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Bedrock Channel Morphology in Relation to Erosional Processes
Abstract
Bedrock channel morphology reflects the interactions between erosive
processes and the resistance of the channel substrate. The controls on
these interactions change with spatial scale. Mineralogy, exposure age
of the substrate, and local heterogeneities are particularly important
in controlling substrate resistance at the micro scale (mm to cm).
Substrate discontinuities created by bedding, joints, and lithologic
contacts become progressively more important at the meso scale (cm to
m), whereas regional structure and baselevel history may dominate
substrate resistance at the macro scale (m to km). In a similar manner,
turbulent fluctuations that create localized abrasion and cavitation are
more important at the micro and meso scales, whereas longitudinal
patterns of unit and total stream power exert a stronger influence on
channel morphology at the macro scale. Most studies of bedrock channel
morphology have described meso-scale erosional features. In the absence
of direct measurements, investigators have inferred both the erosive
processes that produced the observed features, and the controls on the
location of the features. Fluvial erosion of bedrock may occur via; (1)
corrosion, or chemical weathering and solution, (2) corrasion, or
abrasion by sediment in transport along the channel, or (3) cavitation
and other hydrodynamic forces associated with flow turbulence. Very few
direct measurements of rate exist for any of these erosive processes.
Bedrock channel morphologies may be divided into multiple or single
flowpath channels, and subdivided on the basis of sinuosity, uniformity
of bed gradient, and uniformity of erosion across a cross section. These
categories may be used to infer dominant erosional processes and
relative rates of erosion, but we cannot yet predict the occurrence of
specific channel morphologies as a function of driving and resisting
forces. In this context, the traditional assumption that substrate
dominates bedrock channel morphology may be too restrictive.
... Richardson and Carling (2005, p.1) stated, "the morphology of surface bedrock channels is still a relatively neglected area of research within the study of fluvial systems". Anyhow, there has been a big evolution in this research area from Wohl's (1998) and Richardson and Carling's (2005) seminal attempts to describe bedrock river forms to more recent efforts by Lamb et al. (2015), Beer et al. (2017), Velázquez et al. (2016) to derive processes from forms in this environment. ...
... The formation and development of bedrock river features at different spatial scales are the result of a combination of extrinsic factors (such as stream hydraulics) and intrinsic bedrock factors such as lithology and heterogeneities (Wohl 1998) in combination with surface alteration processes, as for example wetting-drying cycles (Inoue et al. 2017). As Hall et al. (2012) suggested, "rock decay" encompasses energy transfer processes (a term they prefer to weathering), the influence of rock properties and product removal (erosion). ...
... Erosional processes dominate the morphology of most bedrock channels and to be able to understand erosion fully, the scale factor needs to be taken into account. Erosion ranges from small spatial scale (micro-scale, millimetre to centimetre) with erosional characteristics as abrasion, flacking or plucking (small size grains), meso-scale (centimetre to metre) with selective erosion that produce potholes, grooves, knickpoints, inner-channels and other meso-features and finally macro-scale (metre to kilometre) with morphologies at reach, basin-scale (Wohl 1998). ...
Morphometric characterisation is particularly relevant in the study of geomorphological heritage. 3D modelling techniques have been proven as very useful tools to recognise, characterise and valorise geomorphosites. Bedrock rivers account for one of the most outstanding aspects within geomorphological heritage due to the amount of distinctive and attractive geomorphological features associated to them and the high preservation of sculpted forms. Digital elevation models (DEMs) have made it increasingly possible to establish accurate morphometric indices and establish clearer connections between forms and processes. This paper reviews different methodologies to obtain DEMs on bedrock rivers. This review goes from DEM analysis at multiple spatial scales to introduce optical microrugosimeter as the latest technical development to facilitate micromorphometric analysis. Micromorphometric analysis opens the scope for improving the knowledge we have on trans-scale issues in bedrock rivers. Micromorphometric analysis also opens a new layer of information that enriches the public’s valuation of geodiversity of geomorphosites by increasing its didactic and interpretative potential.
... The study of morphology in bedrock channels remains poorly developed in comparison to other fluvial systems. Wohl (1998) and Richardson and Carling (2005) divided large-scale features into multiple and single-flow path channels and sub-types depending on bed gradient and crosssectional geometry. Richardson and Carling (2005) further divided small-scale features into five groups (concave features, convex and undulating surfaces, composite, solutional forms, and tool marks). ...
