happens at the system or network scale. The crowded landscape concept dictates that linkages and connections among ESS components are inevitable. The connection selection concept holds that the linkages among components are advantageous to the network and are selected for and thereby preserved and enhanced. These network advantages are illustrated via algebraic graph theory. For a given number of components in an ESS, as the number of links or connections increases, spectral radius, graph energy, and algebraic connectivity increase. While the advantages (if any) of increased complexity are unclear, higher spectral radii are directly correlated with higher graph energy. The greater E(g) is associated with more intense feedback in the system, and tighter coupling among components. This in turn reflects advantageous properties of more intense cycling of water, nutrients, and minerals, as well as multiple potential degrees of freedom for individual components to respond to changes. The increase of algebraic connectivity reflects a greater ability or tendency for the network to respond in concert to changes.
... Flow-driven biophysical processes occur at different hierarchical levels (Thorp et al., 2008). Thus, responses to flow alterations within river networks are inherently complex, where physical and ecological processes strongly interact, often in a non-linear manner (Phillips, 2010). In this way, larger-scale physical factors (i.e. ...
Hydropeaking leads to major anthropogenic disturbance of river networks worldwide. Flow variation imposed by hydropeaking may significantly affect macroinvertebrate assemblages within the river network. As such, the responses of macroinvertebrate assemblages to hydropeaking are expected to be complex and vary across spatial and temporal scales as well as ecological organization levels. To unpack this complexity, we assessed the interplay of geomorphic and hydrological variables as drivers of the responses of macroinvertebrate assemblages to hydropeaking. Specifically, we studied different levels of ecological organization of macroinvertebrate assemblages in two functional process zones (FPZs; Sub-Andean and Central Valley Gravel Dominated) subjected to different flow management in two Chilean Andean river networks. Hydropeaking caused significant reduction of macroinvertebrate abundances in both FPZs and at all ecological organization levels with the exception of one feeding guild (scrapers). Furthermore, the response of macroinvertebrate assemblage variance was stronger in the Central Valley Gravel Dominated FPZ. Both geomorphic and hydrological variables drove macroinvertebrate assemblage responses. However, the effects of the principal geomorphic variables operated at valley (meso) spatial scale and main hydrological variables operated at sub-daily (micro) temporal scale. Therefore, to minimize the effects of hydropeaking on macroinvertebrate assemblages, flow management should consider reduction of sub-daily variability. Furthermore, placement of new barriers should take into account not only their position within the river network but also their effects downstream that strongly depend on characteristics of river valley.
... Um dos principais meios utilizados para a avaliação da condição de equilíbrio do relevo tem sido o sistema fluvial e suas respostas sobre as vertentes deduzidas da organização de materiais e formas. Mudanças nos sistemas fluviais tais como impactos humanos, mudança climática, ou deformação tectônica são frequentemente assumidos como fatores que afastam os rios do equilíbrio (Phillips, 2010). No entanto, distinguir a ação individual de tais causas na complexidade multicausal dos sistemas geomorfológicos é um desafio a ser continuamente enfrentado no trabalho de investigação científica. ...
The concept of equilibrium has been frequently used in geomorphological
studies in different contexts of drivers, processes and relief shapes. It is a concept
with different meanings, which entail various underlying theoretical approaches to the
definition. Nevertheless seem to converge towards an equivalence of forces manifest
in a given geomorphological system. In this paper, which is part of the author’s doctoral
thesis, the main approaches given to the concept of equilibrium by different authors have
been characterized from the 19th to the early 21st century. Among the conclusions of
this review include: (i) the concept of equilibrium in geomorphology is derived from the
Newtonian conception of equality (balance) of forces; (ii) such a Newtonian conception
is common to many authors from the 19th century to now in the metodological terms
although there is different treatment on how to assess equilibrium condition of the relief;
(iii) the challenging issue around the relief equilibrium analysis and its operationalization
in terms of shapes, materials and processes.
... Methods using analyses have not been previously applied on Mars, most likely due to the lack of uplift to balance out erosion, which resulted in a transient retreating wall. However, even on Earth achieving steady state is rare (Phillips, 2010). As mentioned in section 3.1, in the case of U = 0, any gradient variations in a profile will be due to spatial heterogeneity in E or K. On Earth, channel concavity and steepness indexes can be used to infer climatic conditions and/or erosion rates, respectively, and in the absence of uplift and significant lithological contrasts these methods are appropriate for examining climatic controlling processes on Martian crater rim channels. ...
Alluvial fans on Mars, which are primarily sourced from erosional alcoves incised into crater rims, record a period of increased surface runoff which ended >1 Ga. However, we lack quantitative constraints on the frequency and duration of river-forming processes and the climatic conditions that accompanied these long-term habitable episodes. Here we use bedrock erosion and sediment transport models to show that the cumulative time span of wet activity (i.e., nonzero erosion and deposition) was between 100 years to 1 Myr excluding dry years. We use Context Camera (CTX) digital elevation models to compile a data set of >200 channels upstream of depositional fans and determine key fluvial geometry metrics. Results from calculating Mars stream power parameters are compared to great escarpment channels and globally distributed bedrock rivers on Earth. Although Martian channel profile morphologies fall within the range of those on Earth, they are slightly less concave-up (concavity index = 0.35 ± 0.16) and steeper for a given drainage area(reference steepness index = 0.09 ± 0.03, for reference drainage area, Ar = 1 ×10^7 m2). Timescales depend strongly on poorly constrained variables such as erodability and grain size. Channel morphologies, intermittencies, spatial distributions, and orientations collectively suggest an arid climate and a source from snowmelt on steep crater rims, possibly from obliquity-paced insolation variations or orographic accumulation. Derived timescales are consistent with erosion rates and intermittencies observed in arid environments on Earth and do not support short-lived or catastrophic triggers for the warm climate conditions (such as impacts or individual volcanic eruptions).
... Esta tradição continua na pesquisa geográfica crítica contemporânea. Por exemplo, Clark e Richards (2002), Fryirs e Brierley (2009), Phillips (2010), Rhoads et al. (1999, e Wohl e Merritts (2007) demonstram a maneira como as estruturas de referência aparentemente objetivas da geomorfologia fluvial são imbuídas de presunções de valores sobre a relevância das escalas humanas nas mudanças ambientais. As noções da "naturalidade" dos canais fluviais são tão normativas e contextuais quanto "científicas", e moldam a gestão ambiental de maneiras específicas. ...
Um artigo de opinião, recentemente publicado, reacendeu o debate sobre se a composição interdisciplinar que atualmente existe na área da Geografia é um vestígio da história ou uma atual e potencial fonte de vitalidade intelectual. Neste artigo nós adotamos esta última postura e destacamos os benefícios da integração prolongada da Geografia Física e Geografia Humana crítica. Por razões políticas e pragmáticas, nós denominamos esta área de pesquisa e prática indissociáveis de Geografia Física Crítica (GFC). A GFC combina a atenção crítica às relações de poder com o conhecimento profundo das ciências biofísicas ou tecnológicas a serviço da transformação social e ambiental. Argumentamos que a pesquisa da GFC, quando realizada por indivíduos ou equipes, podem melhorar a qualidade intelectual e expandir a relevância política da Geografia Física e da Humana críticas, dado que é cada vez mais impraticável analisar sistemas naturais e sociais separadamente: as paisagens sociobiofísicas são tanto produto de relações desiguais de poder, do legado histórico do colonialismo e das disparidades raciais e de gênero, quanto de fatores físicos como a hidrologia, ecologia e alterações climáticas. Neste texto, nós apresentamos os trabalhos existentes em GFC, discutimos os principais benefícios de um engajamento crítico integrador na pesquisa, no ensino e na extensão; e oferecemos nossas reflexões coletivas sobre como fazer uma GFC viável.
... Analysis of river morphology provides an important means to explore the modern landscape and tectonic evolution of basin-mountain transition zones (e.g., Clift & Blusztajn, 2005;Phillips, 2010;Twidale, 2004;Whipple, 2004). River incision drives regional erosion patterns and affects the thermal structure of the crust (e.g., Finnegan et al., 2008;Whipple, 2004), and rivers transport sediments and play a significant role in the source-to-sink system (e.g., Zhang et al., 2012;J. ...
The Bayin River, the largest river in the northeastern Qaidam basin, plays an important role in the source‐to‐sink system and landscape evolution at the basin‐mountain boundary between the Qilian Mountains and the Qaidam basin in the northern Tibetan Plateau. In this study, we conduct field observation, topographic analysis, zircon U–Pb dating, and apatite (U–Th)/He dating to constrain the landscape and tectonic evolution of the Bayin River watershed. Bedrock zircon U–Pb dating indicates the age group of 420–450 Ma for far‐source sediments and the age group of >1,700 Ma for near‐source sediments in the Bayin River watershed. Detrital zircon U–Pb dating results from the Mesozoic and Cenozoic strata in the Bayin River watershed reveal that the most important source transition occurred during the Cretaceous. The Zongwulong Mountains gradually uplifted throughout the Cenozoic, along with decreasing far source and increasing near source based on detrital zircon U–Pb dating. Rapid uplift occurred across the Qilian Mountains during the late Cenozoic, leading to high normalized steepness indices, young apatite (U–Th)/He ages, and deep incised valleys at the basin‐mountain transition zone. The knickpoints caused by the latest headward erosion just reach an elevation of ~3,800 m on the river longitudinal profiles, indicating that the latest uplift magnitude is ~300–400 m relative to the basin surface of the Qaidam basin. Elevation distribution and apatite (U–Th)/He ages reveal that river incision leads to high relief in the Zongwulong Mountains and influences its tectonic evolution.
