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Air-photo based change in channel width in the Minnesota River basin: Modes of adjustment and implications for sediment budget
Abstract
The Minnesota River and major tributaries have experienced large increases in discharge over the past century. Aerial photograph-based measurements of channel width were made for the 1938–2015 period at 16 multibend subreaches by digitizing the area between vegetation lines and dividing by centerline length. Results show considerable increases in width for the main stem (0.62 ± 0.10%/y) and major tributaries (0.31 ± 0.08%/y) but are inconclusive for smaller channels (width < 25 m). Width change for a 146.5-km reach of the lower Minnesota River between 1938 and 2008 is similar to that from the subreach-scale analysis. Widening was associated with lateral centerline movement and temporal change in at-a-station hydraulic geometry for water surface width, indicating that widening is associated with cross-sectional change and not simply upward movement of the vegetation line. Digital elevation model analysis and regional hydraulic geometry show that the main stem and larger tributaries account for the vast majority (~ 85%) of bankfull channel volume. High-order channels are thus disproportionately responsible for sediment production through cross section enlargement, although floodplains or off-channel water bodies adjacent to these channels likely represent important sediment sinks. Because channel enlargement can play an important role in sediment production, it should be considered in sediment reduction strategies in the Minnesota River basin and carefully evaluated in other watersheds undergoing long-term increases in discharge.
... The Minnesota River near Mankato is an ideal location to zero this model because detailed field data are available to estimate input and output parameters (Lauer et al., 2017;Lenhart et al., 2013). However, due to rapid channel widening (Lauer et al., 2017), the present Minnesota River is not adequate for the application of an equilibrium model. ...
... The Minnesota River near Mankato is an ideal location to zero this model because detailed field data are available to estimate input and output parameters (Lauer et al., 2017;Lenhart et al., 2013). However, due to rapid channel widening (Lauer et al., 2017), the present Minnesota River is not adequate for the application of an equilibrium model. Significant changes in channel geometry of the Minnesota River did not occur prior to 1966, and a large shift in cropping practices (increased efficiency of drain tiles) has been observed since 1938 (Lenhart et al., 2013). ...
... The mean water depth at bankfull flow is 4.47 m (Johannesson & Parker, 1985) and the predicted equilibrium L f is 4.48 m. Measured B t is 75 m (Lauer et al., 2017) and the model equilibrium result varies between 71 and 81 m. Finally, F f,s is equal to 0.40 based on field data (Lauer et al., 2017) and to 0.41 according to the model. ...
Equilibrium geometry of single‐thread rivers with fixed width (engineered rivers) is determined with a flow resistance relation and a sediment transport relation, if characteristic discharge, sediment caliber and supply are specified. In self‐formed channels, however, channel width is not imposed, and one more relation is needed to predict equilibrium geometry. Specifying this relation remains an open problem. Here we present a new model that brings together a coherent train of research progress over 35 years to predict equilibrium geometry of single‐thread rivers from the conservation of channel and floodplain material. Predicted channel geometries are comparable with field observations. In response to increasing floodplain width, sand load and grain size, the equilibrium slope increases, bankfull depth and width decrease. As the volume fraction content of mud in the sediment load increases, bankfull width‐to‐depth ratio and slope decrease suggesting that mud load has a strong control on channel patterns and bankfull geometry.
... We further examine the relative importance of extremely high, short duration flows versus moderately high, longer duration flows. The model is applied to the actively meandering Minnesota River, MN, USA (Figure 1a), the channel of which has been subject to widening primarily due to increased stream flow over more than four decades (Figures 1c,d;Lenhart et al., 2013;Lauer et al., 2017). The model application is extended to address the difference between the time scale of cross-sectional profile adjustment and that of longitudinal profile adjustment, which is also observed in the Minnesota River. ...
... Modern observed bankfull channel characteristics near Jordan, MN are: bankfull discharge Qbf = 700 m 3 /s, bankfull width Bbf = 107 m, bankfull depth Hbf = 5 m, and down-channel bed slope Sc = 0.00022, respectively (Li et al., 2015;Lauer et al., 2017). Lauer et al. (2017) have reported that the Minnesota River and its major tributaries have been subject to widening at an average rate of 0.62 %/yr (relative to the average width between 2000 and 2009) since the 1930s. ...
... Modern observed bankfull channel characteristics near Jordan, MN are: bankfull discharge Qbf = 700 m 3 /s, bankfull width Bbf = 107 m, bankfull depth Hbf = 5 m, and down-channel bed slope Sc = 0.00022, respectively (Li et al., 2015;Lauer et al., 2017). Lauer et al. (2017) have reported that the Minnesota River and its major tributaries have been subject to widening at an average rate of 0.62 %/yr (relative to the average width between 2000 and 2009) since the 1930s. Similarly, Lenhart et al. (2013) have reported that rapid widening of the main stem of the Minnesota River began later than 1966, which is after an extreme flood event of 1965 (Strub, 1965;Novotny & Stefan, 2007). ...
We consider the evolution of the hydraulic geometry of sand‐bed meandering rivers. We study the difference between the timescale of longitudinal river profile adjustment and that of channel width and depth adjustment. We also study the effect of hydrological regime alteration on the evolution of bankfull channel geometry. To achieve this, a previously‐developed model for the spatiotemporal co‐evolution of bankfull channel characteristics, including bankfull discharge, bankfull width, bankfull depth and down‐channel bed slope is used. In our modeling framework, flow variability is considered in terms of a specified flow duration curve. Taking advantage of this unique feature, we identify the flow range responsible for long‐term bankfull channel change within the specified flow duration curve. That is, the relative importance of extremely high short duration flows compared to moderately high longer duration flows is examined. The Minnesota River, MN, USA, an actively meandering sand‐bed stream, is selected for a case study. The longitudinal profile of the study reach is still in adjustment toward equilibrium since the end of the last glaciation, while its bankfull cross‐section is rapidly widening due to hydrological regime change in the last several decades. We use the model to demonstrate that the timescale for longitudinal channel profile adjustment is much greater than the time scale for cross‐sectional profile adjustment due to a lateral channel shift. We also show that hydrological regime shift is responsible for the recent rapid widening of the Minnesota River. Our analysis suggests that increases in the 5%‐25% exceedance flows play a more significant role in recent bankfull channel enlargement of the Minnesota River than increase in either the 0.1% exceedance flow or the 90% exceedance flow.
... River channels are among the most dynamic landforms on Earth's surface, sweeping laterally across valley bottoms-often in subtle and sometimes catastrophic-ways over event-, decadal-, and millennial timescales. Remotely-sensed imagery is increasingly used to delineate channel boundaries to measure changes in river planform such as lateral migration (Hickin and Nanson, 1984;Gurnell et al., 1994;Gaeuman et al., 2005b;Lauer and Parker, 2008;Constantine et al., 2014;Donovan et al., 2015Donovan et al., , 2016Morais et al., 2016), as well as channel width (Winterbottom, 2000;Pavelsky and Smith, 2008;Swanson et al., 2011;Downs et al., 2013;Lauer et al., 2017). These measurements provide a basis for understanding effective management strategies for erosion along the riparian corridor (Micheli et al., 2004;Piégay et al., 2005), providing input to sediment budgets (Allmendinger et al., 2007;Belmont et al., 2011;Smith et al., 2011), and automated characterization of single-versus multi-threaded river planforms (Rowland et al., 2016). ...
... The latter is particularly relevant for short-term measurements of smaller changes that are rendered obsolete if uncertainty is overestimated (Liro, 2015;Lea and Legleiter, 2016;Donovan and Belmont, 2019). However, when uncertainty cannot be properly quantified, results should be constrained using upper and lower bounds of uncertainty (Kiiveri, 1997;Crosetto and Tarantola, 2001;Donovan et al., 2015;Passalacqua et al., 2015;Lauer et al., 2017) or simply highlighting locations where the measurement is more or less a reflection of noise (i.e., random variability) to provide an estimate of reliability for end users. Using probability and fuzzy positional boundaries has been proposed as a generic approach to estimate probabilistic positional uncertainty in GIS (Kiiveri, 1997;Wheaton et al., 2010). ...
... Regardless of the method used, channel-margin delineations are most often defined by the edge of riparian vegetation in order to eliminate variability arising from fluctuating water levels (Winterbottom, 2000;Gaeuman et al., 2003;Nelson et al., 2013;Rowland et al., 2016;Werbylo et al., 2017). The exception to this is when using delineations to estimate discharge or to determine if/how channel width has changed for a given flow value, in which case, variability as a function of water stage is desired (Bjerklie et al., 2005;Smith and Pavelsky, 2008;Lauer et al., 2017). Vegetated channel-margin delineations are more prone to error and inconsistency for braided and anastomosing systems, where width-related metrics are more sensitive to stage (Werbylo et al., 2017). ...
... The MRB drains approximately 43,400 km 2 of south-central Minnesota, South Dakota, and Iowa, USA ( Figure 1). The Minnesota River itself is disproportionately small (~100 m wide; Lauer et al., 2017) compared to its wide valley (up to 2 km wide; Lenhart et al., 2013) carved by outflows from glacial Lake Agassiz near the end of the last glaciation (Clayton & Moran, 1982;Matsch, 1983). Incision on the mainstem Minnesota River created knickpoints that have propagated upstream on all major tributaries, leading to deeply incised lower tributary valleys in an otherwise relatively flat postglacial landscape (Belmont, 2011;Gran et al., 2009Gran et al., , 2013. ...
... There are three main reasons for this: (1) Large-scale conversion of prairie to primarily row-crop agriculture with accompanying changes in surface and subsurface drainage has left the MRB strongly altered from its pre-European settlement state, affecting its connectivity and hydrologic response (Figures 2a and 2b;Foufoula-Georgiou et al., 2015;Lenhart et al., 2012;Schottler et al., 2014). Analyses of available discharge, precipitation, and land use/land cover data have demonstrated increasing flows over time on the mainstem Minnesota River and its tributaries Kelly et al., 2017;Novotny & Stefan, 2007); (2) The deeply incised postglacial valleys are particularly sensitive to changes in hydrology, and the strong geomorphic response in channels has important ramifications for erosion and sediment loading Cho, 2017;Gran et al., 2013;Lauer et al., 2017;Lenhart et al., 2018;Vaughan et al., 2017); and (3) Dramatic increases in fertilizer input, coupled with the drainage of~80% of historic wetlands in the region since European settlement, have contributed to high loads of nitrogen and phosphorus throughout the watershed creating numerous local and downstream water quality challenges, including drinking water contamination, algal blooms, hypoxic zones, and harm to aquatic life (Boardman, 2016;Hansen et al., 2018;USACE, U.S. Army Corps of Engineers St. Paul District, M, 2004). ...
... ArcGIS shapefiles of source and sink delineations including ravines, bluffs, channel centerlines (in 1938 and 2008), and lakeshed and watershed boundaries in the GBERB are available in Bevis and Gran (2017) with full descriptions of mapping methods in the metadata and Bevis (2015). Rates of erosion were determined through a variety of methods including change detection analyses of lidar and georeferenced historical air photos along channel corridors Day et al., 2013b;Lauer et al., 2017;Passalacqua et al., 2012), repeat aerial and terrestrial lidar on river bluffs (Day et al., 2013b(Day et al., , 2013aSchaffrath et al., 2015), repeat structure-from-motion (SfM) photogrammetry and time-lapse photography of river bluffs , and autosampler monitoring of ravines Gran et al., 2011). Sediment fingerprinting (described below) provided an independent constraint on sediment partitioning. ...
Observatory-scale data collection efforts allow unprecedented opportunities for integrative, multidisciplinary investigations in large, complex watersheds, which can affect management decisions and policy. Through the National Science Foundation-funded REACH (REsilience under Accelerated CHange) project, in collaboration with the Intensively Managed Landscapes-Critical Zone Observatory, we have collected a series of multidisciplinary data sets throughout the Minnesota River Basin in south-central Minnesota, USA, a 43,400-km ² tributary to the Upper Mississippi River. Postglacial incision within the Minnesota River valley created an erosional landscape highly responsive to hydrologic change, allowing for transdisciplinary research into the complex cascade of environmental changes that occur due to hydrology and land use alterations from intensive agricultural management and climate change. Data sets collected include water chemistry and biogeochemical data, geochemical fingerprinting of major sediment sources, high-resolution monitoring of river bluff erosion, and repeat channel cross-sectional and bathymetry data following major floods. The data collection efforts led to development of a series of integrative reduced complexity models that provide deeper insight into how water, sediment, and nutrients route and transform through a large channel network and respond to change. These models represent the culmination of efforts to integrate interdisciplinary data sets and science to gain new insights into watershed-scale processes in order to advance management and decision making. The purpose of this paper is to present a synthesis of the data sets and models, disseminate them to the community for further research, and identify mechanisms used to expand the temporal and spatial extent of short-term observatory-scale data collection efforts.