... River geometry refers to the shape of the active channel. In contrast to the numerous classification schemes proposed for alluvial river channels, only one classification has been published for reach-scale bedrock channel geometry (Wohl 1998;Wohl and Merritt 2008). This classification distinguishes channels with multiple and single flow paths, longitudinally uniform and variable bed gradients, and cross-sectionally uniform and variable bed elevation (Fig. 4). ...
... Multiple-flow path bedrock rivers are rare in nature. These have been named scablands (Baker 1973) and may result from insufficient channel capacity, domal uplift along the channel, or strong joint control (Wohl 1998). Bedrock rivers with single flow paths are more common, but multiple-channel bedrock rivers can occur in highly jointed bedrock and/or upstream from large waterfalls. ...
The Sobrarbe Geopark (Huesca, Spain) has one of the best examples in the world of diverse forms of bedrock rivers. Lithological variability as well as a complex tectonic history exerts an important control on examples of most of the bedrock river typologies described in scientific literature. This legacy constitutes an important added value to an area protected, in the case of national or natural parks, mostly based on biological aspects or, in the case of a Geopark, based primarily on paleontological, tectonic, or stratigraphical characteristics. Bedrock rivers are fragile systems that support distinctive ecosystems. A detailed survey of more than 200 sites allowed us to determine the high geodiversity that bedrock rivers offer in the Sobrarbe Geopark and to illustrate the relevance of bedrock features in terms of value added to the park. Past glacial activity, together with lithology, influences river morphologies at the scale of entire river networks. Glacial retreat controls tributaries that subsequently develop tall waterfalls in former hanging valleys; step-pool sections are developed in narrow, incised reaches upstream of a knickpoint; pool-riffle sections are located in locally steep reaches with glacial frontal moraines; knickpoints reflect local lithological changes; and undulating and pothole channel morphologies occur upstream from knickpoints. This variety of features in the Sobrarbe Geopark can be used for designing geotouristic routes that take advantage of the fact that tourists with diverse interests visit the Geopark, including those who practice adventure sports including hiking and canyoning, as well as tourists seeking a beautiful landscape.
... Although sediment transport may seem like a simple process, the origin and evolution of the erosional features in bedrock river channels remain poorly understood. As a result, numerous studies have focused on the processes that involve the formation of those sculpted shapes (Barnes, 1956;Allen, 1971;Wohl, 1998;Johnson & Whipple, 2007;Cook, Turowski, & Hovius, 2013). Bedrock fluvial systems are essentially characterised by boulder deposits, which have been accumulated under conditions of high flow turbulence from upstream source areas (Goode & Wohl, 2010). ...
... Another important aspect that must be considered in bedrock systems is the channel morphology. According to Wohl (1998), there is a close connection between the erosional process and the resistance of the material that composes the channel substrate. Recent studies indicate that this interaction is controlled by other factors, such as climate, lithological heterogeneity, rock mineral composition, regional structural pattern, topographic gradient and turbulent fluctuation (Burbank & Anderson, 2001;Wohl & Achyuthan, 2002;Kobor & Roering, 2004;Hovius & Stark, 2006;Johnson & Whipple, 2010;Turowski, 2012;Cook et al., 2013;Wilson, Wilson, Hovius, & Turowski, 2013). ...
... These authors conducted exhaustive reviews with regard to the hydraulic driving forces, physical resistances of the substrate and morphological features. As previously noted by Wohl (1998) and Whipple, DiBiase, & Crosby (2013), the natural occurrence of these sculpted forms is particularly interesting to illustrate the general implications of the fluvial erosion dynamic process on the river channel incision and landscape evolution. ...