... Las planicies de inundación son definidas como áreas relativamente planas, aledañas a los cauces activos de los ríos, que se inundan durante crecientes (Syvitski, Overeem, Brakenridge, Hannon, 2012)SRTM and MODIS. La baja pendiente de las planicies de inundación favorece la deposición de sedimentos, debido a que las velocidades de flujo disminuyen y la energía requerida para transportar dichos sedimentos ya no es suficiente (Phillips, 2010). Eventos de inundación también arrastran y depositan grandes cantidades de sedimentos de todos los tamaños. ...
La variabilidad en la configuración geomorfológica, la abundancia del recurso hídrico y la amplia gama en la cual se presentan las variables ambientales a lo largo del territorio nacional, dan origen a una diversidad en los tipos de humedales. Esta variabilidad de condiciones dificulta la identificación y la diferenciación de esta clase de hábitats en el contexto del paisaje, sin embargo, recientemente se ha desarrollado un gran esfuerzo institucional enfocado a la comprensión de estos hábitats. En el presente trabajo queremos exponer algunos aspectos relacionados con el componente físico, importantes para entender la dinámica del agua en el contexto de los humedales del Altiplano del Oriente antioqueño. Entre estos aspectos figuran las variaciones en la precipitación, evapotranspiración, geomorfología y la relación intrínseca con los acuíferos, factores principales que determinan el funcionamiento hidrológico de estos ecosistemas.
... In an attempt to provide a physical interpretation of these results, we can refer to the notion of rivers as hydraulic entities. Channel networks must develop to redistribute moisture, and sediment transport is a byproduct of hydrological processes [Phillips, 2010]. Soil moisture redistribution occurs through hydraulic selection principles: channels develop through concentrations of water and associated shear stress. ...
The theory of optimal channel networks (OCNs) explains the existence of self-similarities in river networks by multiple optimality principles, namely, (i) the minimum energy expenditure in any link, (ii) the equal energy expenditure per unit area of channel anywhere, and (iii) the minimum total energy expenditure (TEE). These principles have been used to generate OCNs from 2-D networks. The existing notion of OCN considers the concavity of river longitudinal profiles as a priori condition. Attempts to generate OCNs starting from a random 3-D digital elevation model (DEM) and minimizing solely TEE have failed to reproduce concave profiles. Yet alternative approaches can be devised from the three optimality principles, for instance, focusing on the local energy expenditure (LEE). In this paper, we propose a Multiobjective modeling framework for Riverscape Exploration (MoRE) via simultaneous optimization of multiple OCN criteria. MoRE adopts a multiobjective evolutionary algorithm and radial basis functions to efficiently guide DEM elevation variations required to shape 3-D OCNs. By minimizing both TEE and the variance in LEE, MoRE successfully reproduces realistic on-DEM, OCN-based riverscapes, for the first time. Simulated networks possess scaling laws of upstream area and length and river longitudinal profile resembling those of real river networks. The profile concavity of generated on-DEM OCNs emerges as a consequence of the minimization of TEE constrained to the equalization of LEE. Minimizing TEE under this condition generates networks that possess specific patterns of LEE, where the scaling of slope with basin area resembles the patterns observed in real river networks.
... Focusing on obstacle marks (state III) in the laboratory flume, scour depth, -width, -length and ridge width clearly showed a non-linear development towards a steady state, while ridge height and -length did not. This kind of temporal development of bedform geometries is consistent with the notion of a 'relaxation-time equilibrium' as discussed by Phillips (2010) in a fluvial-geomorphological context. Also, steady state scour depths, -widths, -lengths and ridge widths are significantly correlated with each other, while ridge heights and lengths are not. ...
A series of 51 experiments was conducted in a laboratory flume to investigate hydromorphological processes at submerged solitary boulder-like obstacles. As part of a programme to validate the experimental results, five sites with obstacle marks at large boulders were surveyed in two gravel-bed ephemeral streams located in eastern Spain. Three hydromorphological system states (I–III) were observed, each exhibiting characteristic dynamic interactions between hydraulic and sedimentary processes: (I) wake-vortex domination and absent scouring; (II) wake-vortex domination and downstream scouring; (III) horseshoe-vortex domination, frontal scouring and ridge-like sediment accumulation at the lee-side. The latter system state (III) comprises ‘typical’ fluvial obstacle marks. In mobile alluvial beds, obstacles may tilt upstream into the scour hole when frontal scour depths exceed 0.6 times obstacle length, favouring the rapid achievement of steady state conditions (state III). Field and experimentally modelled obstacle marks are related to each other when frontal scour depth, -width and -length as well as ridge width are used as morphometric variables. An analytical model was adapted and successfully applied for determination of these variables at the steady state condition. The model was also used to assess the critical threshold for the formation of obstacle marks. It integrates mean flow velocity, water depth, obstacle height and width, obstacle shape, median grain size relative to obstacle width and grain size distribution.
... Optimal behavior is related to feedbacks when they either reinforce optimal phenomena that happen to occur, or mitigate against suboptimal trends. For example, when more rapid or efficient material use or cycling in ecological systems confers a survival or reproductive advantage, this positively reinforces the trend toward maximizing cycling rates (e.g.(2008)used the term " survival of the most stable " to describe the iterative adjustments, and others have also invoked a process of hydraulic selection (more efficient flow paths are preferentially formed and enhanced) in the formation of fluvial channels (Leopold 1994;Twidale 2004;Phillips 2010a).Ulanowicz (1997)presented similar arguments (i.e., stability is positively related to persistence) for ecological systems. To the extent optimal patterns are based on probability or feedback considerations, both imply selection in the sense that the optimal patterns are more likely be preserved, reinforced, or replicated. ...
Metanarratives are critiqued and even rejected by many geographers and geoscientists. Yet, despite the inescapable role of geographical and historical contingency in physical geography, metanarratives are helpful, perhaps even necessary, in part because equifinality is common in Earth surface systems (ESS). Similarity of forms and patterns implies a possible single underlying cause. However, by definition the similar outcomes of equifinality are not the result of the same underlying processes, indicating that any encompassing construct must be in the form of a metanarrative. An effective metanarrative need not be strictly true, but should be useful in explanation, and its implications subject to empirical verification. Metanarratives should also be simplifying rather than complexifying. An example proposed here is the principle of efficiency selection: the most efficient pathways and modes of mass and energy flux are preferentially preserved and enhanced. This explains and unifies optimality principles proposed for a variety of ESS. Efficiency selection is testable based on observations and simplifying in that it encompasses a number of situations with a single concise proposition. According to the principle of efficiency selection, apparent optimality in ESS is neither teleological nor deterministically inevitable, but rather an emergent property.
... Within these imposed conditions, hydrology, sediment regime and vegetation conditions (flux boundary conditions) act on the valley bottom to determine the range of process behaviour that is possible, and the range of types of rivers that can form in that setting (Brierley and Fryirs, 2005;Hough-Snee et al., 2015). The extent to which various forms of confinement occur determines the degrees of freedom within which the river has the capacity to adjust (Phillips, 2008(Phillips, , 2010. Correctly interpreting the valley setting using measures of valley margin and valley bottom confinement is critical to correctly interpreting the contemporary character and behaviour of that river. ...
... The sediment bed load transport deeply affects river morphology, and at the same time, it can endanger river bank stability (Whitaker and Potts, 2007;Phillips, 2010). Lisle (1982) analysed natural gravel bed channels showing that the sediment transport contributes to modify pools and riffles morphologies. ...
... Valley bottoms can be bound by bedrock hillslopes, or by other landforms along the margins of valleys such as alluvial fans, terraces (abandoned floodplains), moraines or piedmonts . The shape, size and extent of a valley bottom in relation to the associated stream channel's width and position on the valley floor are reflected in the degree of river confinement, which in turn dictates the types of rivers that can form (Brierley and Fryirs, 2013) as well as their character, behavior and capacity to adjust (Phillips, 2008;Phillips, 2010). As such, correctly delineating the valley bottom and interpreting the valley-setting is critical to correctly interpreting river character and behavior . ...
The shape, size and extent of a valley bottom dictates the form and function of the associated river or stream. Consequently, accurate, watershed-wide delineation of valley bottoms is increasingly recognized as a necessary component of watershed management. While many valley bottom delineation approaches exist, methods that can be effectively applied across entire drainage networks to produce reasonably accurate results are lacking. Most existing tools are designed to work using high resolution topography data (i.e. > 2 m resolution Digital Elevation Model (DEM)) and can only be applied over relatively short reach lengths due to computational or data availability limitations. When these precise mapping approaches are applied throughout drainage networks (i.e. 102 to 104 km), the computational techniques often either do not scale, or the algorithms perform inconsistently. Other tools that produce outputs at broader scale extents generally utilize coarser input topographic data to produce more poorly resolved valley bottom approximations. To fill this methodology gap and produce relatively accurate valley bottoms over large areas, we developed an algorithm that accepts terrain data from one to 10 m with slope and valley width parameters that scale based on drainage area, allowing for watershed-scale valley bottom delineation. We packaged this algorithm in the Valley Bottom Extraction Tool (V-BET) as an open-source ArcGIS toolbox for ease of use. To illustrate V-BET's scalability and test the tool's robustness across different physiographic settings, we delineated valley bottoms for the entire perennial drainage network of Utah as well as twelve watersheds across the interior Columbia River Basin (totaling 55 400 km) using 10 m DEMs. We found that even when driven with relatively coarse data (10 m DEMs), V-BET produced a relatively accurate approximation of valley bottoms across the entire watersheds of these diverse physiographic regions.
... Within these imposed conditions, hydrology, sediment regime and vegetation conditions (flux boundary conditions) act on the valley bottom to determine the range of process behaviour that is possible, and the range of types of rivers that can form in that setting (Brierley and Fryirs, 2005;Hough-Snee et al., 2015). The extent to which various forms of confinement occur determines the degrees of freedom within which the river has the capacity to adjust (Phillips, 2008(Phillips, , 2010. Correctly interpreting the valley setting using measures of valley margin and valley bottom confinement is critical to correctly interpreting the contemporary character and behaviour of that river. ...