... Fluxes of fine sediment (clay, silt and fine sand) in particular have been directly affected by dam construction, urbanization, agriculture, fire suppression, mining, dredging, and logging [6]. Pervasive changes in watershed hydrology, due to anthropogenic climate change as well as land and water management actions, have indirectly amplified and damped sediment loading [8][9][10][11]. Such alterations in riverine fine sediment fluxes have important implications for channel and floodplain morphology [12,13], nutrient and contaminant transport [14][15][16], and aquatic habitat [17,18]. ...
... Thus, a suite of conservation measures are currently being considered to reduce sediment loading [28]. The hydroclimate of south central Minnesota, like large swaths of the Midwest, is becoming wetter and ongoing increases in artificial agricultural drainage continue to increase river runoff [10,11], creating more erosive flows. ...
... Several studies document strong coupling between discharge and erosion of near-channel sediment sources (NCSS), such as streambanks and bluffs, at broad spatial (10 3 + km 2 ) and temporal (semiannual to decadal) scales in the Minnesota River basin [7,9,10,[29][30][31]. However, mechanistic linkages between NCSS erosion and streamflows have received less attention at finer spatial (10 1 -10 2 m 2 ) and temporal (daily to seasonal) scales. ...
Using a combination of Structure from Motion and time lapse photogrammetry, we document rapid river blufferosion occurring in the Greater Blue Earth River (GBER) basin, a muddy tributary to the sediment-impaired Minnesota River in south central Minnesota. Our datasets elucidated dominant blufffailure mechanisms and rates of bluffretreat in a transient system responding to ongoing streamflow increases and glacial legacy impacts. Specifically, we document the importance of fluvial scour, freeze-thaw, as well as other drivers of blufferosion. We find that even small flows, a mere 30% of the two-year recurrence interval flow, are capable of causing blufferosion. During our study period (2014-2017), the most erosion was associated with two large flood events with 13-and 25-year return periods. However, based on the frequency of floods and magnitude of bluffface erosion associated with floods over the last 78 years, the 1.2-year return interval flood has likely accomplished the most cumulative erosion, and is thus more geomorphically effective than larger magnitude floods. Flows in the GBER basin are nonstationary, increasing across the full range of return intervals. We find that management implications differ considerably depending on whether the blufferosion-runoffpower law exponent, γ, is greater than, equal to, or less than 1. Previous research has recommended installation of water retention sites in tributaries to the Minnesota River in order to reduce flows and sediment loading from river bluffs. Our findings support the notion that water retention would be an effective practice to reduce sediment loading and highlight the importance of managing for both runofffrequency and magnitude.
... In contrast, sediment delivery is high for bank erosion in the Lower Minnesota River. Since the banks are much taller in the Lower Minnesota River (3-4 m) than in tributaries (typically 0.5-3 m), it accounts for a disproportionate amount of the channel-derived sediment (Lauer et al. 2017). ...
... Bluffs and large stream banks contribute the majority of sediment making it out of the MRB. By location it is the Blue Earth and Le Sueur watersheds in the south central basin (Fig. 2) contributing most of the bluff sediment and the lower main channel contributing the most streambank sediment (Lauer et al. 2017). For these reasons sediment load reduction plans in the MRB have focused on reducing channel sediment loading through watershed management to reduce flow and targeted stream bank and bluff stabilization projects. ...
... Consequently, plans to reduce bank and bluff sediment must consider these areas. In terms of bank sediment contributions, the lower main stem Minnesota River supplies a disproportionate amount of sediment, due to its large channel size and easily eroded alluvial channel materials (Lenhart et al. 2013;Lauer et al. 2017) although substantial sediment is also stored in the Lower Minnesota River floodplain. Targeting tall, actively eroding banks may be helpful in reducing sediment load, and provides additional benefits where infrastructure is threatened. ...
In many agricultural watersheds, channel erosion is a major source of sediment. In these watersheds, a comprehensive approach to reducing channel erosion can aid in the feasibility of meeting water quality goals. In the Minnesota River Basin (MRB), a large agricultural watershed, increased peak flows have contributed to greater sediment loading from within the channel. This study focuses on three components of channel restoration that are important in developing a strategic framework: sediment source reduction, working with private landowners, and the economics of restoration. A synthesis of sediment research was done to assess the sediment reduction benefit of restoration projects. While field erosion is the largest source of gross erosion, most of the sediment from fields is not delivered to the river. In the MRB, sediment from stream banks and bluffs is the dominant source, thus requiring water storage and/or stream management in order to reduce sediment load. Landowners favored edge-of-field practices that minimize the conversion of farmland and have minimal government intervention. Economic analysis of restoration showed that the most cost-effective restoration projects reduced large amounts of sediment on short stretches of lower-order streams. However ecological benefits, project sustainability, and local cultural issues suggest a role for smaller stream restoration projects. Sediment reduction estimates and assessment of ecosystem service benefits from stream restoration projects need to be further developed.
... The measurements derived through the interpretation of aerial images and orthophotomaps at a 1:25000 (1997), (Williams and Thesis, 1978;Atkins, 1979). The levees were delineated manually along each stage of study (Lauer et al., 2017). We applied the Digital Elevation Model (DEM) of high resolution of the study area, and data from the Aster GDEM to limit the drainage basin. ...
... However, it is discussed that the granulometric difference responds to the selective transport of the materials as a response to the hydrodynamic variations produced in the direction of the current (Manassero et al., 2004). One notion that is more generalized sustains that the distribution of size of any sediment exported down water depends on the distribution of the size of the sediment eroded from the banks, as well as the deposits of bars (Lauer et al., 2017). However, in the Apure river, it may be inferred that the source area of these materials is complex, for being the Apure a collector of all the rivers that flow in the Northwestern Plains; however, the materials that filled up the depression to get to form the present plain have two different sources: one in the Colombo-Venezuelan Andes, and the other in the Guiana Massif (MARN-Hidromet, 2001). ...
The Apure river is the largest tributary of the Orinoco river basin in the Venezuelan plains, it transports considerable amounts of sediments in the range of coarse sand to clay, and its distribution is fundamental to understand the processes of sedimentation in the different fluvial environments. However, little attention has been given to its sedimentary features. We present a field investigation of the different streams of the anastomosis system, which are controlled by a change in the underlying lithostratigraphy of the channel. We analyze the grain size distribution of the sediments in the different fluvial environments to understand the patterns of evacuation and the transport mechanisms under conditions of variable energy. We examine seven sections distributed in an area of 65 km², where the distribution of the morphosedimentary environments from a combination of data of Landsat 8 images, Geographic Information Systems (GIS) and sedimentological samples in the field were stablished. We found a predominance of fine to medium silt (71%) in relation to the clay (18%) and sand (11%). This variation is mainly associated to the existence of controls of lithological origin. The C-M diagram suggest that the sediments were transported in a graded to uniform suspension. This study reveals that sorting and transport of sediment in the anastomosis system of the study area is mainly influenced by the hydrodynamic of the flow stream. This study requires additional validation; however, it supports that changes in the amount and distribution of grain size within the fluvial system affect the morphology of the channel in anastomosis rivers, and it help to improve the present understanding of the sedimentological records and the geomorphological changes, determine the stability of the channel, and its environmental effects.
... We evaluate curvature-migration relationships using channel change along centerlines derived from aerial photographs spanning approximately 180 km of the Minnesota River between the towns of Mankato and Fort Snelling, near the confluence with the Mississippi River. Six sets of images (1937, 1951, 1964, 1980, 1991, and 2013) were available along this portion of the river, which has been the focus of multiple recent geomorphic studies due to its unique geomorphic history and longitudinally contrasting sediment dynamics (Call et al., 2017;Lauer et al., 2017;Lenhart et al., 2013;Libby, 2017;Kelly & Belmont, 2018). About 13,400 years ago, the outpouring of glacial Lake Agassiz caused 70 m of incision of the mainstem Minnesota River Valley (Clayton & Moran, 1982;Lepper et al., 2007;Matsch, 1983;Shay, 1967), which has resulted in multiple knickpoints and exposure of highly erodible glacial sediments along tributary valleys (Belmont, 2011;Gran et al., 2013;Jennings, 2010). ...
... In addition, the river has been responding to contemporary land use and precipitation changes over the last 80 years, which have increased flows by 50%-250% (Belmont, Dogwiler, & Kumarasamy, 2016;Kelly et al., 2017;Foufoula-Georgiou et al., 2015;Novotny & Stefan, 2007;Schottler et al., 2014). Recent increases in flow from artificial drainage have amplified rates of lateral channel migration Libby, 2017) and increased channel widths by 52% since 1938 (Lauer et al., 2017;Schottler et al., 2014). ...
River meander migration plays a key role in the unsteady “conveyor belt” of sediment redistribution from source to sink areas. The ubiquity of river meandering is evident from remotely sensed imagery, which has allowed for long‐term, high‐resolution studies of river channel change and form‐process relationships. Empirical, experimental, and theoretical research approaches have described two distinct relationships between channel curvature and river channel migration rates. In this study, we employ a novel application of time‐series algorithms to calculate migration rates and channel curvature at sub‐meander bend length scales using 6 decades of aerial imagery spanning 205 km of the Minnesota River and Root River, Minnesota, USA. Results from the Minnesota River provide the first empirical evidence demonstrating how migration‐curvature relations break down for rivers with low sediment supply, which is supported by the Root River data set. This not only highlights the importance of sediment supply as a driver of river migration, but also supports a simple means to detect river reaches lacking sediment supply. Furthermore, results from both rivers demonstrate that sub‐meander bend measurement scales are most appropriate for studying channel migration rates and further indicate that a quasi‐linear relationship—rather than the more commonly inferred peaked relationship—exists between channel curvature and migration rates. The highest migration rates are associated with the highest measured channel curvatures in our data set, after accounting for a spatial lag of 2.5±0.20.3 channel widths. These findings are consistent with flume experiments and empirical data across diverse geologic and climatic environments.
... In the fluvial context, remote-sensed data opportunely provides spatial information on historical lateral dynamics of river channels (Bollati et al., 2014;Cadol et al., 2010;Comiti et al., 2011;Gurnell et al., 1994;Hajdukiewicz and Wyżga, 2019;Wesley Lauer et al., 2017). This is of crucial importance for e.g. ...
... Firstly, a key outcome is the ability of MC simulations to actually detect low-magnitude planform changes in mid-sized river channels. This positive achievement thus 10 overcomes the main difficulty related to the use of classic planimetric methods in such settings (Piégay et al., 2005), as recently highlighted by Wesley Lauer et al. (2017), who failed detecting noticeable changes in mid-sized active channels (width<25 m). Secondly, as for river restoration, our methodological framework should help constructing robust scenarios of future river management, especially those based on past planform changes (e.g., Marçal et al., 2017). ...
Abstract. Remote-sensed data in the fluvial context are extensively used to document historical planform changes. However, geometric and delineation errors inherently associated with these data can result in poor or even misleading interpretation of measured changes, especially (rates of) channel lateral migration. It is thus fundamental to take a spatially-variable (SV) error affecting remote-sensed data into account. In the wake of recent key studies using this SV-error as a level of detection, we introduce a new framework to evaluate the significance of measured channel migration. Going beyond their linear metric (i.e. migration vectors between diachronic river centrelines), we assess this significance through the channel polygon method yielding a surfacic metric (i.e. quantification of eroded, deposited, or eroded/deposited surfaces).
Our study area is an active wandering mid-sized river: the lower Bruche, a ∼ 20 m wide sub-tributary of the Rhine in eastern France. Within our four test sub-reaches, the active channel is digitised using diachronic orthophotos (1950; 1964) and the sub-reach specific SV-error affecting the data is interpolated with an Inverse Distance Weighting (IDW) technique. A main novelty of our approach consists then in running Monte-Carlo (MC) simulations to randomly translate active channels and propagate geometric and delineation errors according to the SV-error. This eventually leads to the production of a Surface of Detection (SoD), which allows evaluating the significance of measured surfacic changes. Putting the SoD into practice in the lower Bruche shows that only 37 % of the total surfacic measured changes are significant. Our results suggest that (i) orthophotos are affected by a significant SV-error, (ii) the latter strongly affects the significance of measured changes and (iii) the significance is strongly dependent on the magnitude of surfacic changes. Taking the SV-error into account is strongly recommended, regardless of the remote-sensed data used (orthophotos or aerial photos), especially in the case of mid-sized rivers (< 30 m width) and/or low amplitude river planform changes (< 1000 m<sup>2</sup>/yr). We finally insist on the transposability of our approach as we use well-established tools (IDW, MC): this opens new perspectives in the fluvial context (e.g. multi-thread river channels) for robustly assessing surfacic changes.