span style="font-size: 10.5pt; font-family: 'Times New Roman','serif'; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;" lang="EN-US">The Juqueriquerê River channel was formed in a Precambrian crystalline basement. The lithological association is largely composed of ancient metamorphic and igneous rocks, with several overlapping tectonic episodes. Field surveys along the upper and middle course allowed for cataloguing a wide variety of fluvial erosion features. A sizable amount of morphological features have been sculpted on different types of rocks, including furrows, potholes, percussion marks, polishing and smoothing boulders as the most representative. The sizes and shapes of these scour marks are also diverse, and their study has provided important results for better understanding the erosive processes. Given their wide variety, the erosive morphological features offer an excellent opportunity to explore the mechanisms of fluvial erosion and evaluate their effective capacity to remove cobbles and boulders in bedrock river systems.</span
... The major erosional mechanisms postulated to control the morphology and genesis of bedrock channels are (i) abrasion (Sharpe and Shaw, 1989;Kor et al., 1991;Sjogren and Rains, 1995;Hancock et al., 1998;Wohl, 1998;Whipple et al., 2000a;Dietrich, 2001, 2004;Johnson and Whipple, 2007;Wilson et al., 2013;Wilson and Lavé, 2014); (ii) plucking (Baker, 1974(Baker, , 1978(Baker, , 1979Baker and Komar, 1987;Sharpe and Shaw, 1989;Hancock et al., 1998;Whipple et al., 2000aWhipple et al., , 2000bLamb and Fonstad, 2010;Anton et al., 2015); (iii) cavitation (Baker, 1974;Baker and Costa, 1987;Wohl, 1992Wohl, , 1998Baker and Kale, 1998;Hancock et al., 1998;Whipple et al., 2000aWhipple et al., , 2000b; (iv) dissolution or corrosion (Sharpe and Shaw, 1989;Wohl, 1992Wohl, , 1998Whipple et al., 2000a); (v) fluid stressing (Allen, 1971;Sjogren and Rains, 1995;Richardson and Carling, 2005;Carling et al., 2009;Wilson and Lavé, 2014); and (vi) physical weathering (Sharpe and Shaw, 1989;Whipple et al., 2000aWhipple et al., , 2000bCarling et al., 2009). Of these, abrasion and plucking are considered the most important processes, with plucking effective when rocks are fractured and exhibit discontinuities, whilst abrasion is thought to dominate in massive rock with weak jointing (Hancock et al., 1998;Whipple et al., 2000a;Chatanantavet and Parker, 2009;Lamb and Fonstad, 2010). ...
... The major erosional mechanisms postulated to control the morphology and genesis of bedrock channels are (i) abrasion (Sharpe and Shaw, 1989;Kor et al., 1991;Sjogren and Rains, 1995;Hancock et al., 1998;Wohl, 1998;Whipple et al., 2000a;Dietrich, 2001, 2004;Johnson and Whipple, 2007;Wilson et al., 2013;Wilson and Lavé, 2014); (ii) plucking (Baker, 1974(Baker, , 1978(Baker, , 1979Baker and Komar, 1987;Sharpe and Shaw, 1989;Hancock et al., 1998;Whipple et al., 2000aWhipple et al., , 2000bLamb and Fonstad, 2010;Anton et al., 2015); (iii) cavitation (Baker, 1974;Baker and Costa, 1987;Wohl, 1992Wohl, , 1998Baker and Kale, 1998;Hancock et al., 1998;Whipple et al., 2000aWhipple et al., , 2000b; (iv) dissolution or corrosion (Sharpe and Shaw, 1989;Wohl, 1992Wohl, , 1998Whipple et al., 2000a); (v) fluid stressing (Allen, 1971;Sjogren and Rains, 1995;Richardson and Carling, 2005;Carling et al., 2009;Wilson and Lavé, 2014); and (vi) physical weathering (Sharpe and Shaw, 1989;Whipple et al., 2000aWhipple et al., , 2000bCarling et al., 2009). Of these, abrasion and plucking are considered the most important processes, with plucking effective when rocks are fractured and exhibit discontinuities, whilst abrasion is thought to dominate in massive rock with weak jointing (Hancock et al., 1998;Whipple et al., 2000a;Chatanantavet and Parker, 2009;Lamb and Fonstad, 2010). ...