Valley setting and confinement (or lack thereof) are primary controls on river character and behaviour. Although thereare various proxies for valley confinement, direct measures that quantify the nature and extent of confinement are generally lackingand/or inconsistently described. As such they do not lend themselves to consistent analysis over large spatial scales. Here we clearlydefine forms of confinement to aid in quantification of degrees of confinement. Types of margin that can induce confinement aredifferentiated as a valley margin, valley bottom margin, and/or anthropogenic margin. Such margins sometimes overlap and sharethe same location, and in other situations are separated, giving immediate clues as to the valley setting. We apply this frameworkto examples from Australia, United States and New Zealand, showing how this framework can be applied across the spectrum ofriver diversity. This method can help to inform interpretations of reach-scale river behaviour, highlighting the role of antecedentcontrols on contemporary forms and processes. Clear definitions of confinement are shown to support catchment-scale analysis ofriver patterns along longitudinal profiles, and appraisals of the geomorphic effectiveness of floods and sediment flux in catchments(e.g. process zone distribution, lateral sediment inputs and (dis)connectivity).
... RS data for the first time offer the possibility to generate data at basin and beyond regional scales opening to the possibility to create structured geo-database of HYMO data. Soon, it will be possible to systematically compare river functional types across basins with the same metrics and to investigate those drivers like hydrological forcing, sediment supply, and historical contingency responsible for these differences (Phillips 2010). All that will allow to setting quantifiable river management targets and to effectively monitoring the rate of success of the implemented measures. ...
This part provides a series of applications of some of the methods reported in the Part 1. The document is organised in three chapters. In Chapter 1, the Morphological Quality Index (MQI) and the Morphological Quality Index for monitoring (MQIm) have been applied to eight case studies. Chapter 2 presents the application of semi-automated procedures based on remote sensing datasets for monitoring and characterising channel forms to the River Orco (Italy). In Chapter 3, the Hydromorphological Evaluation Tool (HYMET) is applied to the Drau Ruver (Austria).
... 1. Setup of sufficient superelevation of a channel belt compared to its surrounding flood basin [Jones and Schumm, 1999;Mohrig et al., 2000;Slingerland and Smith, 2004]: the setup will develop as channel aggradation as the sediment-carrying capacity of a channel becomes smaller than the sediment inflow [Bryant et al., 1995;Phillips, 2010]. 2. A trigger initiating a breach: this typically involves a peak discharge event that lasts for several days and a weakened bank. ...
The Yellow River, China, experienced >1000 levee breaches during the last 3000 years. A reduced-complexity model is developed in this study to explore the effects of climate change and human activity on flood levels, levee breaches, and river avulsions. The model integrates yearly morphological change along a channel belt with daily river fluxes and hourly evolution of levee breaches. Model sensitivity analysis reveals that under natural conditions, super-elevation of the channel belt dominates flood frequency. When there is significant human-accelerated basin erosion and breach repair, the dominant factors shift to a combination of mean annual precipitation, super-elevation, critical shear stress of weak channel banks, and the time interval between breach initiation and its repair. The effect of precipitation on flood frequency is amplified by land use changes in the hinterland, particularly in the erodible Loess Plateau. Uncertainty analysis estimates the most likely values of the dominant factors for six historical periods between 850 BC and AD 1839, which are used to quantitatively reconstruct flood dynamics. During 850 BC to AD 1839, when the sediment load increased fourfold, the breach recurrence interval was shortened from more than 500 years to less than 6 years, and the breach outflow rate increased ~27 times. River management practices during AD 1579 to AD 1839 focused on levees and triggered a severe positive feedback of increased levee heights and flood hazard exacerbation. Raising the levee heights proved to be ineffective for sustainable flood management.
... Today only a few researchers (e.g. Church 1996Church , 2010Phillips, 1995Phillips, , 2010Phillips, , 2014 address either theoretical or field evidential material concerned with the generation of large-scale complex landforms and their long-term nested developments that so preoccupied previous generations. Some do see material flux modelling as a unifying theme (Coulthard et al., 2002;Murray et al., 2009;Fryirs, 2013). ...
The terms geomorphology uses reveal much about its identity, history, perceptions, external inspirations and methods. Current usage also involves unstated ambiguities. The paper critically reviews the origins, development and nature of the lexicon in English: for discretely identified landforms, for landform sets and hierarchies, and for discussing grand theories. These reflect multiple purposes from the genetically diagnostic to the morphologically descriptive, together with the development of purposeful cognitive and hierarchical classifications, and new terminologies used in the modelling of complex emergent systems. Debate as to the changing and multiple meaning of words is a normal outcome. Metaphors, verbal or otherwise, play a large part in conceptualization. Recent terms relate especially to stimulating technological advances in survey, analysis and modelling. There is now resort to the visual as much as to the verbal for model presentations, together with a much-increased use of initialisms and similar abbreviated word forms. This article is protected by copyright. All rights reserved.
... Several authors suggest that water flow in catchments and catchment structure is in accordance with different candidate optimality principles that characterize the associated energy conversions and related thermodynamic limitations (Phillips, 2006;Paik and Kumar, 2010;Phillips, 2010). Woldenberg (1969) showed that basic scaling relationships of river basins can be derived from optimality assumptions regarding stream power. ...
According to Dooge (1986) intermediate-scale catchments are systems of organized complexity, being too organized and yet too small to be characterized on a statisti-cal/conceptual basis, but too large and too heterogeneous to be characterized in a deterministic manner. A key require-ment for building structurally adequate models precisely for this intermediate scale is a better understanding of how dif-ferent forms of spatial organization affect storage and release of water and energy. Here, we propose that a combination of the concept of hydrological response units (HRUs) and thermodynamics offers several helpful and partly novel per-spectives for gaining this improved understanding. Our key idea is to define functional similarity based on similarity of the terrestrial controls of gradients and resistance terms con-trolling the land surface energy balance, rainfall runoff trans-formation, and groundwater storage and release. This might imply that functional similarity with respect to these specific forms of water release emerges at different scales, namely the small field scale, the hillslope, and the catchment scale. We thus propose three different types of "functional units" – specialized HRUs, so to speak – which behave similarly with respect to one specific form of water release and with a characteristic extent equal to one of those three scale levels. We furthermore discuss an experimental strategy based on exemplary learning and replicate experiments to identify and delineate these functional units, and as a promising strategy for characterizing the interplay and organization of water and energy fluxes across scales. We believe the thermodynamic perspective to be well suited to unmask equifinality as in-herent in the equations governing water, momentum, and en-ergy fluxes: this is because several combinations of gradients and resistance terms yield the same mass or energy flux and the terrestrial controls of gradients and resistance terms are largely independent. We propose that structurally adequate models at this scale should consequently disentangle driving gradients and resistance terms, because this optionally allows Published by Copernicus Publications on behalf of the European Geosciences Union. 4636 E. Zehe et al.: HESS Opinions: Thermodynamic reinterpretation of the HRU concept equifinality to be partly reduced by including available ob-servations, e.g., on driving gradients. Most importantly, the thermodynamic perspective yields an energy-centered per-spective on rainfall-runoff transformation and evapotranspi-ration, including fundamental limits for energy fluxes asso-ciated with these processes. This might additionally reduce equifinality and opens up opportunities for testing thermody-namic optimality principles within independent predictions of rainfall-runoff or land surface energy exchange. This is pivotal to finding out whether or not spatial organization in catchments is in accordance with a fundamental organizing principle.
The characteristics of dynamic and delayed channel geomorphic responses to changing hydrological processes are still insufficiently understood, especially in large dammed rivers and segments farther downstream from the dam. Geomorphic effects farther downstream from the dam could be easily underestimated. This study investigated the hydrological changes and channel adjustments in the Chenglingji–Jiujiang Reach (CJR), ~450 km downstream from the Three Gorges Dam. The post‐dam evolution developed from early low‐intensity alternating erosion/deposition to later intense erosion. This is because the increasing sediment starvation caused by the dam and attenuated upstream sediment replenishment produced streams with excess transport capacity for the farther downstream channel, where the early mild evolution and fine‐sandy riverbed guaranteed adequate sediment availability. Additionally, the increasing geomorphic contribution of low‐to‐medium flows, coupled with revetment resistance to lateral adjustments, promoted migration of the major adjustment area from above the bankfull channel to below the medium‐flow channel and characterized erosion by one‐way incision with no evident channel narrowing/widening. The net increase in reach‐averaged thalweg incision depth during a later period (2.85 m) was 35 times larger than that during an earlier period (0.08 m). Channel morphogenetic intra‐annual hydrological processes were identified to contain all discharges smaller than 39 000 m ³ /s, and discharges of 30 000–39 000 m ³ /s exerted maximum geomorphic effects. On this basis, a method for estimating the cumulative erosion volume at equilibrium was proposed and integrated into the Delayed Response Model. The encouraging simulation and prediction results indicate that geomorphological adjustments within the CJR were greatly influenced by the hydrological conditions of current year and previous 4 years, while the impacts of extreme and rare events depended more on the consistency of their hydrological behaviours with channel evolutionary trends than on their temporal distances. This work facilitates the understanding and prediction of channel self‐adjustments farther downstream from the dam.