... Analyses of geomorphic responses to fluvial perturbation have been approached in many ways. But aside from small-scale analog models (e.g., Baynes et al., 2018;Lisle et al., 1997;Schumm, 1973;Schumm et al., 1987) and numerical models (e.g., Limaye & Lamb, 2016;Malatesta et al., 2017), few studies actually document long-term channel evolution following disturbance of a dynamically stable river (e.g., East et al., 2017;Gellis et al., 2017;Gran et al., 2013;Kasai, 2006;Kasai et al., 2004;Lauer et al., 2017;Leopold, 1973;Meyer et al., 1992Meyer et al., , 1995Moody et al., 1999;Moody & Meade, 2014, 2018Tunnicliffe et al., 2018), and even fewer record spatiotemporal variations of the relative efficacies of vertical versus lateral adjustments in the fluvial system (e.g., East et al., 2018;Meyer & Martinson, 1989;Moody & Meade, 2018). This is not surprising; few rivers exist where substantial (kilometers-long) reaches are effectively reset and reestablished over human (decadal) timescales or where it has been possible to sustain multiyear, let alone multidecadal, monitoring. ...
... This new phase space diagram demonstrates lateral adjustments dominate most of the conceptual phase space domains representing geomorphic adjustments in alluvial rivers. The dominance of lateral over vertical adjustments is consistent with channel evolution observations across a broad array of alluvial systems (e.g., Gurnell et al., 1994;Jacquet et al., 2017;James, 1999;Lauer et al., 2017;Mertes et al., 1996;Vericat et al., 2017). ...
A 2.5‐km³ debris avalanche during the 1980 eruption of Mount St. Helens buried upper North Fork Toutle River valley and reset the fluvial landscape. Since then, a new drainage network has evolved. Cross‐sectional surveys repeated over nearly 40 years at 16 locations along a 20‐km reach of river valley document channel evolution. We analyze spatial and temporal changes in channel morphology using two new metrics: (1) a shape index that defines the degree of U‐shaped or V‐shaped valley geometry and (2) an alluvial phase space diagram that relates bed degradation or aggradation to increases or decreases in cross‐sectional area. Unlike a simple, linear response model previously proposed, our analysis reveals channel development has been distinctly nonlinear and nonsequential. Rather than following a sequential trajectory of (1) channel initiation and incision, (2) aggradation and widening, and (3) episodic scour and fill with little change in bed elevation, long‐term channel evolution has been more complex with vertical and lateral adjustments intertwined throughout. Our analysis reveals channel evolution has followed a complex trajectory that has migrated nonsequentially through several phase space domains including degradation and aggradation with widening and narrowing, bed‐level fluctuations with little change in cross‐section area, and changes in cross‐sectional area with little change of bed elevation. Persistent channel widening and reworking of the channel bed are responsible for maintaining elevated sediment delivery from this basin. Elevated sediment delivery is likely to persist until valley floor widths greatly exceed that of the channel migration zone, and/or channel slopes and valley walls stabilize.
... Other methods for estimating the rate of sediment transport, such as sediment budgets and the morphologic method, are considerably more resource intensive but can provide useful information if applied correctly. For example, sediment budgets provide an overview of the transfer of sediment between various sections of the network that can be used to contextualize morphologic responses (Lauer et al., 2017) and predict contaminant pathways through a watershed (Walling & Collins, 2008). Sediment budgets are constructed by estimating sediment inputs (soil erosion, bank erosion, tributaries) and outputs from GIS analyses and field measurements (Allmendinger et al., 2007;Chen et al., 2018). ...
Fluvial geomorphology, which describes the form and processes of rivers, is increasingly being incorporated into river assessment procedures. However, the complexity of geomorphic processes makes a single universal and standardized assessment protocol a challenging and possibly impractical task. In this paper, we present a set of recommendations for choosing appropriate river assessment procedures and measuring geomorphic indicators to effectively capture important geomorphic processes required to support river management goals. We outline steps for building a river assessment procedure based on an adaptive approach rather than a one‐size‐fits‐all approach, where the geomorphic indicators, spatial and temporal scale, and methodologies used are carefully chosen based on the goals of the management project; the assessment aims to support. Guidance for choosing the appropriate geomorphic indicators is based on their significance (usefulness in characterizing the system), ease of measurement, and temporal scale needs. We also present recommendations on measurement techniques for each indicator while highlighting recent technological and methodological advancements that help overcome resource challenges often faced in river assessment. Given the wealth of scientific and technological developments in the field of geomorphology, it is possible to improve how geomorphic form and function are measured and incorporated into river assessments that support watershed management goals.
... Tal parâmetro é extensivamente utilizado em estimativas de outros atributos do sistema fluvial, como descarga e concentração de sedimentos (ELMI; TOURIAN;SNEEUW, 2015;PEREIRA et al., 2018). Estudos na Geomorfologia Fluvial integram a variação da largura do canal em análises das mudanças da forma em planta de rios (ASSUMPÇÃO; MARÇAL, 2012;WESLEY LAUER et al., 2017). Medições da largura do canal podem ser facilmente estimadas através do Sensoriamento Remoto. ...
A largura do canal pode ser afetada por alterações no regime hidrológico ou por intervenções antrópicas. Eventos de corridas de lama e de detritos podem resultar em assoreamento de canal, ampliando sua largura, conforme exemplificado no processo que ocorreu em 2011 no trecho de planície do rio Jacareí (Litoral do Paraná). Objetiva-se analisar as alterações na largura do canal do rio Jacareí decorrentes do assoreamento induzido por evento extremo e pelas intervenções antrópicas posteriores. Propõem-se um roteiro metodológico para mensuração comparativa da largura de canais fluviais em ambiente SIG. O rio Jacareí teve significativas alterações na largura no período analisado, dividido em trechos alargados tanto pelo intenso assoreamento como por intervenções antrópicas. No trecho alargado pelo assoreamento, constatou-se um processo de retorno a condições de equilíbrio antes de o sistema ser perturbado pelo evento de 2011. Já no trecho do canal afetado por intervenções antrópicas, as larguras se mantiveram estáveis, provavelmente em função das características da obra efetuada (abertura e desassoreamento).
... Formation of river island and its gradual growth bifurcates the single thread river into multi-thread river and for capacitating water flow widening of bifurcated channels eroding the fragile part of the bank is very usual in fluvial process (Kleinhans et al. 2011;Lauer et al. 2017). Such erosion processes again contribute huge sediment load for island/ char formation (Baki and Gan 2012;Rashid and Rahman 2021). ...
The Ganga/Ganges river bank erosion forced thousands of people to become landless and environmental refugees. river islands that emerged in this river segment may be alternative habitats for the environmental refugees. The present article intends to map Charland (river island), and the stability of Charland (island) to show the habitable area using time-series satellite images since 1987–2016. Instead of the factor-based approach usually used by scholars, in factor data sparse conditions, a result (charland planform)-based approach has been introduced for examining the stability of charland. From the analysis, it is found that the total area of the charland area excluding mainland charland is 237.64 km² and 286.82 km², respectively in the upper and lower reaches of the Farakka barrage during the post-monsoon period, whereas these are 261.21 km² and 311.48 km² during the pre-monsoon season. The seasonally submerged charland area is 48.22 km² due to the swelling of river water during the monsoon season. Only 22.9% of the area can be treated as stable charland since their appearance is consistent and they can be used for rehabilitation. Seasonally emerged, fertile charland could be used for agriculture during the pre-monsoon season.
... Storage of runoff within a SuDS system is essential for providing the extended detention of flows for water quality treatment, as well as for peak flow attenuation of larger flows for flood protection downstream of the site. Runoff storage can be provided within an on-site system through the use of structural controls and/or non-structural features and landscaped areas [22][23][24][25]. ...
Urban drainage systems are in transition from functioning simply as a transport system to becoming an important element of urban flood protection measures providing considerable influence on urban infrastructure sustainability. Rapid urbanization combined with the implications of climate change is one of the major emerging challenges. The increased concerns with water security and the ageing of existing drainage infrastructure are new challenges in improving urban water management. This study carried out in the Seixal area in Portugal examines flood risk analyses and mitigation techniques performed by computational modelling using MIKE SHE from the Danish Hydraulic Institute (DHI). Several scenarios were compared regarding flood risk and sustainable urban drainage systems (SuDS) efficiency. To obtain a more accurate analysis, the economic viability of each technique was analyzed as well through (i) life cost analysis and (ii) taking into account the damages caused by a certain type of flood. The results present that the best scenario is the one that will minimize the effects of great urbanization and consequently the flood risk, which combines two different measures: permeable pavement and detention basin. This alternative allows us to fully explore the mitigation capacity of each viable technique, demonstrating a very important improvement in the flood mitigation system in Seixal.
... These long-term rates can result from relatively steady erosional processes or may integrate punctuated large disturbances that deviate significantly from the long-term average (e.g., landslides, wildfires, large storm events). Whereas sediment delivery research has largely focused on hillslope and mass wasting processes (e.g., Cashman et al., 1995;Croke & Hairsine, 2006;Graham & O'Green, 2016), recent research has highlighted the importance of near-channel sediment sources within the stream network corridor (Belmont, 2011;Donovan et al., 2015;Lauer et al., 2017;Stout et al., 2014;Vaughan et al., 2017;Walter & Merritts, 2008). ...
The watersheds along the North Coast of California span a wide range of geologic settings, tectonic uplift rates, and historic timber harvest activity. Known trends in how each of these factors influence erosion rates provides an opportunity to examine their relative importance. We analyzed 71 watersheds within nine larger river basins, investigated the factors influencing suspended sediment rating curves (SRCs), investigated how SRCs varied among our study watersheds, and used Random Forest modeling (RFM) to determine which environmental characteristics and land management metrics influence SRC shapes, vertical offsets, and slopes. While SRCs typically take the form of a power function, they also can exhibit threshold or peak relationships. First, we found both power and threshold relationships for the SRCs within our study watersheds. Second, the SRC offsets and slopes systematically varied with regional tectonic uplift. Third, SRC offsets increased in several watersheds following intensive timber harvest events and SRC slopes decreased due to a greater relative increase in suspended sediment concentration at lower flows than higher flows. Our RFM correctly classified 96% of the SRC shapes using two near‐channel metrics; near‐channel precipitation‐sensitive deep‐seated landslide susceptibility and near‐channel soil erodibility. Our RFM models also showed that timber harvest activity and near‐channel local relief can explain 40% of the variability in SRC offsets, whereas tectonic uplift rates, millennial‐scale erosion rates, and precipitation patterns explain 40% of the variability in SRC slopes.
... There were large variations in sizes of floodplains studied. Larger rivers typically have larger floodplains [54]. Although floodplain-to-watershed area ratios would have been helpful for assessing nutrient removal efficiency, the watershed area was seldom mentioned. ...
Nutrient loads in fresh and coastal waters continue to lead to harmful algal blooms across the globe. Historically, floodplains—low-lying areas adjacent to streams and rivers that become inundated during high-flow events—would have been nutrient deposition and/or removal sites within riparian corridors, but many floodplains have been developed and/or disconnected. This review synthesizes literature and data available from field studies quantifying nitrogen (N) and phosphorus (P) removal within floodplains across North America and Europe to determine how effective floodplain restoration is at removing nutrients. The mean removal of nitrate-N (NO3−-N), the primary form of N in floodplain studies, was 200 (SD = 198) kg-N ha−1 year−1, and of total or particulate P was 21.0 (SD = 31.4) kg-P ha−1 year−1. Based on the literature, more effective designs of restored floodplains should include optimal hydraulic load, permanent wetlands, geomorphic diversity, and dense vegetation. Floodplain restorations along waterways with higher nutrient concentrations could lead to a more effective investment for nutrient removal. Overall, restoring and reconnecting floodplains throughout watersheds is a viable and effective means of removing nutrients while also restoring the many other benefits that floodplains provide.
... Both Lea and Legleiter (2016) and Donovan et al. (2019) developed a LoD for a linear metric (Fig. 1a) implemented in the Planform Statistics Toolbox (Lauer, 2006), which reports fluvial planform changes as a linear adjustment. However, by conflating river banks onto a unique centreline (Fig. 1a), a linear metric can oversimplify geomorphological changes. ...
Remotely sensed data from fluvial systems are extensively used to document historical planform changes.
However, geometric and delineation errors inherently associated with these data can result in poor or even misleading
interpretation of measured changes, especially rates of channel lateral migration. It is thus imperative to take into account a
spatially variable (SV) error affecting the remotely sensed data. In the wake of recent key studies using this SV error as
a level of detection, we introduce a new framework to evaluate the significance of measured channel migration. Going beyond
linear metrics (i.e. migration vectors between diachronic river centrelines), we assess significance through a channel polygon
method yielding a surficial metric (i.e. quantification of eroded, deposited, or eroded-then-deposited surfaces).Our study area is a mid-sized active wandering river: the lower Bruche, a ∼ 20 m wide tributary of the Rhine in eastern France.