... The major erosional mechanisms postulated to control the morphology and genesis of bedrock channels are (i) abrasion (Sharpe and Shaw, 1989;Kor et al., 1991;Sjogren and Rains, 1995;Hancock et al., 1998;Wohl, 1998;Whipple et al., 2000a;Dietrich, 2001, 2004;Johnson and Whipple, 2007;Wilson et al., 2013;Wilson and Lavé, 2014); (ii) plucking (Baker, 1974(Baker, , 1978(Baker, , 1979Baker and Komar, 1987;Sharpe and Shaw, 1989;Hancock et al., 1998;Whipple et al., 2000aWhipple et al., , 2000bLamb and Fonstad, 2010;Anton et al., 2015); (iii) cavitation (Baker, 1974;Baker and Costa, 1987;Wohl, 1992Wohl, , 1998Baker and Kale, 1998;Hancock et al., 1998;Whipple et al., 2000aWhipple et al., , 2000b; (iv) dissolution or corrosion (Sharpe and Shaw, 1989;Wohl, 1992Wohl, , 1998Whipple et al., 2000a); (v) fluid stressing (Allen, 1971;Sjogren and Rains, 1995;Richardson and Carling, 2005;Carling et al., 2009;Wilson and Lavé, 2014); and (vi) physical weathering (Sharpe and Shaw, 1989;Whipple et al., 2000aWhipple et al., , 2000bCarling et al., 2009). Of these, abrasion and plucking are considered the most important processes, with plucking effective when rocks are fractured and exhibit discontinuities, whilst abrasion is thought to dominate in massive rock with weak jointing (Hancock et al., 1998;Whipple et al., 2000a;Chatanantavet and Parker, 2009;Lamb and Fonstad, 2010). ...
... There are several accounts of the mechanisms of fluvial erosion in bedrock channels, and it is not necessary to describe the processes in detail here. The interested reader is referred to the useful works of Allen (1971a), Selby (1985), Hancock et al. (1998), Wohl (1998), Wende (1999) and Whipple et al. (2000). However, brief definitions of the various processes are provided here for reference (see also Richardson and Carling, 2005). ...
... However, the formation of inner channels has been observed experimentally through the simple incision of a plane bed (Shepherd and Schumm, 1974;Kodama and Ikeda, 1984;Wohl and Ikeda, 1997). A common mechanism of knickpoint retreat in homogeneous rocks is that of pothole growth and coalescence in the reach immediately above the knickpoint (Wohl, 1998;Wohl et al., 1999). This results in inner channels and slot canyons with convoluted wall topography (Elston, 1917(Elston, , 1918Jennings, 1985;Wohl, 1999;Kunert and Coniglio, 2002) in which undulations on opposite sides are generally out of phase (Wohl, 1993;Wohl et al., 1999;Wohl and Achyuthan, 2002). ...
... However, an inner channel has also been observed with in-phase wall undulations . A stair-like succession of knickpoints may be termed a cataract (Wohl, 1998). Knickpoint migration is also an important mechanism of erosional escarpment retreat (Weissel and Seidl, 1998). ...
High-energy fluid flows such as occur in large water floods can produce large-scale erosional landforms on Earth and potentially on Mars. These forms are distinguished from depositional forms in that structural and stratigraphical aspects of the sediments or bedrock may have a significant influence on the morphology of the landforms. Erosional features are remnant, in contrast to the depositional (constructional) landforms that consist of accreted waterborne sediments. A diversity of erosional forms exists in fluvial channels on Earth at a range of scales that includes the millimetre and the kilometre scales. For comparison with Mars and given the present-day resolution of satellite imagery, erosional landforms at the larger scales can be identified. Some examples include: periodic transverse undulating bedforms, longitudinal scour hollows, horseshoe scour holes around obstacles, waterfalls, plunge pools, potholes, residual streamlined hills, and complexes of channels. On Earth, many of these landforms are associated with present day or former (Quaternary) proglacial landscapes that were host to jökulhlaups (e.g. Iceland, Washington State Scablands, Altai Mountains of southern Siberia), while on Mars they are associated with landscapes that were likely host to megafloods produced by enormous eruptions of groundwater. The formative conditions of some erosional landforms are not well understood, yet such information is vital to interpreting the genesis and palaeohydraulic conditions of past megaflood landscapes. Correct identification of some landforms allows estimation of their genesis, including palaeohydraulic conditions. Kasei Valles, Mars, perhaps the largest known bedrock channel landscape, provides spectacular examples of some of these relationships.
... It acts according to a series of variables: a) the resistance of clasts in transport relative to the resistance of the bedrock substrate, b) the grain size distribution and volume of transported sediment, c) the hydraulics of the flow, which determinate the location, magnitude and frequency of sediment and bedrock abrasive contacts. According to Wohl (1998), the highest abrasion ratios occur during floods, when turbulent waters are highly loaded with large amounts of sediment in suspension. In the studied reach, although we are not able to indentify themain forming factors, flute marks and pseudo-ripples are formed. ...