River and floodplains are process-response systems that geomorphic states are typically results of hydrologic drivers and geologic setting. Large sand-bed braided rivers geomorphology shows more sensitive dynamic evolution and significant space-time difference. However, few investigations have been focused on it. Here we show that river characteristics including expansion-contraction and elevation heterogeneity of the river cross sections affect erosion and deposition partially and cause spatially differential variation of morphology in the Yanni Wetland, in the middle of Yarlung Zangbo–Brahmaputra River (YBR), and interannual hydrology process is main responsible for temporal river morphology variation as the different impact of each HIs on river morphology. We found the river morphology varied significantly with main channel shifting and branches removing over the past 35 years. Within HIs, bankfull flow (Qbf) is the channel forming discharge equal to 5.6 year flood approximately. The sub-bankfull flow (Qsf), and main channel full flow (Qmf) are channel maintaining discharge. Affected by the broad width, main channel straighten was a result of years of large floods over Qbf. Small and moderate floods within the Qsf and Qbf are also important as the morphology adjustments were mainly observed in sub-channels. And effective discharges with the Nash approach (Q1/2) are more suitable in braided river compared with Wolman-Miller approach (Qef). Our research highlighted complexity response of landform and hydrology to river morphological variations in sand-bed braided riverine wetland. It provides reference basis for studying the ecological importance of flood dispatches and complements the scarcity of research data on morphological evolution in Tibet. And a new method for assessing the HIs with remote sensing was established that is convenient with high accuracy and suitable for wide-valley river with multiple channels. We anticipate our method to be a new view to analyze ecological hydrology, and promote better managing river ecosystems.
Meander cutoffs and oxbow lakes are very common features of fluvial landscapes that add complexity and diversity to floodplain alluvial architecture and riverine habitats. Following initial cutoff, sediment accumulates within the entrance and exit of the original bend forming plugs that eventually disconnect the abandoned bend from the main channel. While studies have examined the sedimentology of these plugs once they have fully disconnected from the abandoned bend, fewer studies have detailed the sedimentological processes occurring during the early stages following cutoff initiation. Furthermore, recent studies have highlighted the importance that planform geometry plays in the evolution of neck cutoffs. This study examines the spatial depositional patterns of two neck cutoffs on the White River in central Arkansas, USA, that remain hydrologically connected to the main channel, providing a unique opportunity to research sedimentological processes before disconnection of the abandoned bend. Sediment cores were collected in key locations of each cutoff, including the entrance and exit of the abandoned bends, abandoned bend apices, and newly developed cutoff bars in the downstream channel. The cores were logged and interpreted and grain‐size analyses were performed. In addition to sediment cores, repeat high‐resolution multibeam echo sounding surveys were conducted roughly four hours apart to estimate bedload transport rates and patterns of bedload routing through each cutoff. Results from this research are different from previous studies. Sediment core results show a pattern of deposition typically associated with lower diversion angle chute cutoffs instead of higher diversion angle neck cutoffs. Previous research has indicated that plugging of abandoned bends drives disconnection from the active channel; however, this research shows that disconnection is more associated with the prevention of sediment from being delivered to the abandoned bends due to flow being pulled away from the abandoned bends and the evolving channel morphology.
Darwinian natural selection acting on individuals is one of only several types of selection influencing landscape evolution. Ecological filtering and abiotic selection (including the least action principle and preferential flows) apply. The overarching principle is one of efficiency selection, whereby more efficient, stable, and durable forms, structures, patterns, networks, and flux pathways are more likely to occur, grow, and persist than less efficient ones. Particularly important forms are gradient selection, favoring steeper and faster flow paths; resistance selection, whereby more resistant features are preferentially preserved; biogeochemical selection, which favors more rapid elemental cycling; network selection, which makes more efficient flux and interaction networks more likely; and thermodynamic selection, reflecting the advantages of energy use efficiency. Efficiency selection is highly local, however, one of several reasons that landscapes and environmental systems are not always inevitably becoming more efficient overall. A case study illustrating selection principles is given.
Cambridge Core - Geomorphology and Physical Geography - River Dynamics - by Bruce L. Rhoads
Neck cutoffs and their resultant oxbow lakes are important and prominent features of riverine landscapes. Detailed field‐based research focusing on the morphologic evolution of neck cutoffs is currently insufficient to fully characterize cutoff evolution. High‐resolution bathymetric data were collected over 3 years for the purpose of determining channel morphology and morphologic change on three actively evolving neck cutoffs. Results indicate the following general trends in morphologic adjustment: (1) a longitudinal bar in the upstream meander limb that develops near the entrance to the abandoned bend; (2) a deep scour hole in the downstream meander limb immediately downstream of the cutoff channel; (3) erosion of the bank opposite the cutoff in the downstream meander limb; (4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb; and (5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. The results presented herein were used to develop a conceptual model that depicts the morphologic evolution of highly curving neck cutoffs. The findings of this research are combined with recent analyses of the three‐dimensional flow structure through neck cutoffs to provide a mechanistic explanation for the morphodynamics of neck cutoffs. © 2019 John Wiley & Sons, Ltd. This study focuses on morphologic changes following neck cutoffs on meandering rivers when the cutoff channel is not straight but highly curving. Investigation of three neck cutoffs on the White River in Arkansas reveals key similarities in the morphologic evolution that are summarized with a conceptual model. The model depicts the stages following cutoff initiation prior to plugging, with the development of a pronounced cutoff bar, erosion of the bank opposite the cutoff, and downstream rotation of the resulting bend.
Understanding evolution of soils and landforms (and other Earth surface systems) has itself evolved from concepts of single-path, single-outcome development to those that recognize multiple possible developmental trajectories and different maturely developed states. Soil geomorphology and pedology should now move beyond showing that multiple trajectories are possible to investigating why some evolutionary pathways (EPs) are common and persistent, whereas others are rare and transient. A typology of EPs is developed and applied to soil formation in the North Carolina coastal plain. Some EP are impossible because of violation of generally applicable laws or absence of necessary conditions; others are currently impossible, having occurred in the past but requiring conditions that no longer exist. Improbable paths are possible but rare, because necessary circumstances involve rare events or boundary conditions. Inhibited EPs are also possible but rare because of resistance factors or feedbacks that prevent or inhibit them. Transient paths may be common but are not long-lived or well preserved and are thus rarely observed. Recurring but nonrepeating EPs occur in different locations but are irreversible in any given location and cannot recur except in the case of system-resetting disturbance or new inputs. Recurring EPs are not inhibited or self-limited, occur in different locations, and may be repeated because of ongoing or recurrent processes or conditions. Selected path types occur in multiple situations, but with increased probability due to feedbacks or responses that encourage or enhance recurrence and/or persistence. The case study shows examples of all possible EP types.
This document is a collection of Jonathan Phillips’ Geoscience Blog posts from its inception (29 May 2014) through 2 July 2017. The major sections include (1) How it's Done; (2) Earth Surface System Theory 1: Equilibrium & Otherwise; (3)Earth Surface System Theory 2: Nonlinear Dynamics, Complexity, Self-Organization, Power Laws; (4) Earth Surface System Theory 3: Optimality & Selection; (5) Forest Biogeomorphology; (6) Climate & Sea-Level Rise; (7) Coevolution; (8) Rivers & Streams; (9) Environmental Management; (10) Geomorphology; (11) Soil, Regolith & Karst
The organization of drainage basins shows some reproducible phenomena, as exemplified by self-similar fractal river network structures and typical scaling laws, and these have been related to energetic optimization principles, such as minimization of stream power, minimum energy expenditure or maximum "access". Here we describe the organization and dynamics of drainage systems using thermodynamics, focusing on the generation, dissipation and transfer of free energy associated with river flow and sediment transport. We argue that the organization of drainage basins reflects the fundamental tendency of natural systems to deplete driving gradients as fast as possible through the maximization of free energy generation, thereby accelerating the dynamics of the system. This effectively results in the maximization of sediment export to deplete topographic gradients as fast as possible and potentially involves large-scale feedbacks to continental uplift. We illustrate this thermodynamic description with a set of three highly simplified models related to water and sediment flow and describe the mechanisms and feedbacks involved in the evolution and dynamics of the associated structures. We close by discussing how this thermodynamic perspective is consistent with previous approaches and the implications that such a thermodynamic description has for the understanding and prediction of sub-grid scale organization of drainage systems and preferential flow structures in general.
Published by the American Geophysical Union as part of the Water Resources Monograph Series, Volume 19. What are the forms and processes characteristic of mountain rivers and how do we know them? Mountain Rivers Revisited, an expanded and updated version of the earlier volume Mountain Rivers, answers these questions and more. Here is the only comprehensive synthesis of current knowledge about mountain rivers available. While continuing to focus on physical process and form in mountain rivers, the text also addresses the influences of tectonics, climate, and land use on rivers, as well as water chemistry, hyporheic exchange, and riparian and aquatic ecology. With its numerous illustrations and references, hydrologists, geomorphologists, civil and environmental engineers, ecologists, resource planners, and their students will find this book an essential resource. Ellen Wohl received her Ph.D. in geology in 1988 from the University of Arizona. Since then, she has worked primarily on mountain and bedrock rivers in diverse environments.
Energy expenditure plays a key role in the development of the topological structure of river networks and the adjustment of their channel geometry at different timescales through processes of runoff generation, sediment transport, erosion, and deposition. The combination of local energy expenditure in individual channel reaches and global energy expenditure in the interconnected drainage system as a whole determines the overall energetic peformance of the river basin. Primary variables of energy expenditure (stream power) and their distributions in real river networks and those generated by optimality hypotheses, in particular the optimal channel network concept, are presented and discussed.
The physical, chemical, and biological transformations in the Earth's Critical Zone – from the atmosphere–vegetation interface down to the bottom of the aquifer – shape soils, vegetation, landscapes, and biogeochemical cycling. To sustain these transformations , free energy – the part of energy that can be used to perform physical, chemical, or biological work – needs to be generated and supplied continuously. This chapter provides an overview of the basis to characterize the Critical Zone as a ther-modynamic system and illustrates the relevance of upper thermodynamic limits to Critical Zone processes. We first describe how the relevant forms of energy are formulated in thermodynamic terms and how the laws of thermodynamics restrict the direction and magnitude of these transformations. These principles are then illustrated by three examples of how thermodynamic limits restrict (1) turbulent exchange between the land surface and the atmosphere, (2) mechanical weathering from periodic heating and cooling of the ground, and (3) the transport of sediments by river flow. We then outline the relevance of these considerations to hydropedology in the context of the drivers for pedogenesis and the generation and maintenance of preferential flow in soils and landscapes. We conclude with a summary and a brief prospectus for future work relating to thermodynamic limits and optimality related to these limits within the Critical Zone and its applications in hydropedology.