Within our four test sub-reaches, the active channel is digitised using diachronic orthophotos (1950 and 1964), and the
SV error affecting the data is interpolated with an inverse-distance weighting (IDW) technique. The novelty of our approach arises from then
running Monte Carlo (MC) simulations to randomly translate active channels and propagate geometric and delineation errors
according to the SV error. This eventually leads to the computation of percentage of uncertainties associated with each of the
measured planform changes, which allows us to evaluate the
significance of the planform changes. In the lower Bruche, the uncertainty associated with the documented changes ranges from
15.8 % to 52.9 %.Our results show that (i) orthophotos are affected by a significant SV error; (ii) the latter strongly
affects the uncertainty of measured changes; and (iii) the significance of changes is dependent on both the magnitude and the
shape of the surficial changes.
Taking the SV error into account is strongly recommended even in orthorectified aerial photos,
especially in the case of mid-sized rivers ( < 30 m width) and/or low-amplitude river planform changes ( < 1 m2 m−1 yr−1).
In addition to allowing detection of low-magnitude planform changes, our approach
is also transferable as we use well-established tools (IDW and MC): this opens new perspectives in the fluvial context
(e.g. multi-thread river channels) for robustly assessing surficial channel changes.
... In recent years, remote sensing technology has become a tool in geomorphology for change detection in rivers and their floodplain dynamics at the strategic scale because of their capability to cover the spatiotemporal variation of river channels [48]. High-resolution aerial photographs have often been used to manually delineate river channel boundaries [49]; however, aerial imagery is very costly, and historical aerial time-series images are barely accessible for most cases. Landsat satellite images are freely available data resources for detecting river channels and have been used in previous studies. ...
River channel occupation has made cities in the mountainous areas more vulnerable to floodwater out of river channels during rapid global urbanization. A better understanding of the influence of river channel occupation on urban flood disasters can serve as a reference in planning effective urban flood control strategies. In this study, taking a flood event that occurred on July 26th, 2017 in a city on the Loess Plateau as an example, field surveys, dynamics detection of the river channel using remote sensing technology, and scenario simulations with a two-dimensional flow and sediment model were utilized to quantitatively analyze the impacts of river channel occupation on urban inundation and sedimentation. The results show that river channel dynamics reduced by construction can be successfully detected using the combination of high-resolution images and Landsat time-series images. The variation of the water level-discharge relationship caused by the narrowing of the river channel and the increase of the flood-water level caused by water-blocking bridges/houses result in a significant reduction of the flood discharge capacity. The contribution of the narrowing of the river channel was 72.3% for the total area inundated by floodwater, whereas 57.2% of urban sedimentation was caused by the construction of bridges/houses within the river channel. Sustainable flood mitigation measures were also recommended according to the investigations and research findings in this study in order to reduce the social, environmental and economic damages caused by floods.
... European-American settlement across the Midwestern United States led to widespread wetland drainage and land use conversion to agriculture [1], with important consequences for the hydrology of agricultural regions [2][3][4][5]. Many of these regions have experienced stream flow increases [5,6], and these increased flows drive increases in erosion and sediment loading from stream banks and Water 2018, 10, 1053 2 of 21 bluffs [7][8][9][10]. In rivers where excess sediment leads to water quality impairments, there is a need to reduce erosion. ...
Climate change, land clearing, and artificial drainage have increased the Minnesota River Basin’s (MRB) stream flows, enhancing erosion of channel banks and bluffs. Accelerated erosion has increased sediment loads and sedimentation rates downstream. High flows could be reduced through increased water storage (e.g., wetlands or detention basins), but quantifying the effectiveness of such a strategy remains a challenge. We used the Soil and Water Assessment Tool (SWAT) to simulate changes in river discharge from various water retention site (WRS) implementation scenarios in the Le Sueur watershed, a tributary basin to the MRB. We also show how high flow attenuation can address turbidity issues by quantifying the impact on near-channel sediment loading in the watershed’s incised reaches. WRS placement in the watershed, hydraulic conductivity (K), and design depth were varied across 135 simulations. The dominant control on site performance is K, with greater flow reductions allowed by higher seepage rates and less frequent overflowing. Deeper design depths enhance flow reductions from sites with low K values. Differences between WRS placement scenarios are slight, suggesting that site placement is not a first-order control on overall performance in this watershed. Flow reductions exhibit power-law scaling with exceedance probability, enabling us to create generalized relationships between WRS extent and flow reductions that accurately reproduce our SWAT results and allow for more rapid evaluation of future scenarios. Overall, we show that increasing water storage within the Le Sueur watershed can be an effective management option for high flow and sediment load reduction.
... Differentiating between these two sources is often very difficult, and yet is essential for identifying and achieving water quality standards Trimble and Crosson, 2000;Wilcock, 2009). Sediment sources derived from near or within the channel itself (e.g., bank erosion from channel widening) are particularly sensitive to changes in streamflows (Lauer et al., 2017;Schottler et al., 2014;Lenhart et al., 2013). Bank erosion is a significant sediment source in many alluvial rivers, contributing as much as 80 to 96 % of the sediment that comprises a river's total sediment load (Kronvang et al., 2013;Palmer et al., 2014;Schaffrath et al., 2015;Simon et al., 1996;Stout et al., 2014;Willett et al., 2012). ...
Complete transformations of land cover from prairie, wetlands, and
hardwood forests to row crop agriculture and urban centers are thought to
have caused profound changes in hydrology in the Upper Midwestern US since
the 1800s. In this study, we investigate four large
(23 000–69 000 km2) Midwest river basins that span climate and land
use gradients to understand how climate and agricultural drainage have
influenced basin hydrology over the last 79 years. We use daily, monthly, and
annual flow metrics to document streamflow changes and discuss those changes
in the context of precipitation and land use changes. Since 1935, flow,
precipitation, artificial drainage extent, and corn and soybean acreage have
increased across the region. In extensively drained basins, we observe 2 to 4
fold increases in low flows and 1.5 to 3 fold increases in high and extreme
flows. Using a water budget, we determined that the storage term has
decreased in intensively drained and cultivated basins by 30–200 % since
1975, but increased by roughly 30 % in the less agricultural basin. Storage
has generally decreased during spring and summer months and increased during
fall and winter months in all watersheds. Thus, the loss of storage and
enhanced hydrologic connectivity and efficiency imparted by artificial
agricultural drainage appear to have amplified the streamflow response to
precipitation increases in the Midwest. Future increases in precipitation are
likely to further intensify drainage practices and increase streamflows.
Increased streamflow has implications for flood risk, channel adjustment, and
sediment and nutrient transport and presents unique challenges for
agriculture and water resource management in the Midwest. Better
documentation of existing and future drain tile and ditch installation is
needed to further understand the role of climate versus drainage across
multiple spatial and temporal scales.
Floods often cause changes in the hydro-geomorphology of riverbeds and banks. These changes need to be closely monitored to find a balance and exchange between lateral and vertical erosion and deposition, upstream local sediment supply, and a stream’s transport capacity. Low-frequency cross-sectional field surveys cannot map hard-to-reach locations. Innovative techniques, such as small unmanned aerial vehicles (UAVs), must be employed to monitor these processes. This research compared historical data with a UAV survey and the Pix4DMapper structure-from-motion (SfM) program to assess the longitudinal, lateral, and vertical changes of Sidere Creek in the eastern Black Sea, Türkiye. Digitization was undertaken using 2011–2015–2017 Google Earth photographs, 1960s topographic maps, and 2023 orthomosaics. ArcGIS 10.6 was used to delineate the centerlines (thalweg), left/right banks, alluvial bars, active channel widths, and channel confinement layers. Channel Migration Toolbox and CloudCompare were utilized for analyzing lateral and vertical morphological changes, respectively. The active channel migrated 25.57 m during 1960–2011, 15.84 m during 2011–2015, 6.96 m during 2015–2017, and 5.79 m during 2017–2023. Left-bank channel confinement rose from 2.4% to 42% and right-bank channel confinement from 5.9% to 34.8% over 63 years. Neither stream meandering nor sinuosity index changed statistically. Active channel boundary widths varied from 149.79 m to 9.46 m, averaging 37.3 m. It can be concluded that UAV surveys can precisely measure and monitor the stream channel longitudinal, lateral, and vertical morphological changes at a lower cost and in less time than previous methods.
The Diamond Fork River in central Utah, USA experienced extreme flow augmentation via transbasin flows. Beginning in 1916, irrigation water was delivered through a tributary, Sixth Water Creek, with daily summer flows regularly exceeding a 500‐year flood at the point of introduction. Flows were dramatically reduced by management action in 2004 but with mandated minimum flows. We examined the geomorphic response of Sixth Water and Diamond Fork using aerial imagery, lidar, topographic cross sections, and sediment transport measurements. River channel response varied with valley confinement and with position in the watershed, which determined the magnitude of augmented flow relative to natural floods and the amount of sediment supply. Confined, steep sections of Sixth Water incised many meters into bedrock, whereas partially confined and unconfined sections developed a braided form even under conditions of general incision. With the removal of flow augmentation, smaller natural floods on Sixth Water are unable to transport bed material and the channel remains static. On the alluvial lower Diamond Fork, a lower slope, upstream sediment supply, and larger natural floods produced a dynamic shifting channel that widened in response to natural floods. After flow augmentation, a coarsened bed is partially mobile and channel narrowing appears to be limited by artificial baseflow, which prevent vegetation establishment in the channel.
In the second half of the twentieth century, after the end of the Second World War, a considerable anthropogenic pressure was observed in most of the Mediterranean territories. This process has caused the expansion of existing settlements and the construction of numerous new towns, often located very close to rivers. A frequent consequence of this process is the transformation of several rivers through planform changes, narrowing, channelization and culverting to recover spaces where inhabited centers expanded, and the construction of structures interacting with rivers. This issue is very important in territories such as the Metropolitan City of Reggio Calabria, in southern Italy, which is an interesting case study due to the considerable anthropogenic pressures observed in the last 70 years. The main goal of this paper is to evaluate the effects of anthropogenic pressure in the last 70 years on some rivers of the Metropolitan City of Reggio Calabria in terms of the following issues: planform changes, channelization, culverting, and the presence of structures and infrastructures interacting with rivers. The specific goals of this paper are the quantification of the effects of anthropogenic pressure on the rivers of the study area analyzing sixteen parameters, the identification of possible conditions of hydraulic hazard through the analysis of past events, and the proposal of structural and non-structural mitigation interventions. In many rivers of the study area, the significant effects of anthropogenic pressure are visible through rivers that pass above highways, barred rivers, rivers replaced by roads and numerous crossing roads with a missing levee.
The forces associated with flooded rivers can be incredibly destructive, causing significant damage to widespread areas. Urbanization, tiling of agricultural land, and an increasing trend in heavy precipitation are some of the suspected contributors to more frequent large-scale flooding of river basins across the United States. With this increasing commonality of flooding, a growing number of transmission structures are being put at risk. Fast flowing water carrying pieces of debris or ice can potentially damage or destroy structures within its path. Many existing transmission poles within new or changing flood zones are not robust enough to stand up to these forces. The damage can lead to costly repairs and lengthy outages because of the difficulties that accompany traversing and constructing in the flooded terrain. This paper is a case study of a structure failure along the Minnesota River. It includes an investigation into the factors that contributed to the structure failure and takes a detailed look at the difficulties associated with post-failure reconstruction. By comparing the flow data collected on the Minnesota River Basin over the past century to land use and precipitation trends, this paper draws some conclusions as to why such noticeable changes have occurred in the basin and explores the impacts those changes have had on transmission infrastructure.
Complete transformations of land cover from prairie, wetlands, and hardwood forests to row crop agriculture and urban centers are thought to have caused profound changes in hydrology in the Upper Midwestern US since the 1800s. In this study, we investigate four large (23,000–69,000 km²) Midwest river basins that span climate and land use gradients to understand how climate and agricultural drainage have influenced basin hydrology over the last 79 years. We use daily, monthly, and annual flow metrics to document streamflow changes and discuss those changes in the context of precipitation and land use changes. Since 1935, flow, precipitation, artificial drainage extent, and corn and soybean acreage have increased across the region. In extensively drained basins, we observe 2 to 4 fold increases in low flows and 1.5 to 3 fold increases in high and extreme flows. Using a water budget, we determined that the storage term has decreased in intensively drained and cultivated basins by 30 %–200 % since 1975, but increased by roughly 30 % in the less agricultural basin. Storage has generally decreased during spring and summer months and increased during fall and winter months in all watersheds. Thus, the loss of storage and enhanced hydrologic connectivity and efficiency imparted by artificial agricultural drainage appear to have amplified the streamflow response to precipitation increases in the Midwest. Future increases in precipitation are likely to further intensify drainage practices and increase streamflows. Increased streamflow has implications for flood risk, channel adjustment, and sediment and nutrient transport and presents unique challenges for agriculture and water resource management in the Midwest. Better documentation of existing and future drain tile and ditch installation is needed to further understand the role of climate versus drainage across multiple spatial and temporal scales.