Resumen En aquest article s'analitzen les actuacions de restauració fluvial dutes a terme a la Confederació Hi-drogràfica del Xúquer entre 2008 i 2009, especialment les intervencions de restauració geomorfològica, la qual cosa ens permet una sèrie de consideracions crítiques sobre les obres. Es fan també algunes reflexions teòriques sobre els canvis ambientals i antròpics esdevinguts al llarg del segle XX i les implicacions que tenen sobre la restauració fluvial, en la mesura que aporten complexitat i incertesa a la restauració. En particular s'analitzen algunes actuacions de restauració que comprometen la connectivitat hidrològica i sedimentària, que són especialment febles en rius efímers. PaRaules clau: restauració fluvial, canvis ambientals, canvis antròpics, connectivitat sedimentària, connectivitat hidrològica abstRact This paper presents a critical review of the geomorphological river restoration works carried out in the Júcar River Basin between 2008 and 2009, considering the impacts on the hydro-geomorphic systems. The scope of the environmental and anthropogenic changes occurred in the Mediterranean rivers throughout the twentieth century adds considerable complexity and uncertainty to the restoration actions. In particular several considerations are done about some restoration works that compromise the hydrological and sediment connectivity, which are particularly weak in ephemeral rivers.
Delayed response means that channels cannot achieve a new equilibrium state immediately following disruption; the channel requires a response time or relaxation time to reach equilibrium. It follows that the morphological state of fluvial system represents the cumulative effects of all previous disturbances and environmental conditions. A unique feature of the delayed response model for bankfull discharge is that the model is capable of representing the cumulative effects of all previous flow conditions when applied to predict the path/trajectories of bankfull discharge in response to altered flow regimes. In this paper, the delayed response model was modified by readjusting the weight for the initial boundary conditions and introducing a variable β with respect to time. The modified model was then applied to the bankfull discharge calculations for three selected river reaches of the Yellow River, with each reach having different geomorphic settings and constraints. Results indicated that the modified model can predict accurately the bankfull discharge variation in response to changes in flow discharge and sediment load conditions that have been dramatically altered in the past. Results also demonstrated the strong dependence of current bankfull discharge on the previous years' flow conditions, with the relaxation time varied from 2 to 14 years, meaning that the bankfull discharge was not only affected by the flow discharge and sediment load in the current year, but also by those in previous 1 to 13 years. Furthermore, the relaxation time of bankfull discharge adjustment was inversely proportional to the long-term average suspended sediment concentrations, and this may be explained by fact that high sediment concentrations may have a high potential to perform geomorphic work and there is more sediment readily available to shape the channel boundary and geometry.
This paper describes the relationship between the statistics of bedload transport flux and the time scale over which it is sampled. A stochastic formulation is developed for the probability distribution function of bedload transport flux, based on the Ancey et al. [2008] theory. An analytical solution for the variance of bedload transport flux over differing sampling time scales is presented. The solution demonstrates that the time-scale dependence of the variance of bedload transport flux reduces to a three-regime relation demarcated by an intermittency time scale (tI) and a memory time scale (tc). As the sampling time scale increases, this variance passes through an intermittent stage (<<tI), an invariant stage (tI < t < tc) and a memoryless stage (> > tc). We propose a dimensionless number (Ra) to represent the relative strength of fluctuation, which provides a common ground for comparison of fluctuation strength among different experiments, as well as different sampling time scales for each experiment. Our analysis indicates that correlated motion and the discrete nature of bedload particles are responsible for this three-regime behavior. We use the data from three experiments with high temporal resolution of bedload transport flux to validate the proposed three-regime behavior. The theoretical solution for the variance agrees well with all three sets of experimental data. Our findings contribute to the understanding of the observed fluctuations of bedload transport flux over mono/multiple-size grain beds, to the characterization of an inherent connection between short-term measurements and long-term statistics, and to the design of appropriate sampling strategies for bedload transport flux.
This opinion paper proposes a novel framework for exploring how spatial organization alongside with spatial heterogeneity controls functioning of intermediate scale catchments of organized complexity. Key idea is that spatial organization in landscapes implies that functioning of intermediate scale catchments is controlled by a hierarchy of functional units: hillslope scale lead topologies and embedded elementary functional units (EFUs). We argue that similar soils and vegetation communities and thus also soil structures "co-developed" within EFUs in an adaptive, self-organizing manner as they have been exposed to similar flows of energy, water and nutrients from the past to the present. Class members of the same EFU (class) are thus deemed to belong to the same ensemble with respect to controls of the energy balance and related vertical flows of capillary bounded soil water and heat. Class members of superordinate lead topologies are characterized by the same spatially organized arrangement of EFUs along the gradient driving lateral flows of free water as well as a similar surface and bedrock topography. We hence postulate that they belong to the same ensemble with respect to controls on rainfall runoff transformation and related vertical and lateral fluxes of free water. We expect class members of these functional units to have a distinct way how their architecture controls the interplay of state dynamics and integral flows, which is typical for all members of one class but dissimilar among the classes. This implies that we might infer on the typical dynamic behavior of the most important classes of EFU and lead topologies in a catchment, by thoroughly characterizing a few members of each class. A major asset of the proposed framework, which steps beyond the concept of hydrological response units, is that it can be tested experimentally. In this respect, we reflect on suitable strategies based on stratified observations drawing from process hydrology, soil physics, geophysics, ecology and remote sensing which are currently conducted in replicates of candidate functional units in the Attert basin (Luxembourg), to search for typical and similar functional and structural characteristics. A second asset of this framework is that it blueprints a way towards a structurally more adequate model concept for water and energy cycles in intermediate scale catchments, which balances necessary complexity with falsifiability. This is because EFU and lead topologies are deemed to mark a hierarchy of "scale breaks" where simplicity with respect to the energy balance and stream flow generation emerges from spatially organized process-structure interactions. This offers the opportunity for simplified descriptions of these processes that are nevertheless physically and thermodynamically consistent. In this respect we reflect on a candidate model structure that (a) may accommodate distributed observations of states and especially terrestrial controls on driving gradients to constrain the space of feasible model structures and (b) allows testing the possible added value of organizing principles to understand the role of spatial organization from an optimality perspective.
A recent opinion piece rekindled debate as to whether geography's current interdisciplinary make-up is a historical relic or an actual and potential source of intellectual vitality. Taking the latter position, we argue here for the benefits of sustained integration of physical and critical human geography. For reasons both political and pragmatic, we term this area of intermingled research and practice critical physical geography (CPG). CPG combines critical attention to power relations with deep knowledge of biophysical science or technology in the service of social and environmental transformation. We argue that whether practiced by individuals or teams, CPG research can improve the intellectual quality and expand the political relevance of both physical and critical human geography because it is increasingly impractical to separate analysis of natural and social systems: socio-biophysical landscapes are as much the product of unequal power relations, histories of colonialism, and racial and gender disparities as they are of hydrology, ecology, and climate change. Here, we review existing CPG work; discuss the primary benefits of critically engaged integrative research, teaching, and practice; and offer our collective thoughts on how to make CPG work. © 2013 Canadian Association of Geographers / L' Association canadienne des géographes.
In recent decades views of change, disturbance, response, and recovery in geomorphology have expanded considerably. Conceptual frameworks emphasizing single-path, single-outcome trajectories of change have been supplemented - not replaced - by multi-path, multi-outcome perspectives. Geomorphology has also seen a transition from the idea of normative standards such as characteristic, (steady-state) equilibrium, zonal, and mature forms to the recognition that some systems may have multiple potential characteristic or equilibrium forms - and that some may have no particular normative state at all. These trends are not presented as a replacement of outmoded ideas, but rather as a broadening of approaches. The single-path single-outcome frameworks can generally be viewed as special cases of the broader pluralistic analytical structures. In this context, two perspectives - an adaptation of White's hazards matrix, and the landscape sensitivity concept - are suggested which lend themselves to studies of recent and contemporary changes in earth surface systems. These perspectives can be synthesized into a framework for the assessment of geomorphic changes and responses based on the 'four Rs': response (reaction and relaxation times), resistance (relative to the drivers of change), resilience (recovery ability, based on dynamical stability), and recursion (positive and/or negative feedbacks).
Equilibrium is a single word that embraces multiple concepts. The largely qualitative use of equilibrium within geomorphology has fostered imprecision and even outright error; as a result the term, for many, has degenerated to the status of a non-corrigible regulative principle. Although a few geomorphologists make precise use of equilibria terminology, their precision is invariably lost on the larger audience.Equilibrium is associated with force in dynamics, with energy in thermodynamics (and probability by extension), and with pure numerical behavior in mathematics. In General System Theory, equilibrium is derived from thermodynamics but applied, by analogy, almost exclusively to mass. In addition, a purely geomorphic version of equilibrium (dynamic equilibrium) stems from the work of G. K. Gilbert (1876; 1877) and is based on sediment flux at the basin scale. Unfortunately, Gilbert's concept/term has been distorted by some geomorphologists in their attempt to establish linkage between equilibria concepts, especially between those relating to energy and mass.This paper reviews and reconsiders the various equilibria concepts. An updated version of Gilbert's dynamic equilibrium based on sediment transfer proposed by Ahnert (n.d.), and termed by us “mass flux equilibrium,” holds promise. In addition, the purely mathematical concept of equilibrium outlined by Howard (1988) has considerable utility, provided union with geomorphological theory is achieved. Given the difficulty of measuring thermal energy in the field, its inefficient use in nature, and the uncertainty with which it and form can be related, thermodynamic equilibria concepts would appear, presently, to offer geomorphologists only very limited opportunities.
A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979-2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental landmasses, zonal means over land, hemispheric land means, and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high-frequency information taken from atmospheric reanalyses. Results for E - P are contrasted with those from atmospheric moisture budgets based on 40-yr ECMWF Re-Analysis (ERA-40) data. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land, especially in the Tropics and subtropics.
A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated foods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by a collapse of the turbulence structure by sediment-induced density gradients, whereas erosion prevails when this sediment is largely held in suspension due to hindered settling. Observed patterns of erosion and sedimentation during these floods can be well reproduced using a numerical model in which sediment-induced density effects and hindered settling are included. Based on these results a refined definition of hyperconcentrated flow is proposed.
Recent theoretical models suggest that topographic characteristics of bedrock channels are products of interactions among tectonics, substrate resistance, and the climatically modulated erosive ability of the river. The degree to which these factors influence the form of channel profiles is poorly quantified at present. Here we investigate bedrock channels developed across the southern flank of the Santa Ynez Mountains, California. Uniform climate and systematic variations in lithology and rock uplift rate along the range allow comparison of channel morphology between (1) channels eroding rocks of uniform and nonuniform strength and (2) channels experiencing differences in tectonic forcing. We combine field observations, surveys, and analysis of digital data to determine topographic and hydraulic characteristics of bedrock channels. At a constant rock uplift rate, streams flowing from resistant to less resistant bedrock exhibit highly concave profiles and increased gradients along lower reaches relative to channels developed in uniform bedrock. These effects are interpreted as responses to (1) an increase in substrate resistance to channel incision in the upper reaches and (2) transport-limited gradients along lower reaches. Comparisons of channels developed across uniform lithology but experiencing an approximately sevenfold difference in rock uplift rate reveal an approximately twofold increase in gradient and an approximately threefold decrease in width. In this landscape the combined channel adjustments of gradient and width are consistent with a fluvial incision model in which channel incision rate is linearly proportional to mean bed shear stress.
Channel geometry exerts a fundamental control on fluvial processes. Recent work has shown that bedrock channel width (W) depends on a number of parameters, including channel slope, and is not only a function of drainage area (A) as is commonly assumed. The present work represents the first attempt to investigate the consequences, for landscape evolution, of using a static expression of channel width (W ~ A0.5) versus a relationship that allows channels to dynamically adjust to changes in slope. We consider different models for the evolution of the channel geometry, including constant width-to-depth ratio (after Finnegan et al., Geology, v. 33, no. 3, 2005), and width-to-depth ratio varying as a function of slope (after Whittaker et al., Geology, v. 35, no. 2, 2007). We use the Channel-Hillslope Integrated Landscape Development (CHILD) model to analyze the response of a catchment to a given tectonic disturbance. The topography of a catchment in the footwall of an active normal fault in the Apennines (Italy) is used as a template for the study. We show that, for this catchment, the transient response can be fairly well reproduced using a simple detachment-limited fluvial incision law. We also show that, depending on the relationship used to express channel width, initial steady-state topographies differ, as do transient channel width, slope, and the response time of the fluvial system. These differences lead to contrasting landscape morphologies when integrated at the scale of a whole catchment. Our results emphasize the importance of channel width in controlling fluvial processes and landscape evolution. They stress the need for using a dynamic hydraulic scaling law when modeling landscape evolution, particularly when the uplift field is non-uniform.
We construct a theory in which we interpret stable, self-similar, intermediate asymptotic solutions to PDEs representing conservation of mass over stream long profiles as graded streams. The theory applies to alluvial channels for which the transport of sediment is modeled by qs∝qγwSδ, where qw is the discharge of water and S the slope, and to bedrock channels for which the transport of sediment is modeled by (∂qs/∂x)∝qγwSδ, where x is the distance downstream. The parameters α and β of these similarity solutions, z(x, t)=ταF(x/τβ), where τ=τ0+ωt and t is time, may be derived using dimensional analysis and are representable in terms of conditions of sediment removal at the lower boundary of the profile and (γ, δ). Conditions on the physical realizability of profiles lead to constraints on admissible values of (γ, δ, α, β, ω). All alluvial self-similar profiles and an important subset of bedrock self-similar profiles are stable and act as transient attractors when boundary conditions are unchanging. Their forms are independent of the details of their initial conditions. The remaining bedrock channels are unstable because of the spontaneous emergence of shocks that migrate upstream as breaks in slope. Two regimes of profile behavior exist: for ω0, corresponding to low energy environments, profiles have decelerating relative loss rates and infinite life. Changes in profile elevations over time may be decomposed into upper boundary and lower boundary effects controlled, respectively, by ωα and ωβ and depending ultimately on γ and δ. We examine explicit profiles for geomorphically important sets of boundary conditions: desert mountain/pediment (α+β=0); hanging valley (α=0); fixed Davisian base-level (β=0); and steady state (α=1), for alluvial and bedrock channels and various tectonic conditions. We investigate numerically the effects of changes from hanging valley to fixed base level boundary conditions, showing that profiles associated with the hanging valley attractor retreat upstream as profiles associated with the fixed based level attractor replace them from below. We demonstrate numerically the extension of the theory to streams with tributaries. The theory provides the natural definition of grade for our models, and indicates that it is a real and deep property of the associated profiles. The theory also resolves many contentious issues concerning the concept of grade. We conjecture that defining grade in terms of stable, self-similar, intermediate asymptotic solutions to conservation equations generalizes over reasonable extensions to the transport conditions of our models.
Recent experimental and theoretical studies support the notion that bed
load in mountain rivers can both enhance incision rates through wear and
inhibit incision rates by covering the bed. These effects may play an
important role in landscape evolution and, in particular, the response
of river channels to tectonic or climatic perturbation. We use the
channel-hillslope integrated landscape development (CHILD) numerical
model with two different bedrock incision models that include the dual
role of the sediment flux to explore the transient behavior of fluvial
landscapes. Both models predict that steady state channel slopes
increase in landscapes with higher rock uplift rates. However, the
incision models predict different transient responses to an increase in
uplift rate, and the behavior of each incision model depends on both the
magnitude of change in uplift rate and the local drainage area. In some
cases, the transient channel behavior is indistinguishable from that
predicted for transport-limited alluvial rivers. In other cases,
knickpoints form in some or all of the drainage network, as predicted by
the detachment-limited stream power model. In all cases the response in
the lower parts of the network is highly dependent on the response in
the upper parts of the network as well as the hillslopes. As the upper
parts of the network send more sediment downstream, channel incision
rates may rise or fall, and slopes in the lower parts of the channel
may, in fact, decrease at times during the transient adjustment to an
increase in rock uplift rate. In some cases, channel incision in the
upper parts of the network ceases during the transient while the
hillslopes adjust to the new uplift rate; drainage density may also
change as a function of uplift rate. Our results suggest that if the
sediment flux strongly controls bedrock incision rates, then (1) the
transient fluvial response will take longer than predicted by the
detachment-limited stream power model, (2) changes in channel slope may
be much more complex than predicted by the detachment-limited stream
power model, and (3) changes in the fluvial system will be closely tied
to sediment delivery from the hillslopes. Importantly, our results
outline quantitative differences in system behavior produced by
competing models and provide a framework for identifying locations in
natural systems where differences in channel morphology can be used to
discern between competing fluvial erosion models.
The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.
The analysis of longitudinal profiles of river channels and terraces in the southern Central Massif border, between the Aude and the Orb, allows the detection of anomalies caused by lithology and/or tectonic distortions. The rivers which have abnormally high slope and non-lithological knickzones indicate the main uplifted zones: the Montagne Noire and the Saint-Chinian ridge. A geomorphological and sedimentological analysis of detrital deposits was carried out as a basis for correlating the different formations, reconstructing the palaeodrainage and finding the main uplift and fluvial incision stages. During the Miocene, uplift remains limited as it is shown by the correlative fine deposits in the Languedocian piedmont. The Messinian incision (5.7–5.3 Ma) does not cross the Saint-Chinian ridge. On the other hand, fluvial incision becomes widespread in the Montagne Noire during the Upper Pliocene (3.4–2 Ma) when coarse deposits overlie either the Pliocene clay in the Orb palaeovalley or the Messinian conglomerates at the Cesse outlet. An Upper Pliocene uplift of the Montagne Noire and of the Saint-Chinian ridge is the cause of this incision and also of the diversion of the Cesse towards the Aude. Where the uplift rate was higher than incision rate, knickzones have developed like in the Avant-Monts south-side. The knickzones of lithological origin maintain a strong vertical stability during all the river incision stages. On the other hand, those of tectonic origin or base level lowering record upstream migration and their rate of retreat is controlled by the river discharge. As incision occurs only during the cold/temperate transition periods during the Quaternary, upward erosion slowly migrates (15 km since the Upper Pliocene, on the Orb) and so does not reach the riverheads.
Realistic physical models of meandering rivers have proven extremely difficult to produce, particularly in comparison to the formation of braided rivers in the laboratory. Here we address the question of why such realistic model meanders are so difficult to reproduce, through the most realistic physical modeling of meandering channels yet achieved. This paper demonstrates that cohesion is a key variable in the development and maintenance of single-thread channels. In particular, cohesion must be sufficient to force the planform away from a braided state but low enough for active migration to continue and for the avoidance of a gradual reduction and eventual cessation of planform movement (ossification). The enhanced realism of the experiments also enables the processes of meander evolution, and critically the resultant alluvial architecture, to be examined in a physical model for the first time. Planform history can be linked to deposits, and this process-product linkage enables the depositional development of the experimental deposits to be compared against, and to test, existing models of bedding geometries within point bars. Here we document three mechanisms for bend cutoff, provide new process explanations for certain modes of bend evolution in coarse-grained meandering rivers, examine the geometries and spatial distribution of alluvial architecture, and demonstrate that existing models of point-bar geometry successfully reproduce the larger-scale aspects of point-bar accretion in rivers dominated by episodic unit-bar accretion.