Complete transformations of land cover from prairie, wetlands, and hardwood forests to homogenous row crop agriculture scattered with urban centers are thought to have caused profound changes in hydrology in the Upper Midwestern US since the 1800s. Continued intensification of land use and drainage practices combined with increased precipitation have caused many Midwest watersheds to exhibit higher streamflows today than in the historical past. While changes in crop type and farming practices have been well documented over the past few decades, changes in artificial surface (ditch) and subsurface (tile) drainage systems have not. This makes it difficult to quantitatively disentangle the effects of climate change and artificial drainage intensification on the observed hydrologic change, often spurring controversial interpretations with significant implications for management actions. In this study, we investigate four large (23,000–69,000 km2) Midwest river basins that span climate and land use gradients to understand how climate and agricultural drainage have influenced basin hydrology over the last 79 years. We use daily, monthly, and annual flow metrics to document streamflow changes and discuss those changes in the context of climate and land use change. While we detect similar timing of precipitation and streamflow changes in each basin, overall the magnitude and significance of precipitation changes are much less than we detect for streamflows. Of the basins containing greater than 20 % area drained by tile and ditches, we observe 2 to 4 fold increases in low flows and 1.5 to 3 fold increases in high and extreme flows. Monthly precipitation has increased slightly for some months in each basin, mostly in fall and winter months (August – March), but total monthly streamflow has increased in all months for the Minnesota River Basin (MRB), every month but April for the Red River Basin (RRB), September-December and March in the Illinois River Basin (IRB), and no months in the Chippewa River basin (CRB). Using a water budget, we determined that the soil moisture/groundwater storage term for the intensively drained and cultivated MRB, IRB, and RRB, has decreased by about 200 %, 100 %, and 30 %, respectively while increased by roughly 30 % in the largely forested CRB since 1975. We argue that agricultural land use change, through wetland removal and artificial drainage installation, has decreased watershed storage and amplified the streamflow response to precipitation increases in the Midwest. Highly managed basins with large reservoirs and urban centers, such as the Illinois River basin (IRB), may be able to buffer some of these impacts better than largely unregulated systems such as the Minnesota River (MRB) and Red River of the North (RRB) basins. The reported streamflow increases in the MRB, IRB, and RRB are large (18 %–318 %), and should have important implications for channel adjustment and sediment and nutrient transport. Acknowledging both economic benefits and apparent detrimental impacts of artificial drainage on river flows, sediments, and nutrients, we question whether any other human activity has comparably altered critical zone activities, while remaining largely unregulated and undocumented. We argue that better documentation of existing and future drain tile and ditch installation is greatly needed.
Complete transformations of land cover from prairie, wetlands, and
hardwood forests to row crop agriculture and urban centers are thought to
have caused profound changes in hydrology in the Upper Midwestern US since
the 1800s. In this study, we investigate four large
(23 000–69 000 km2) Midwest river basins that span climate and land
use gradients to understand how climate and agricultural drainage have
influenced basin hydrology over the last 79 years. We use daily, monthly, and
annual flow metrics to document streamflow changes and discuss those changes
in the context of precipitation and land use changes. Since 1935, flow,
precipitation, artificial drainage extent, and corn and soybean acreage have
increased across the region. In extensively drained basins, we observe 2 to 4
fold increases in low flows and 1.5 to 3 fold increases in high and extreme
flows. Using a water budget, we determined that the storage term has
decreased in intensively drained and cultivated basins by 30–200 % since
1975, but increased by roughly 30 % in the less agricultural basin. Storage
has generally decreased during spring and summer months and increased during
fall and winter months in all watersheds. Thus, the loss of storage and
enhanced hydrologic connectivity and efficiency imparted by artificial
agricultural drainage appear to have amplified the streamflow response to
precipitation increases in the Midwest. Future increases in precipitation are
likely to further intensify drainage practices and increase streamflows.
Increased streamflow has implications for flood risk, channel adjustment, and
sediment and nutrient transport and presents unique challenges for
agriculture and water resource management in the Midwest. Better
documentation of existing and future drain tile and ditch installation is
needed to further understand the role of climate versus drainage across
multiple spatial and temporal scales.
Complete transformations of land cover from prairie, wetlands, and hardwood forests to row crop agriculture and urban centers are thought to have caused profound changes in hydrology in the Upper Midwestern US since the 1800s. In this study, we investigate four large (23,000–69,000 km²) Midwest river basins that span climate and land use gradients to understand how climate and agricultural drainage have influenced basin hydrology over the last 79 years. We use daily, monthly, and annual flow metrics to document streamflow changes and discuss those changes in the context of precipitation and land use changes. Since 1935, flow, precipitation, artificial drainage extent, and corn and soybean acreage have increased across the region. In extensively drained basins, we observe 2 to 4 fold increases in low flows and 1.5 to 3 fold increases in high and extreme flows. Using a water budget, we determined that the storage term has decreased in intensively drained and cultivated basins by 30 %–200 % since 1975, but increased by roughly 30 % in the less agricultural basin. Storage has generally decreased during spring and summer months and increased during fall and winter months in all watersheds. Thus, the loss of storage and enhanced hydrologic connectivity and efficiency imparted by artificial agricultural drainage appear to have amplified the streamflow response to precipitation increases in the Midwest. Future increases in precipitation are likely to further intensify drainage practices and increase streamflows. Increased streamflow has implications for flood risk, channel adjustment, and sediment and nutrient transport and presents unique challenges for agriculture and water resource management in the Midwest. Better documentation of existing and future drain tile and ditch installation is needed to further understand the role of climate versus drainage across multiple spatial and temporal scales.
Hydrology in many agricultural landscapes around the world is changing in unprecedented ways due to the development of extensive surface and subsurface drainage systems that optimize productivity. This plumbing of the landscape alters water pathways, timings, and storage, creating new regimes of hydrologic response and driving a chain of environmental changes in sediment dynamics, nutrient cycling, and river ecology. In this work we non-parametrically quantify the nature of hydrologic change in the Minnesota River Basin, an intensively managed agricultural landscape, and study how this change might modulate ecological transitions. During the growing season when climate effects are shown to be minimal, daily streamflow hydrographs exhibit sharper rising limbs and stronger dependence on the previous-day precipitation. We also find a changed storage-discharge relationship and show that the artificial landscape connectivity has most drastically affected the rainfall-runoff relationship at intermediate quantiles. Considering the whole year, we show that the combined climate and land-use change effects reduce the inherent nonlinearity in the dynamics of daily streamflow, perhaps reflecting a more linearized engineered hydrologic system. Using a simplified dynamic interaction model that couples hydrology to river ecology, we demonstrate how the observed hydrologic change and/or the discharge-driven sediment generation dynamics may have modulated a regime shift in river ecology, namely extirpation of native mussel populations. We posit that such non-parametric analyses and reduced complexity modeling can provide more insight than highly parameterized models and can guide development of vulnerability assessments and integrated watershed management frameworks. This article is protected by copyright. All rights reserved.
The bankfull geometry of alluvial rivers is thought to be controlled by water and sediment supply, and characteristic sediment size. Here we demonstrate a novel finding: when bankfull shear velocity and bankfull depth are correlated against bed material grain size and bed slope, they are to first order independent of grain size and dependent on water viscosity. We demonstrate this using a similarity collapse for bankfull Shields number as a function of slope and grain size, obtained with data for 230 river reaches ranging from silt-bed to cobble-bed. Our analysis shows that bankfull Shields number increases with slope to about the half power. We show that the new relation for bankfull Shields number provides more realistic predictions for the downstream variation of bankfull characteristics of rivers than a previously used assumption of constant bankfull Shields number.
Delineation of river-flow boundaries constitutes an important step in various river related studies, including river hydraulic modeling, flow-width estimations, and river and floodplain habitat mapping and assessment. Increasing the level of automation of delineation of flow boundaries from synoptic remote-sensing images provides great potential, by reducing the labor cost, especially for studies focusing on long river reaches and those examining flow changes over time. This article investigates the boundary delineation of river channel flow from aerial photographs using Support Vector Machine (SVM) and image-derived ancillary data layers. It also includes a quantitative evaluation of delineation accuracy. The findings show that SVM performs satisfactory delineations of the boundaries, and the ancillary data layers generated using edge detectors and spatial domain texture statistics particularly increase delineation accuracy. Moreover, a multiscale evaluation scheme allows for examining the performance of SVM for the whole river reach, as well as that for the subriver sections with varied geomorphic and environmental conditions.
Traditionally, flows that did not cause flooding were thought to be inconsequential for agricultural watershed management. However, flow volume plays an important role in flow dura-tion and Total Maximum Daily Loads (TMDLs), particularly for nitrate-nitrogen. Prolonged below-bankfull flows may also increase bank saturation and the frequency of mass wast-ing, leading to increased sediment and phosphorous load-ing and reduced index of biotic integrity scores. Low, mean, and high flows below the bankfull elevation have increased in many upper midwestern watersheds in the past 30 years, although large floods have not increased significantly at most of our study sites. Using the indicators of hydrologic altera-tion suite of statistical metrics, we found that streamflow has increased in agricultural watersheds (> 67% agricultural land use) in annual mean flow, most monthly median values, and many flow duration metrics during the 1980–2009 time pe-riod compared to 1940–1979. As a percentage, flow has increased most in December and least in August through Oc-tober. At the same time, the streamflow-to-precipitation (Q:P) ratio has increased in the past three decades compared to the previous several decades. The overall change in Q:P, the timing of increased flow, and the reduced streamflow vari-ability, as measured by the coefficient of variation, suggest a mechanism of subsurface tile flow and/or increased ground-water flow. Management actions are needed in agricultural watersheds of the Upper Midwest to reduce water volume as well as peak flow to meet TMDL requirements.
Excessive loading of fine sediment is a prominent cause of river impairment, not only due to direct effects on biota and habitat but because sediment is often laden with excess nutrients, metals, and toxic substances. Determining the sources and transport pathways of sediment has proven challenging. The Root River watershed in southeastern Minnesota was listed under section 303d of the U.S. Clean Water Act as having forty-three impaired reaches, raising these questions: Where is the fine sediment coming from? What proportions of the sediment are from uplands versus near-channel erosion? How much of the excess sediment loading is caused by modern land use and water management versus the legacy of past land use? Managing fine sediment at the watershed scale requires that we identify potential sources and sinks throughout the watershed, measure source contributions, and understand transport pathways of fine sediment. Here we utilize sediment fingerprinting techniques involving long- and short-lived radionuclide tracers, specifically beryllium-10 (10Be), excess lead-210 (210Pbex), and cesium-137 (137Cs), in combination with other supporting data sets to address the preceding questions. We document a shift in hydrologic regime and that sediment fluxes are sensitive to both magnitude and sequence of flood events. Geomorphic analysis indicates that many river reaches have accessible near-channel sources that contribute the dominant proportion of the washload flux in subwatersheds. Lastly, geochemical tracer analyses of floodplains and hillslope soils indicate that historic erosion has been variable across the landscape and the majority of suspended sediment in the river today is sourced from floodplains and terraces.
Purpose
The science of sediment fingerprinting has been evolving rapidly over the past decade and is well poised to improve our understanding, not only of sediment sources, but also the routing of sediment through watersheds. Here, we discuss channel–floodplain processes that may convolute or modify the sediment fingerprinting signature of alluvial bank/floodplain sources and explore the use of nonconservative tracers for differentiating sediment derived from surface soil erosion from that of near-channel fluvial erosion.
Materials and methods
We use a mathematical model to demonstrate the theoretical effects of channel–floodplain exchange on conservative and nonconservative tracers. Then, we present flow, sediment gauging data, and geochemical measurements of long- (meteoric beryllium-10, 10Be) and short-lived (excess lead-210 and cesium-137, 210Pbex and 137Cs, respectively) radionuclide tracers from two study locations: one above, and the other below, a rapidly incising knick zone within the Maple River watershed, southern Minnesota.
Results and discussion
We demonstrate that measurements of 10Be, 210Pbex, and 137Cs associated with suspended sediment can be used to distinguish between the three primary sediment sources (agricultural uplands, bluffs, and banks) and estimate channel–floodplain exchange. We observe how the sediment sources systematically vary by location and change over the course of a single storm hydrograph. While sediment dynamics for any given event are not necessarily indicative of longer-term trends, the results are consistent with our geomorphic understanding of the system and longer-term observations of sediment dynamics. We advocate for future sediment fingerprinting studies to develop a geomorphic rationale to explain the distribution of the fingerprinting properties for any given study area, with the intent of developing a more generalizable, process-based fingerprinting approach.