Anabranching rivers have been identified globally, but a widely accepted
and convincing theoretical explanation for their occurrence has remained
elusive. Using basic flow and sediment transport relations, this study
analyzes the mechanisms whereby self-adjusting alluvial channels can
anabranch to alter their flow efficiency (sediment transport capacity
per unit of stream power). It shows that without adjusting channel
slope, an increase in the number of channels can produce a proportional
decrease in flow efficiency, a finding particularly relevant to
understanding energy consumption in some braided rivers. However,
anabranching efficiency can be significantly increased by a reduction in
channel width, as occurs when vegetated alluvial islands or
between-channel ridges form. The counteracting effects of width
reduction and an increasing number of channels can cause, with no
adjustment to slope, an otherwise unstable system (underloaded or
overloaded) to achieve stability. As with other river patterns,
anabranching can be characterized by stable equilibrium or accreting
disequilibrium examples.
The hydrologic sciences must play a major role in improving our understanding of the transport and fate of the vast amount of reactive nitrogen that is being added to the environment by human activities. Detailed understanding of the function of different landscape units will help predict watershed losses of nitrogen. A better understanding of the processes that control denitrification in surface and groundwaters is essential to ascertain total gaseous N loss and the percentage that is nitrous oxide, a greenhouse gas that is accumulating in Earth's atmosphere.
▪ Abstract Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.
Longitudinal profiles of bedrock streams in central Kentucky, and of coastal plain streams in southeast Texas, were analyzed to determine the extent to which they exhibit smoothly concave profiles and to relate profile convexities to environmental controls. None of the Kentucky streams have smoothly concave profiles. Because all observed knickpoints are associated with vertical joints, if they are migrating it either occurs rapidly between vertical joints, or migrating knickpoints become stalled at structural features. These streams have been adjusting to downcutting of the Kentucky River for at least 1.3 Ma, suggesting that the time required to produce a concave profile is long compared to the typical timescale of environmental change. A graded concave longitudinal profile is not a reasonable prediction or benchmark condition for these streams. The characteristic profile forms of the Kentucky River gorge area are contingent on a particular combination of lithology, structure, hydrologic regime, and geomorphic history, and therefore do not represent any general type of equilibrium state. Few stream profiles in SE Texas conform to the ideal of the smoothly, strongly concave profile. Major convexities are caused by inherited topography, geologic controls, recent and contemporary geomorphic processes, and anthropic effects. Both the legacy of Quaternary environmental change and ongoing changes make it unlikely that consistent boundary conditions will exist for long. Further, the few exceptions within the study area–i.e., strongly and smoothly concave longitudinal profiles–suggest that ample time has occurred for strongly concave profiles to develop and that such profiles do not necessarily represent any mutual adjustments between slope, transport capacity, and sediment supply. The simplest explanation of any tendency toward concavity is related to basic constraints on channel steepness associated with geomechanical stability and minimum slopes necessary to convey flow. This constrained gradient concept (CGC) can explain the general tendency toward concavity in channels of sufficient size, with minimal lithological constraints and with sufficient time for adjustment. Unlike grade- or equilibrium-based theories, the CGC results in interpretations of convex or low-concavity profiles or reaches in terms of local environmental constraints and geomorphic histories rather than as “disequilibrium” features.
The stability of flood channels has attracted considerable attention because of their complicated interactions with the prevailing hydrodynamics and importance in ship navigation. This research examines long-term morphodynamic evolution in the Yangtze Estuary from 1861 to 2002 and the equilibrium mechanism of the Xinqiao Channel in the Yangtze Estuary by digitizing 15 selected maritime charts and calculating the volume of the channel. Although the total period of channel development is much longer than the historical data used in this paper, three stages are identified during the study period: the first embryonic stage (66 years), the second formation stage (33 years) and the third equilibrium stage (45 years). Variations in coastline location, channel volume, and hydrodynamics in the channel during the three stages indicate that the channel equilibrium was reached and maintained when the channel direction was aligned with the direction of offshore tidal wave propagation. Variations in river and sediment discharges affect erosion and deposition in the channel and thus channel geometry. However, future reduction in sediment supply by 10–33% due to the ongoing river engineering projects would increase the volume of the Xinqiao Channel only by 1–3%. It seems unlikely that the above change in sediment discharge will disrupt the equilibrium of the Xinqiao Channel.
1] Long memory is a hydrological property that can lead to prolonged droughts or the temporal clustering of extreme floods in a river. Analyses of 28 long (up to 145 years), continuous instrumental runoff series from six European, American, and African rivers reveal that this effect increases downstream. Simulations reproduce the increase qualitatively and show that a river network aggregates short-memory precipitation and converts it into long-memory runoff. In view of projected changes in climate and the hydrological cycle, these findings show that decadal-scale variations in drought or flood risk can be predicted for individual rivers, with higher predictability downstream. Spatial aggregation may also explain the emergence of long memory in other networks, such as the brain or those formed by computers.
Number of stream basin areas (that is, the number of Strahler stream segments) is predicted on the basis of hexagonal model for basin areas. It is suggested that the number of basins per order, balance opposing tendencies for minimum overland work for streams flowing in small basins and maximum work savings in large, as opposed to small, channels. In addition, the entropy of the system approaches the maximum possible. The model agrees with empirical data in cases where the land surface is reasonably uniform with regard to structure and lithology.
Fluvial geomorphology has witnessed a continuing reduction in the time- and space-scales of research, with increasing emphasis on the dynamics of small site-specific river reaches. This shift can be regarded as part of a trend towards the understanding and explanation rather than description of how rivers change, which raises important questions regarding the relevance of such short time-scale and small space-scale research to understanding longer-term aspects of landform behaviour. The methodological challenges that arise from such intensive case study research are illustrated here using a detailed investigation of a river reach. Morphological changes within this reach are shown to be driven by: (i) catchment-scale processes associated with the interaction of discharge and sediment supply waves: and (ii) modification of these processes through morphological controls on erosion and deposition patterns and hence net channel change, The 'morphological conditioning' of channel response reflects the configurational aspects of channel change, and the importance of local characteristics in the understanding of system behaviour, Sensitivity to local conditions implies that short time-scale and small space-scale processes may be critical to channel behaviour, particularly if the system is interpreted in non-linear terms. Although it may be possible to identify statistically averaged stable states, non-linear system behaviour implies that system trajectories are sensitively dependent upon instantaneous system states. Thus, changes between average states can only be understood through an understanding of the sequence of configurational states through which the system evolves.
Increasingly, attempts are being made to predict the responses of natural ecosystems to global changes in climate and land use. It is argued that the model of vegetation dynamics underlying much of the current debate is too simplistic to yield predictions that will be useful at a scale relevant to most land-use decisions. A useful modeling approach will use the state and transition approach to modify transition probabilities in a standard Markov formulation. By linking this approach into a GIS (Geographic Information System) format, spatially-explicit landscape modeling which incorporates spatial and temporal changes in transition probabilities is possible. -from Author
The balance of nature has been a background assumption in natural history since antiquity, but even to the present it has seldom been closely studied. The idea of a balance of nature emerged, but only implicity, in antiquity. During the 17th century, with an increased knowledge of natural history, the idea became a functional assumption, but within a theological rather than ecological context. In the 18th century Linnaeus defined the concept and attempted to make it the foundation of an ecological science. However, it remained tied to theology and was elaborated without critical examination. The existence of agricultural pests, the occasional occurrence of plagues of animals, and the possibility of species having become extinct were kinds of evidence which would have been difficult to reconcile with contemporary concepts. Lamarck was one of the few who perceived some anomaly, and he attempted to save the old concept by arguing that fossils represented early forms of existing species rather than extinct sp...
A critical analysis of the present situation on the global water resources assessment is made. Basic data and methodological approaches used by the author for the assessment and prediction of water resources, water use and water availability on the global scale are briefly described. On the basis of data generalization of the world hydrological network new data are given on the dynamics of renewable water resources of the continents, physiographic and economic regions, selected countries as well as on the river water inflow to the world ocean. The results of the assessments for the 20th century and for the future before 2010–2025 on the water supply for municipal, industrial and agricultural needs as well as an additional evaporation from reservoirs are presented. Loads on water resources and water availability depending on socio-economic and phisiographic factors are analyzed; regions of water scarcity and water resources deficit are discovered. Possible ways of water supply improvement and elimination of water resources deficit in different regions and countries are discussed.
This book outlines a generic set of procedures, termed the River Styles Framework, which provides a set of tools for interpreting river character, behavior, condition, and recovery potential. Applications of the framework generate a coherent package of geomorphic information, providing a physical template for river rehabilitation activities. management and restoration of rivers is a rapidly growing topic for environmental scientists, geologists and ecologists - this book provides a learning tool with which to approach geomorphic applications to river management describes the essential geomorphological principles underlying river behaviour and evolution demonstrates how the River Styles Framework can turn geomorphic theory into practice, to develop workable strategies for restoration and management based on real case studies and authors extensive experience applicable to river systems worldwide synthesises fluvial geomorphology, ecology and management.
Atmospheric vapour flux convergence is introduced for the estimation of the water balance in a river basin. The global distribution of vapour flux convergence, -∇H.Q is estimated using the European Centre for Medium-Range Weather Forecasts global analysis data for the period 1980-1988. Generally, the global runoff estimation using the conventional hydrological water balance is larger than the result by the atmospheric water balance method. Annual freshwater transport is estimated by atmospheric water balance combined with geographical information. The results show that the same order of freshwater is supplied to the ocean from both the atmosphere and the surrounding continents through rivers. The rivers also carry approximately 10% of the global annual freshwater transport in meridional directions as zonal means. -from Authors
The plate tectonics revolution in the earth sciences has provided a valuable new framework for understanding long-term landform development. This innovative text provides a comprehensive introduction to the subject of global geomorphology, with the emphasis placed on large-scale processes and phenomena. Integrating global tectonics into the study of landforms and incorporating planetary geomorphology as a major component the author discusses the impact of climatic change and the role of catastrophic events on landform genesis and includes a comprehensive study of surface geomorphic processes.