Conclusions
We show that measurements of conservative and nonconservative tracers (e.g., long- and short-lived radionuclides) can provide spatially integrated, yet temporally discrete, insights to constrain sediment sources and channel–floodplain exchange at the river network-scale. Fingerprinting that utilizes nonconservative tracers requires that the nonconservative behavior is predictable and verifiable.
Riverine ecosystems are recurrently rejuvenated during destructive flood events and vegetation succession starts again. Poplars (i.e. species from Populus genera) respond to hydrogeomorphological constraints, but, in turn, also influence these processes. Thus, poplar development on bare mineral substrates is not exclusively a one-way vegetative process. Reciprocal interactions and adjustments between poplar species and sediment dynamics during their life cycle lead to the emergence of biogeomorphological entities within the fluvial corridor, such as vegetated islands, benches and floodplains. Based on a review of geomorphological, biological and ecological literature, we have identified and described the co-constructing processes between riparian poplars and their fluvial environment. We have explored the possibility that the modification of the hydrogeomorphological environment exerted, in particular, by the European black poplar (Populus nigra L.), increases its fitness and thus results in positive niche construction. We focus on the fundamental phases of dispersal, recruitment and establishment until sexual maturity of P. nigra by describing the hierarchy of interactions and the pattern of feedbacks between biotic and abiotic components. We explicitly relate the biological life cycle of P. nigra to the fluvial biogeomorphic succession model by referring to the ‘biogeomorphological life cycle’ of P. nigra. Finally, we propose new research perspectives based on this theoretical framework.
Rivers self-organize their pattern/planform through feedbacks between bars, channels, floodplain and vegetation, which emerge as a result of the basic spatial sorting process of wash load sediment and bed sediment. The balance between floodplain formation and destruction determines the width and pattern of channels. Floodplain structure affects the style and rate of channel avulsion once aggradation takes place. Downstream fining of bed sediment and the sediment balance of fines in the pores of the bed sediment provide the ?template? or sediment boundary conditions, from which sorting at smaller scales leads to the formation of distinct channel patterns. Bar patterns provide the template of bank erosion and formation as well as the dynamics of the channel network through bifurcation destabilization. However, so far we have been unable to obtain dynamic meandering in laboratory experiments and in physics-based models that can also produce braiding, which reflects our lack of understanding of what causes the different river patterns.
Alluvial rivers are dynamic elements of the landscape in the Pacific Northwest. They expand, contract, and migrate across the bottom of valleys in response to changing flow and vegetation. Channel dynamics fundamentally structure river and floodplain ecosystems. Human activities that affect channel migration—including river regulation, channel revetments, and land use—have the potential for impacting river ecosystems. Active channel width and lateral movement of the active channel centerline was analyzed in 17 km valley segment of the middle Green River in western Washington from aerial photographs for 26 years of unregulated flows (1936–1961) and 41 years of flood regulation by Howard Hansen Dam. Area-based measures proved more robust for characterizing channel dynamics than cross-sectional measurements, though cross-sectional measurements are useful for resolving local processes. Prior to regulation from 1936 to 1961, the active channel width varied from 82 m to 120 m (median 94 m) and the channel migrated laterally a total of 68 m. After flood regulation from 1961 to 2002, the active channel width was generally smaller, varying from 84 m to 52 m (median 69 m) and the channel migrated 48 m. Streamflow greater than about 250 m3/s are most effective for forcing migration and have been reduced since dam construction. The river has re-occupied areas with increasing frequency since dam construction. High flows are essential to create new channel and floodplain habitats in the middle Green River, but land cover/use and revetments in the river corridor are also influential factors for maintaining channel dynamics.
Over the last century, large-scale water development of the upper Rio Grande in the U.S. and Mexico, and of the Rio Conchos in Mexico, has resulted in progressive channel narrowing of the lower Rio Grande in the Big Bend region. We used methods operating at multiple spatial and temporal scales to analyze the rate, magnitude, and processes responsible for channel narrowing. These methods included: hydrologic analysis of historic stream gage data, analysis of notes of measured discharges, historic oblique and aerial photograph analysis, and stratigraphic and dendrogeomorphic analysis of inset floodplain deposits. Our analyses indicate that frequent large floods between 1900 and the mid-1940s acted as a negative feedback mechanism and maintained a wide, sandy, multi-threaded river. Declines in mean and peak flow in the mid-1940s resulted in progressive channel narrowing. Channel narrowing has been temporarily interrupted by occasional large floods that widened the channel, however, channel narrowing has always resumed. After large floods in 1990 and 1991, the active channel width of the lower Rio Grande has narrowed by 36–52%. Narrowing has occurred by the vertical accretion of fine-grained deposits on top of sand and gravel bars, inset within natural levees. Channel narrowing by vertical accretion occurred simultaneously with a rapid invasion of non-native riparian vegetation (Tamarix spp., Arundo donax) which created a positive feedback and exacerbated the processes of channel narrowing and vertical accretion. In two floodplain trenches, we measured 2.75 and 3.5 m of vertical accretion between 1993 and 2008. In some localities, nearly 90% of bare, active channel bars were converted to vegetated floodplain during the same period. Upward shifts of stage-discharge relations occurred resulting in over-bank flooding at lower discharges, and continued vertical accretion despite a progressive reduction in stream flow. Thus, although the magnitude of the average annual flood was reduced between 40 and 50%, over-bank flooding continued. These changes reflect a shift in the geomorphic nature of the Rio Grande from a wide, laterally unstable, multi-thread river, to a laterally stable, single-thread channel with cohesive, vertical banks, and few active in-channel bars.
1] We investigated the processes of sediment exchange between the Strickland River and its lowland floodplain, documenting (1) the rates, textures, and distributions of sediment accumulation, (2) temporal variations in these rates, (3) the remobilization of sediment by erosive processes, and (4) whether net storage is significant over century timescales. We used 210 Pb geochronology of floodplain cores from 11 transects to measure deposition rates over the past $65 years, finding a decline from 5.5 cm/a (0–10 m distance from the channel) to 0.9 cm/a ($400 m) to $0.1 cm/a (>1 km). Rates are elevated along curved sections of channel and are stable over time, and episodic accumulation is prevalent and may be correlated to floods. Integrated temporally and spatially, the average rate is 1.6 cm/a. Integrating along both sides of $318 river km we studied for the lowland Strickland, we calculate 12–19 Mt of annual sediment accumulation ($0.05 Mt/km or 0.07% of the annual load per km of river length), representing 17–27% of the total annual sediment flux. We used georeferenced Landsat images (1972–2001) to quantify channel migration and the resulting return of sediment to the channel. Mean lateral migration was 5.1 ± 0.8 m/a. Given the floodplain width of $10 km, this implies a waiting time of $1 ka between floodplain formation and subsequent reentrainment of the bank as the channel migrates laterally. This raises the possibility that the net return of material from the floodplain due to channel migration could balance the overbank deposition we observed. The exchange flux between cut bank erosion and point bar deposition is 20–40 Mt, highlighting the significance of sediment recycling ($50% of the total load). Such sediment trapping and recycling affects the transport, storage, and evolution of biogeochemically reactive particles, the evolution of the floodplain, and the morphodynamics of basin infilling.
Changes in climate and agricultural practices are putting pressure on agroenvironmental systems all over the world. Predicting the effects of future management or conservation actions has proven exceptionally challenging in these complex landscapes. We present a perspective, gained from a decade of research and stakeholder involvement in the Minnesota River Basin, where research findings have influenced solutions and policy in directions not obvious at the outset. Our approach has focused on identifying places, times, and processes of accelerated change and developing reduced complexity predictive frameworks that can inform mitigation actions.
Predicting the frequency and aerial extent of flooding in river valleys is essential for infrastructure design, environmental management, and risk assessment. Conventional flood prediction relies on assumptions of stationary flood distributions and static channel geometries. However, non-stationary flow regimes are increasingly observed and changes in flow and/or sediment supply are known to alter the geometry and flood conveyance of alluvial channels. Systematic changes in flows and/or channel geometry may amplify or attenuate the frequency and/or extent of flood inundation in unexpected ways. We present a stochastic, reduced complexity model to investigate such dynamics. The model routes a series of annual peak-discharges through a simplified reach-averaged channel-floodplain cross-section. Channel width, depth, and slope are permitted to adjust annually by a user-specified fraction towards equilibrium geometries predicted based on each year's peak-discharge and sediment supply. Modeled channel adjustments are compared with empirical observations for two rivers in Minnesota, USA that have experienced multiple large floods over the past six-years. The model is then run using six-hypothetical scenarios simulating non-stationary flow regimes with temporal adjustments in the mean and/or variance of the governing peak-flow distributions. Each scenario is run repeatedly while varying parameters that control the amount of fractional adjustment that channel geometries can make annually. Results indicate that the intra-annual mean horizontal width of floodplain inundation primarily depends on the governing peak-flow distribution's coefficient of variation, but the intra-annual frequency of floodplain inundation (i.e. the fraction of modeled years with inundation) primarily depends on the amount of channel adjustment permitted annually.
Repeat surveys of high-resolution topographic data enable analysis of geomorphic change through digital elevation model (DEM) differencing. Such analyses are becoming increasingly common. However, techniques for developing robust estimates of spatially variable uncertainty in DEM differencing estimates have been slow to develop and are underutilized. Further, issues often arise when comparing recent to older data sets, because of differences in data quality. Airborne lidar data were collected in 2005 and 2012 in Blue Earth County, Minnesota (1980km2) and the occurrence of an extreme flood in 2010 produced geomorphic change clearly observed in the field, providing an opportunity to estimate landscape-scale geomorphic change. Initial assessments of the lidar-derived digital elevation models (DEMs) indicated both a vertical bias attributed to different geoid models and localized offset strips in the DEM of difference from poor coregistration of the flightlines. We applied corrections for both issues and describe the methods we used to discern those issues and correct them. We then compare different threshold models to quantify uncertainty. Poor quantification of uncertainty can erroneously over- or underestimate real change. We show that application of a uniform threshold, often called a minimum level of detection, overestimates change in areas where change would not be expected, such as stable hillslopes, and underestimates change in areas where it is expected and has been observed, such as channel banks. We describe a spatially variable DEM error model that combines the influence of slope, point density, and vegetation in a fuzzy inference system. Vegetation is represented with a metric referred to as the cloud point density ratio that assesses the complete point cloud to describe the density of above ground features that may hinder bare-earth returns. We compare the significance of spatially variable versus spatially uniform DEM errors on change detection by thresholding the DEM of Difference at a 95% confidence interval (2σ). Results indicate significant geomorphic change in relatively predictable locations, such as erosion on the outside and deposition on the inside, of bends. Final totals indicated net erosion of [2,625,100]±2,389,000m3 in the county between 2005 and 2012. Of this, 39% was generated from bluffs, 1% from ravines, and the remainder came from banks and floodplain areas.
We propose a conceptual model of vegetation–hydrogeomorphology interactions and feedbacks within river corridors (i.e. river channels and their floodplains) that builds on previous similar hydrogeomorphologically centred models by incorporating hydrogeomorphological constraints on river corridor vegetation from region to reach scales;
defining five dynamic river corridor zones within which different hydrogeomorphological processes are dominant so that plants and physical processes interact in different ways, and considering the potential distribution of these zones longitudinally from river headwaters to mouth, laterally across the river corridor, and in relation to different river planform styles;
considering the way in which vegetation‐related landforms within each zone may reflect processes of self‐organization and the role of particular plant species as physical ecosystem engineers within the context of the dominant hydrogeomorphological processes;
focussing, in particular, upon a ‘critical zone’ at the leading edge of plant–hydrogeomorphological process interactions that is located somewhere within the area of the river corridor perennially inundated by flowing water (zone 1) and the area that is frequently inundated and subject to both sediment erosion and deposition processes (zone 2). Within the critical zone some plant species strongly influence the position and character of the margin between the river channel and floodplain, affecting channel width, channel margin form and dynamics, and the transition from one river planform type to another; and
considering the vegetated pioneer landforms that develop within the critical zone and how their morphological impact needs to be scaled to the river size.
The model is illustrated using three example reaches from rivers within different biogeographical zones of Europe, and its potential application in the context of river management and restoration/rehabilitation is discussed. Copyright © 2015 John Wiley & Sons, Ltd.
Suspended sediment transported in river systems has the ability to degrade water quality and impart negative impacts downstream as material is deposited. This study examines the significance of valley wall slumping as a source of both suspended sediment and phosphorus in the Blue Earth River. Several streambanks on the Blue Earth River in south central Minnesota were repeatedly surveyed from 1997 to 2000 to determine annual rates of streambank slumping. Volumetric changes between digital elevation models from successive topographic surveys were converted into changes in mass, and, subsequently, annual erosion rates were calculated based on surface area. An erosion rate constant was derived based on surface area of surveyed sites and applied to an inventory of eroding escarpments along the entire river to estimate the contribution of streambank slumping to the total suspended sediment (TSS) and total phosphorus (TP) loads in the Blue Earth River. Erosion rates for surveyed sites ranged from 544 to 3,995 t ha-1 yr-1 (242 to 1,782 tons ac-1 yr-1), with an average rate of 2,154 t ha-1 yr-1 (961 tons ac-1 yr-1). The derived erosion rate constant for slumping sites was 0.23 t m-2 yr-1 (0.024 tons ft-2 yr-1). Between 75,804 and 106,213 t (83,536 to 117,047 U.S. tons) of sediment is delivered to the river annually from all eroding escarpments inventoried, with between 28,047 and 39,299 t (30,908 and 43,307 U.S. tons) transported by flow as TSS. Streambank slumping accounts for 31% to 44% of the TSS load at the mouth of the Blue Earth River. The percentage of the TP load originating from streambank slumping is estimated to be from 7% to 10%, with annual contributions of 12 to 17 t P (14 to 19 U.S. tons).
Physical experiments of self-formed river channels and floodplains with live vegetation are pathways for understanding that complement numerical modelling. Recent experiments succeeded in creating braided rivers and dynamic meandering systems with clastic and vegetated floodplains. However, application of the insights gained from such experiments to natural systems depends on understanding potential scale effects, temporal, and spatial. Here we combine review, analysis, and experiments to identify fundamental problems of biomorphological river pattern formation that are open for further research in experiments. We first show by review and analysis that physics-based, linear bar theory predicts negligible spatial scale effects in bar and bend wavelength relative to channel width. Time scaling, on the other hand, remains problematic because it integrates multiple processes of sediment transport, floodplain formation, and bank failure affected by bank stratigraphy and riparian vegetation. As a tentative solution, we secondly present experimental methods to assess bank strength effects that can be used in the design of river pattern experiments. The third issue is that riparian vegetation has often been represented in experiments by uniformly seeded sprouts of a single plant species, whilst spectacularly different patterns are obtained with contrasting seeding protocols, showing the need for other experimental procedures, and alternative riparian species. The main challenge for future experiments is better understanding of temporal scaling of biomorphodynamics.
The Minnesota River carries the largest load of sediment to the Mississippi River in Minnesota, most of which comes from channel sources. This study investigates bank retreat in the lower Minnesota River since 1938. Specifically we asked, How have changes to river form influenced sediment transport and deposition in the lower Minnesota River and how did hydrological and ecological processes affect channel change? It was hypothesized that channel straightening, reduction in floodplain access, and streamflow increases contribute to increased channel-derived sediment load and decreased point bar deposition. Secondly, it was hypothesized that hydrologic changes have reduced woody riparian vegetation on sandbars, further promoting channel widening. To quantify channel sediment and phosphorus loading rates in the lower Minnesota River, we analyzed historic aerial photos for evidence of channel change, we performed long-term monitoring of erosion and deposition rates within the river corridor, and we calculated channel sediment transport rates. Results from this study showed that the Minnesota River has widened by 52% between Mankato and St. Paul since 1938, on average contributing 280,000 Mg of sediment per year and 153 Mg total phosphorus. The river also shortened by 7% since 1938, increasing bankfull shear stress and stream power. Sediment deposition rates in the floodplain have increased since European settlement by an order of magnitude. Ecohydrological studies showed that establishment of woody riparian plants has been inhibited on sandbars by prolonged summer flow duration and scour at high flow, reducing potential point bar growth. Findings from this study will be useful in prioritizing sediment and vegetation management actions. © 2013 American Society of Agricultural and Biological Engineers.
The Minnesota River Valley was carved during catastrophic drainage of glacial Lake Agassiz at the end of the late Pleistocene. The ensuing base-level drop on tributaries created knickpoints that excavated deep valleys as they migrated upstream. A sediment budget compiled in one of these tributaries, the Le Sueur River, shows that these deep valleys are now the primary source of sediment to the Minnesota River. To compare modern sediment loads with pre-European settlement erosion rates, we analyzed incision history using fluvial terrace ages to constrain a valley incision model. Results indicate that even though the dominant sediment sources are derived from natural sources (bluffs, ravines, and streambanks), erosion rates have increased substantially, due in part to pervasive changes in watershed hydrology.
Manuscript received 28 Feb. 2011; accepted 29 Apr. 2011
DOI: 10.1130/G121A.1
*E-mail:
Rivers in watersheds dominated by agriculture throughout the US are impaired by excess sediment, a significant portion of which comes from non-field, near-channel sources. Both land-use and climate have been implicated in altering river flows and thereby increasing stream-channel erosion and sediment loading. In the wetland-rich landscapes of the upper Mississippi basin, 20th century crop conversions have led to an intensification of artificial drainage, which is now a critical component of modern agriculture. At the same time, much of the region has experienced increased annual rainfall. Uncertainty in separating these drivers of streamflow fuels debate between agricultural and environmental interests on responsibility and solutions for excess riverine sediment. To disentangle the effects of climate and land-use, we compared changes in precipitation, crop conversions, and extent of drained depressional area in 21 Minnesota watersheds over the past 70 years. Watersheds with large land-use changes had increases in seasonal and annual water yields of >50% since 1940. On average, changes in precipitation and crop evapotranspiration explained less than one-half of the increase, with the remainder highly correlated with artificial drainage and loss of depressional areas. Rivers with increased flow have experienced channel widening of 10–40% highlighting a source of sediment seldom addressed by agricultural best management practices. Copyright © 2013 John Wiley & Sons, Ltd.
Effectively managing and reducing high suspended sediment loads in rivers requires an understanding of the magnitude of major sediment sources as well as erosion and transport processes that deliver excess fine sediments to the channel network. The focus of this research is to determine the magnitude of erosion from tall bluffs, a primary sediment source in the 2880 km2 Le Sueur watershed, Minnesota, USA. We coupled analyses of seven decades of aerial photographs with four years of repeat terrestrial laser scanning (TLS) to determine erosion rates on bluffs. Together, these datasets provide decadal‐scale retreat rates throughout the entire watershed and high‐resolution geomorphic change detection on a subset of bluffs to both constrain erosion rates and document how environmental conditions affect bluff retreat. Erosion rates from aerial photographs and TLS were extrapolated from 243 and 15 measured bluffs, respectively, to all 480 bluffs in the Le Sueur watershed using multiple techniques to obtain estimates of sediment loading from these features at the watershed‐scale. Despite different spatial and temporal measurement scales, the aerial photograph and TLS estimates yielded similar results for bluff retreat rate and total mass of sediment derived from bluffs, with bluffs in the Le Sueur watershed yielding 135 000 ± 39 000 Mg/yr of fine sediment. Comparing this value to the average annual total suspended solids (TSS) load determined from gauging from 2000 to 2010, we determined that bluffs comprise 57 ± 16% of the total TSS load, making bluffs the single most abundant fine sediment source in the basin. Copyright © 2012 John Wiley & Sons, Ltd.
Accurate mapping of water surface boundaries in rivers is an important step for monitoring water stages, estimating discharge, flood extent, and geomorphic response to changing hydrologic conditions, and assessing riverine habitat. Nonetheless, it is a challenging task in spatially and spectrally heterogeneous river environments, commonly characterized by high spatiotemporal variations in morphology, bed material, and bank cover. In this study, we investigate the influence of channel morphology and bank characteristics on the delineation of water surface boundaries in rivers using high spatial resolution passive remote sensing and a template‐matching (object‐based) algorithm, and compare its efficacy with that of Support Vector Machine (SVM) (pixel‐based) algorithm. We perform a detailed quantitative evaluation of boundary‐delineation accuracy using spatially explicit error maps in tandem with the spatial maps of geomorphic and bank classes. Results show that template matching is more successful than SVM in delineating water surface boundaries in river sections with spatially challenging geomorphic landforms (e.g. sediment bar structures, partially submerged sediment deposits) and shallow water conditions. However, overall delineation accuracy by SVM is higher than that of template matching (without iterative hierarchical learning). Vegetation and water indices, especially when combined with texture information, improve the accuracy of template matching, for example, in river sections with overhanging trees and shadows – the two most problematic conditions in water surface boundary delineation. By identifying the influence of channel morphology and bank characteristics on water surface boundary mapping, this study helps determine river sections with higher uncertainty in delineation. In turn, the most suitable methods and data sets can be selectively utilized to improve geomorphic/hydraulic characterization. The methodology developed here can also be applied to similar studies on other geomorphic landforms including floodplains, wetlands, lakes, and coastlines. Copyright © 2014 John Wiley & Sons, Ltd.
Meandering rivers display active communication between bank erosion and bar deposition processes. How does this occur? How does the river select its width? To answer these questions, we implement a model for meander migration where both bank processes (erosion and deposition) are considered independently. Bank erosion is modeled as erosion of purely non-cohesive bank material damped by natural slump block armoring; channel deposition is modeled via flow-retarded vegetal encroachment. Both processes are tied to a slope-dependent channel forming Shields number; banks with near-bank Shields number below this value undergo deposition, and those above it undergo erosion. Channel-forming Shields number must increase with slope, as dictated by available data and model performance. Straight channel modeling shows that a channel arrives at an equilibrium width from any initial condition. For the channel bend, the river always approaches an asymptotic state where width reduces slowly in time and where bank erosion and deposition occur at nearly equal rates. Before this state is reached, however, the river follows a phase-plane trajectory with four possible regimes: a) both banks erode, b) both banks deposit, c) both banks migrate outward, but with a faster depositing bank (bar push), and d) both banks migrate outward, but with a faster eroding bank (bank pull). The trajectory of migration on the phase plane depends on initial conditions and input parameters controlling the rate of depositional and erosional migration. All input parameters have specific physical meaning, and the potential to be measured in the field.
The fluvial riparian and aquatic patch mosaic varies along rivers according to geomorphological setting, hydrological regime, sediment supply and surface–groundwater connectivity. This relation between physical processes and plants is not unidirectional. Once established, riparian and aquatic plants frequently act as physical ecosystem engineers by trapping and stabilising sediments, organic matter and the propagules of other plant species, modifying the local sedimentary and morphological environment by driving the development of landforms and associated habitats, and so facilitating the rapid establishment of other plants that can in turn reinforce the development of landforms such as river banks, vegetated islands and floodplains. This paper reviews knowledge on the hydrogeomorphological significance of riparian and aquatic vegetation with a particular emphasis on humid temperate, mixed load, gravel bed, floodplain rivers.
Numerous studies have suggested the importance of bank vegetation as a control of channel patterns; however, to data there is no conclusive evidence that vegetation does represent a significant control. An analytical model is developed in order to assess the influence of bank vegetation on channel patterns of alluvial gravel-bed rivers. Three channel types are considered: meandering, wandering, and braided. Bank vegetation effects are quantified in terms of a friction angle varphi'. A new theoretical meandering-braiding transition criterion is formulated that includes varphi', median grain diameter D50, and bank-full discharge Q. The theoretical relation is tested against field data from 137 rivers and successfully discriminates between meandering and braided rivers. Wandering rivers show greater scatter. It is concluded that bank vegetation, as expressed in terms of varphi', does exert significant and quantifiable control on alluvial channel patterns. A simple planform stability diagram is developed to determine the sensitivity of gravel-bed rivers to changes in bank vegetation.
Geomorphology has long been engaged in characterizing the form and understanding the evolution of floodplains. This study primarily examines floodplain width in four tributaries to the Minnesota River in southern Minnesota, namely the Maple, Le Sueur, and Blue Earth Rivers and Seven Mile Creek. The tributary systems are relatively young, having formed after retreat of the Wisconsinan ice sheet, and are rapidly evolving in response to a base level fall of nearly 70m–13,400 YBP. As a result, the lower reaches of all four river channels are aggressively incising. This study applies a new, freely available, open-code plug-in for ArcGIS to measure floodplain width as a function of elevation above the geomorphic top of bank. Systematic changes are observed in all four systems. Above the steep, incising knickzone reaches of each of the tributaries, relatively wide, and mostly unconfined, floodplains are observed. Within the knickzone, floodplains progressively narrow to between 25–60% of their width above the knickzone, despite the fact that sediment loads increase significantly in the downstream direction. A power law relationship is consistently observed between floodplain width and the ratio of local channel slope to contributing drainage area to about the negative one-third power in all four rivers. This same power law relationship is observed for two other rivers that differ significantly from our study systems in climatic and tectonic environment as well as base level history.