The primary goal of this project is to develop a relative chronology of events in the geologic history of the Kentucky River, and to consider the geologic controls on those events. This study utilized published geologic and topographic data, as well as field observations and extensive compilation and comparison of digital data, to examine the fluvial record preserved in the Kentucky River valley in central Kentucky. Numerous fluvial features including abandoned paleovalleys, fluvial terraces and deposits, bedrock benches, and relict spillways between adjacent river valleys were identified during the course of the study. The morphology of the modern valley coincides with bedrock lithology and can be used to describe the distribution and preservation of modern and ancient fluvial deposits and features in the study area. Bedrock lithology is the dominant control on valley morphology and on the distribution and preservation of fluvial deposits and features in the study area. Some stream trends are inherited from the late Paleozoic drainage of the Alleghanian orogeny. More recent inheritance of valley morphology has resulted from the erosion of the river from one lithology down into another lithology with differing erosional susceptibility, thus superposing the meander patterns of the overlying valley style onto the underlying lithology. One major drainage reorganization related to a pre-Illinoisan glacial advance disrupted the northward flow of the Old Kentucky River toward the Teays River system and led to organization of the early Ohio River. This greatly reduced the distance to baselevel, and led to abrupt incision and a change in erosional style for the Kentucky River. The successful projection of valley morphologies on the basis of bedrock stratigraphy, the history of erosion suggested by fission track data and the results of this study, as well as soil thickness and development, all argue against the existence of a midto late-Tertiary, low-relief, regional erosional surface. This study instead hypothesizes that the apparent accordance of ridge-top elevations in the study area is a reflection of a fluvially downwasted late Paleozoic depositional surface.
Rivers differ among themselves and through time. An individual river can vary significantly downstream, changing its dimensions and pattern dramatically over a short distance. If hydrology and hydraulics were the primary controls on the morphology and behavior of large rivers, we would expect long reaches of rivers to maintain characteristic and relatively uniform morphologies. In fact, this is not the case - the variability of large rivers indicates that other important factors are involved. River Variability and Complexity presents a new approach to the understanding of river variability. It provides examples of river variability and explains the reasons for them, including fluvial response to human activities. Understanding the mechanisms of variability is important for geomorphologists, geologists, river engineers and sedimentologists as they attempt to interpret ancient fluvial deposits or anticipate river behavior at different locations and through time. This book provides an excellent background for graduates, researchers and professionals.
A graded river conveys its sediment load without net aggradation or degradation. Grade is thought to represent the equilibrium state of a river system subject to steady allogenic forcing. We address the concept of grade in linked depositional systems through a combination of mathematical modeling and flume experiments. We develop a moving boundary morphodynamic model of fluviodeltaic sedimentation and use it to quantify the conditions necessary for a state of sustained grade in an alluvial-deltaic system developing on a uniformly sloping shelf. To test our theory, we conducted a series of flume experiments in which we constructed laboratory-scale fluviodeltaic systems under specific relative sea level histories. On large spatiotemporal scales, our theoretical and experimental results suggest an alternative view of grade as an intrinsically nonequilibrium state that requires a fall in relative sea level. Under conditions of stillstand or rise in relative sea level, the equilibrium (graded) profile does not exist, and the alluvial river will always aggrade. With a steady sediment supply, alluvial grade can be sustained if relative sea level falls with a specific, square-root-of-time dependence. Common sequence-stratigraphic conceptualizations of alluvial river response to sea level change suggest that rivers aggrade and degrade in response to relative sea level rise and fall, respectively. We show that rivers can remain aggradational if the rate of sea level fall is sufficiently small. The boundary between aggradational and degradational states is the graded state described here: that associated with a relative sea level fall that varies with the square root of time.
The number of stream basin areas (that is, the number of Strahler stream segments) is predicted on the basis of an hexagonal model for basin areas. It is suggested that the number of orders and the number of basins per order balance opposing tendencies for minimum overland work for streams flowing in small basins and maximum work savings in large, as opposed to small, channels. In addition, the entropy of the system approaches the maximum possible. The model agrees with empirical data in cases where the land surface is reasonably uniform with regard to structure and lithology. The necessity for approximate geometric progressions of basin areas with order creates geometric progressions of other basin parameters, leading directly to the power function relationships between variables, known as allometric growth.
This model is identical to one proposed to explain the numbers of market areas per order in a system of cities. In addition, the model predicts the structure of bile ducts in one bovine liver cited in this paper.
A plethora of normative conservation concepts have recently emerged, most of which are ill-defined: biological diversity, biological integrity, ecological restoration, ecological services, ecological rehabilitation, ecological sustainability, sustainable development, ecosystem health, ecosystem management, adaptive management, and keystone species are salient among them. These normative concepts can be organized and interpreted by reference to two new schools of conservation philosophy, compositionalism and functionalism. The former comprehends nature primarily by means of evolutionary ecology and considers Homo sapiens separate from nature. The latter comprehends nature primarily by means of ecosystem ecology and considers Homo sapiens a part of nature. Biological diversity, biological integrity, and ecological restoration belong primarily in the compositionalist glossary; the rest belong primarily in the functionalist glossary. The former set are more appropriate norms for reserves, the latter for areas that are humanly inhabited and exploited. In contrast to the older schools of conservation philosophy, preservationism and resourcism, compositionalism and functionalism are complementary, not competitive and mutually exclusive. As the historically divergent ecological sciences—evolutionary ecology and ecosystem ecology—are increasingly synthesized, a more unified philosophy of conservation can be envisioned.
Japanese rivers where a large amount of sediment accumulates along the middle reaches are today not at grade. Nevertheless, most of their longitudinal profiles can be described by one of the mathematical functions proposed for graded rivers. Through aggradational processes, the shape of longitudinal profile of a river changes with the change in mathematical function type, from exponential, to power, and finally to linear functions. The difference in the type of function best fitting the longitudinal profile of a river reflects variations in fluvial processes and the evolutionary stage of the river. -from Author
This study examined the morphological behavior of a channel with vegetation by means of both laboratory experiments and a numerical simulation. Two sets of laboratory experiments were carried out to elucidate the influence of riparian vegetation on braided rivers with erodible banks. One was for a braided river without vegetation from an initially straight channel and the other was for a river with vegetation, where the density of alfalfa was controlled to form vegetated channels. A numerical simulation was also conducted to investigate the effects of vegetation on the channel morphology considering bank erosion, and the results were compared with those of the laboratory experiments. The experimental results showed that the water depth increased and the width decreased with vegetation. The mainstream was also developed to a high meandering channel and then bifurcated to two channels due to vegetation downstream. This was due mainly to a change of flow direction by the blockade of secondary channels near the vegetated zones and backwater effects near the vegetated banks. As the vegetation density increased, the correlation coefficient of cross-sectional data increased in both the numerical and experimental results, and good agreement between the numerical and experimental results was obtained. This indicates that the channel mobility decreased with increasing vegetation density. The numerical model generated the features of the bed topography obtained in experimental channels
Rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers, but whether periodicities exist in the occurrence of step-pools has not been investigated in a comprehensive way, and the implications for such periodicities have not been explored. Spectral analysis was applied to bed elevation data from streams in the Santa Monica Mountains of southern California to investigate periodicity in the occurrence of step-pools. Results indicate significant step-pool periodicities, confirmed by independent field measurements, that vary from 0.43 channel widths to 2.40 channel widths. Spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step-pool spacing. Underlying periodicities are nevertheless detected, suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms. Periodicity in the occurence of step-pools suggests that step-pools are bedforms that represent a fundamental mutual adjustment between flow, channel morphology, and energy expenditure. A rhythmic channel thalweg supports the notion that step- pools are analogous to meandering in the vertical dimension, and implies that, because step-pool channels are not able to adjust energy expenditure in the plane dimension, they instead adjust boundary roughness in the vertical dimension. A continuum of rhythmic gravel bedforms that displays the results of such adjustments, and that shows how step-pools merge into pools and riffles in the stream channel system, is discussed and illustrated with empirical data from the Santa Monica Mountains. Although many differences exist between steep mountain streams and broad alluvial rivers, the findings of this study suggest that generalized principles of uniform energy expenditure may also apply to rhythmic step-pool streams. These findings are potentially valuable in the management and design of steep channels.
Based on data from 199 stations in the Yellow River drainage basin for which more than 5 years of data are available, the relationship between specific sediment yield (Ys) and drainage area (A) has been studied. This relationship for the Yellow River basin is different from those for many other rivers of the world, both at the scale of whole basin and at local scales. With increasing basin area, the specific sediment yield increases, reaches a maximum, and then declines. The non-linear variation in the Ys–A relationship can be explained by, first, surface material distribution; second, adjustment of the basin at macro time- and space-scales; and the third, the variation of energy expenditure with drainage-basin scale. As the loess deposits in the high-relief headwater areas is thin, it can be exhausted much more rapidly by flowing water erosion than in other areas, so the underlying erosion-resistant bedrock may be exposed much earlier. In many rivers in the study area, bedrock is exposed in the upper part of the drainage basin, and loess appears in the middle part, with a thickness that increases downstream to a peak, followed by a decline. Due to the influence of these spatial patterns of surface material distribution, a spatial pattern of specific sediment yield appears. The non-linear Ys–A relationship may be interpreted as an indication that the fluvial system of the Yellow River is still at the stage of strong adjustment to the environmental change of Pleistocene–Holocene transition, especially to the change of dominant geomorphic agency from wind to flowing water. This non-linear relationship can also be explained by the variation of stream power with the drainage basin scale. The stream power increases with drainage area to a peak value, and subsequently decreases, a trend that is similar to the trend of the Ys–A relationship.