The purpose of this study is threefold: to experimentally evaluate the factors controlling the porosity and permeability of ideal clastic materials; to determine the modifications produced in natural deposits by variations from the ideal; to apply the principles of permeability to the problem of the distribution and localization of mineral deposits in clastic sediments. The effect on porosity of absolute grain-size, variable grain-size, shape of grain, method of deposition, and compaction are experimentally determined for ideal materials and compared with those of river and beach sands and other clastic sediments. Various equations for permeability are discussed; and the controlling factors of temperature, hydraulic gradient, size and shape of grain, uniformity of grain-size, porosity, and stratification are experimentally evaluated for ideal materials, and the modifications caused by natural materials are delimited and discussed. Field observations on the effects of wave and river depositional processes on porosity and permeability are presented. Comparisons are made of the relative permeability of gravel, sand, clay, loess, till, and unsorted alluvium. Changes produced in the original porosity and permeability of clastic sediments by the metamorphic processes of compaction, cementation, and recrystallization are outlined. Finally, the principles governing the flow of fluids through unconsolidated clastic materials are applied to the flow of hydrothermal solutions through rocks of clastic origin in an attempt to use the principles of permeability to explain and predict the distribution and localization therein of hydrothermal mineral deposits.
Riparian vegetation can significantly influence the morphology of a river, affecting channel geometry and flow dynamics. To examine the effects of riparian vegetation on gravel bed braided streams, we conducted a series of physical experiments at the St. Anthony Falls Laboratory with varying densities of bar and bank vegetation. Water discharge, sediment discharge, and grain size were held constant between runs. For each run, we allowed a braided system to develop, then seeded the flume with alfalfa (Medicago sativa), allowed the seeds to grow, and then continued the run. We collected data on water depth, surface velocity, and bed elevation throughout each run using image-based techniques designed to collect data over a large spatial area with minimal disturbance to the flow. Our results show that the influence of vegetation on overall river patterns varied systematically with the spatial density of plant stems. Vegetation reduced the number of active channels and increased bank stability, leading to lower lateral migration rates, narrower and deeper channels, and increased channel relief. These effects increased with vegetation density. Vegetation influenced flow dynamics, increasing the variance of flow direction in vegetated runs and increasing scour depths through strong downwelling where the flow collided with relatively resistant banks. This oblique bank collision also provides a new mechanism for producing secondary flows. We found it to be more important than the classical curvature-driven mechanism in vegetated runs.
We examine relations for hydraulic geometry of alluvial, single-thread gravel bed rivers with definable bankfull geometries. Four baseline data sets determine relations for bankfull geometry, i.e., bankfull depth, bankfull width, and down-channel slope as functions of bankfull discharge and bed surface median sediment size. These relations show a considerable degree of universality. This universality applies not only within the four sets used to determine the forms but also to three independent data sets as well. We study the physical basis for this universality in terms of four relations, the coefficients and exponents of which can be back calculated from the data: (1) a Manning-Strickler-type relation for channel resistance, (2) a channel-forming relation expressed in terms of the ratio of bankfull Shields number to critical Shields number, (3) a relation for critical Shields number as a function of dimensionless discharge, and (4) a ``gravel yield'' relation specifying the (estimated) gravel transport rate at bankfull flow as a function of bankfull discharge and gravel size. We use these underlying relations to explore why the dimensionless bankfull relations are only quasi-universal and to quantify the degree to which deviation from universality can be expected. The analysis presented here represents an alternative to extremal formulations to predict hydraulic geometry.
According to Lacey's formula, the width of a natural channel at bankfull flow is proportional to the root of the discharge. It is a very simple formula that has been confirmed by many authors and which, until now, has had no physical explanation. It appears that Lacey's equation is composed of physical and measurable parameters which agree with field observations. The equation hinges on the fact that the velocity at bankfull discharge is a sole function of the bed material. At bankfull discharge the average velocity is no longer a function of the discharge, as is assumed in regime theory. At discharges below bankfull level the stream velocity is a function of the discharge to the power 1/6. However at bankfull discharge a singularity occurs where the water slope is forced on the slope of the natural levees. The velocity of flow associated with this slope is fully determined by the bed material, and is independent of the discharge. An analytical expression for Lacey's coefficient of proportionality is presented.
Empirical data indicate that hydraulic geometry relationships for single-thread sand-bed rivers (i.e., rivers with median bed-material size between 0.062 and 0.50 mm) can be delineated such that bankfull width, bankfull depth, and channel slope are related in consistent ways to bankfull discharge. Such relationships ought to be the external expression of physical relationships intrinsic to sand-bed river dynamics. In this study, a back-calculation is performed to identify parameters (exponents and coefficients) for three relationships taken to be intrinsic to sand-bed rivers: (1) a generalized Manning-Strickler resistance relationship; (2) a relationship for channel-forming Shields number; and (3) a relationship for sand yield at bankfull flow. To back-calculate parameters for the physical relationships, first the hydraulic geometry relationships are expressed in suitable dimensionless form. Second, the physical relationships are expressed with coefficients and exponents that are analytically related to parameters in the hydraulic geometry relationships. Third, parameters from the hydraulic geometry relationships are used to calculate parameters for the physical relationships. The analysis yields the following results for the sand-bed rivers: (1) no physical basis exists for using an exponent of 1/6 in the resistance relationship; (2) channel-forming Shields number decreases with particle Reynolds number, and thus grain size, in a consistent way; and (3) sand concentration at bankfull flow must decline with increasing bankfull discharge. Although each of these relationships could have been established independently on its own, in this study they have been obtained as the only conclusions consistent with the observed hydraulic geometry relationships and the proposed physical framework. The analysis also yields a useful, dimensionally homogeneous predictive relationship for bankfull discharge as a function of bankfull width, bankfull depth, bed slope, and bed-material median grain size. DOI: 10.1061/(ASCE)HY.1943-7900.0000352. (C) 2011 American Society of Civil Engineers.
A series of laboratory experiments demonstrates that riparian vegetation can cause a braided channel to self-organize to, and maintain, a dynamic, single-thread channel. The initial condition for the experiments was steady-state braiding in non-cohesive sand under uniform discharge. From here, an experiment consisted of repeated cycles alternating a short duration high flow with a long duration low flow, and uniform dispersal of alfalfa seeds over the bed at the end of each high flow. Plants established on freshly deposited bars and areas of braidplain that were unoccupied during low flow. The presence of the plants had the effect of progressively focusing the high flow so that a single dominant channel developed. The single-thread channel self-adjusted to carry the high flow. Vegetation also slowed the rate of bank erosion. Matching of deposition along the point bar with erosion along the outer bend enabled the channel to develop sinuosity and migrate laterally while suppressing channel splitting and the creation of new channel width. The experimental channels spontaneously reproduced many of the mechanisms by which natural meandering channels migrate and maintain a single dominant channel, in particular bend growth and channel cutoff. In contrast with the braided system, where channel switching is a nearly continuous process, vegetation maintained a coherent channel until wholesale diversion of flow via cutoff and/or avulsion occurred, by which point the previous channel tended to be highly unfavorable for flow. Thus vegetation discouraged the coexistence of multiple channels. Varying discharge was key to allowing expression of feedbacks between the plants and the flow and promoting the transition from braiding to a single-thread channel that was then dynamically maintained.
A bank-stability analysis is incorporated into an analytical procedure for modeling the hydraulic geometry of an alluvial gravel-bed channel. The new analysis includes a procedure that calculates the mean bed and bank shear stress as well as assessing the bank stability. The bank-stability criterion accounts for the increased stability of the channel banks due to consolidation of the bank sediment, cementing by fines, and binding of the sediment by root masses. The two key parameters in the bank-stability analysis are the median grain diameter of the bank sediment D50bank, and the modified friction angle of the bank sediment phi[prime]. A sensitivity analysis of the two parameters indicate that the bank stability can exert a large influence on the channel geometry. This is supported by testing the theory on published field data. The bank sediment size was assumed to equal the bed sediment size, and the variation of the friction angle phi[prime] was investigated. The estimated phi[prime] values are seen to increase systematically with the bank vegetation density. The results indicate that increased bank stability induced by the vegetation has a significant influence on channel geometry; the vegetated channels are narrower, deeper, and less steep. For well-developed bank vegetation, the channel widths, depths, and slopes were found to be in the order of 0.6, 1.4, and 0.9 times their respective unvegetated channel dimension.
In the 1960s, extensive meander straightening (rectification) was conducted along 8 km of the Sainte-Marguerite River to facilitate highway construction along the valley bottom. Comparison of long profiles and data on bed pavement caliber from the current river channel and the man-made oxbows sheds light on the multifaceted responses to this anthropogenic perturbation of the river over the last 4 decades: (1) reprofiling consisting of 1 m of bed incision in the upper 4 km coupled with 2 m of bed aggradation in the lower 2 km of the straightened reach; (2) a doubling of the caliber of bed pavement in the zone of maximum degradation; (3) regrowth of three meanders within the reach, increasing its length by 7% (400 m); and (4) net input of 4 × 104 m3 of valley-side sediment from channel erosion into three terraces. Hydraulic and sediment transport reconstructions indicate that reprofiling and pavement coarsening over nearly 4 decades equally contributed to transport reequilibration along the reach, while meander regrowth and lateral sediment inputs from terraces played minor roles.
Aerial photographs from the past 70 years show narrowing of channels in Canyon de Chelly National Monument, which coincided with the establishment and expansion of woody riparian vegetation, primarily non-native tamarisk (Tamarix ssp.) and Russian olive (Elaeagnus angustifolia) and native cottonwood (Populus spp.). Rectified air photo sets were used to map the extent of woody riparian vegetation cover in the canyon bottom as well as the channel planform geometry for 6 years: 1935, 1964, 1975, 1981, 1989 and 2004. In 1935, vegetation covered less than 1 per cent of the canyon bottom and the channel was braided and 91 m wide on average. By 2004, the channel in the upper 75% of the canyon had narrowed to a single thread an average of 6.5 m wide and was lined by riparian vegetation, with vegetation covering up to 45% of the canyon bottom in some reaches. A relative timing index was developed that incorporates information from both channel and vegetation area expansion or contraction into a single variable. Vegetation establishment lagged channel narrowing in the upper reaches of the canyons, but narrowing and vegetation establishment occurred simultaneously in the middle and lower reaches. Narrowing progressed downstream through time, and the lowermost reaches remained braided and unincised in 2006. We interpret these patterns to suggest that riparian vegetation is responding to and interacting with changes in the channel morphology of Canyon de Chelly rather than driving channel change. Copyright © 2010 John Wiley & Sons, Ltd.
Morphologic transport estimates available for a 65-km stretch of Fraser River over the period 1952–1999 provide a unique opportunity to evaluate the performance of bedload transport formulae for a large river over decadal time scales. Formulae tested in this paper include the original and rational versions of the Bagnold formula, the Meyer-Peter and Muller formula and a stream power correlation. The generalized approach adopted herein does not account for spatial variability in flow, bed structure and channel morphology. However, river managers and engineers, as well as those studying rivers within the context of long-term landscape change, may find this approach satisfactory as it has minimal data requirements and provides a level of process specification that may be commensurable with longer time scales. Hydraulic geometry equations for width and depth are defined using morphologic maps based on aerial photography and bathymetric survey data. Comparison of transport predictions with bedload transport measurements completed at Mission indicates that the original Bagnold formula most closely approximates the main trends in the field data. Sensitivity analyses are conducted to evaluate the impact of inaccuracies in input variables width, depth, slope and grain size on transport predictions. The formulae differ in their sensitivity to input variables and between reaches. Average annual bedload transport predictions for the four formulae show that they vary between each other as well as from the morphologic transport estimates. The original Bagnold and Meyer-Peter and Muller formulae provide the best transport predictions, although the former underestimates while the latter overestimates transport rates. Based on our findings, an error margin of up to an order of magnitude can be expected when adopting generalized approaches for the prediction of bedload transport. Copyright © 2005 John Wiley & Sons, Ltd.
River regulation imposes primary changes on flow and sediment transfer, the principal factors governing the alluvial channel regime. In this study, the effect of flow regulation is isolated from sediment delivery. Peace River (Q̄ = 1080m3s−1, increasing to 2110m3s−1 downstream) was regulated in 1967 for hydropower. The gravel-bed reach immediately downstream from the dam has become stable. Gravel accumulates at major tributary junctions, so the river profile is becoming stepped. Further downstream, the river has a sand bed. It can still transport sand, so morphological changes along the channel include both aggradation and channel narrowing by lateral accretation. In the gravel-bed Kemano River (Q̄ = 150m3s−1), the addition of water by diversion from another river caused degradation when additional bed material was entrained below the inflow point. However, the effect became evident only after many years, when a competent flood occurred. The short-term response was channel widening. The time-scale for the response depends on the size of the river and the nature and severity of regulation. In both rivers, significant adjustment will require centuries and will intimately involve the riparian forest.
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths.
The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.