... These knickpoints retreat by headward erosion and occur where the greatest amount of energy dissipates. The distribution of the knickpoints is related to the frequency of joints, lithologic layering and the relations between strata dip and stream flow direction (Wohl, 1998). In the study area the potholes coalescence combined with knickpoints form inner channels. ...
... In the study area the potholes coalescence combined with knickpoints form inner channels. These forms maximise the shear strength and the unit power by concentrating the flow in deep, narrow sections and may be the most efficient way of incision in resistant substrate (Wohl, 1998). ...
En el presente trabajo se analizan las formas y los procesos de erosión del lecho de un meandro de la rambla del Tambuc, encajado en las calizas subtabulares de la plataforma del Caroig (Valencia). El análisis de las formas demuestra el predominio de procesos de abrasión, evorsión y remoción sobre los de corrosión. La incisión producida por el fl ujo del canal (en forma de surcos, marmitas de gigante y canales interiores) ha sido más efecti va que la generada por procesos de disolución (manifi estos en cuencos y rillenkarren). El dominio de los procesos hidrodinámicos, fuertemente condicionados por la estructura, da como resultado la formación de un perfi l longitudinal irregular con numerosas rupturas de pendiente
... Potholes are mesoscale erosional landforms that are observed in a wide range of climate, lithology, and channel type [6]. They are the concavity, developed by the entrained grinders by the action of continued vortex action, while its morphology is characterized by the hemispherical or cylindrical indentations excavated in channel bed [7]- [11]. ...
... The occurrences of multiple erosion features such as potholes grooves observed at the Khari Gorge site are evident of maximum energy dispense where the flow depth and velocity may have been high during high precipitation periods ( Figure 6). River born mechanical weathering of bedrock depends on the hydraulics of flow along with nature and amount of abrasive tool incorporated with rock mass strength [6]. The primary purpose of the study was to evaluate the physical properties of bedrock and associated structures in bedrock to bring out the pothole morphology. ...
The potholes are perceptible erosional features associated with bedrock channels. They play an essential role in bedrock incision studies, but little work has been published on the development of potholes, especially in Bedrock Rivers in India. The present site-specific study aims to analyze the role of physical properties of bedrock and associated structures in the process of development of potholes and to classify the potholes based on their dimensions. The Khari River gorge formed over Mesozoic Sandstone with six bedrock terraces exposed along a confined segment of the river. The site is manifested by strath terraces, grooves, potholes, and knick points. Three stretches of Khari Gorge have been studied in detail for this purpose. The pothole dimensions, joint orientations, rock mass strength, physical properties of rock, and placing of potholes to present active channels were measured. The analysis shows the inverse relationship between the size of pothole and rock properties. The distance of potholes and active channels govern the essential factor and manifest the role of flow hydraulics. The high density of matured potholes near active channels over immature potholes supports primary control of hydraulics of flow over rock properties.
... Yet, bedrock channel development, the processes of excavation and the controls on channel morphology are critical for understanding the nature and pace of long-term catchment change (Tinkler and Wohl, 1998). The nature and rate of bedrock incision vary significantly with environment and timescale, depending on lithology, drainage basin area, channel gradient, climatic regime, and whether the tectonic regime is active or passive (e.g., Schumm and Chorley, 1983;Wohl et al., 1994;Wohl, 1998). Establishing the processes and rates of change associated with bedrock channels has recently gained momentum (Tinkler and Wohl, 1998). ...
... Establishing the processes and rates of change associated with bedrock channels has recently gained momentum (Tinkler and Wohl, 1998). Three major themes have emerged: the mechanics of bedrock channel incision; relationships between erosional processes and bedrock channel morphology; and the longitudinal relationships between bedrock and alluvial reaches (Baker and Kale, 1998;Hancock et aI., 1998;Wohl, 1998). ...
Landform-sediment-process assemblages associated with four gorges and their corresponding downstream boulder fans in the alpine periglaciofluvial system of the Storutla river, Jotunheimen, southern Norway, are described. The potential volume of frost-weathered sediment excavated from the gorges is compared using a sediment-budget approach to calculate the volume of angular sediment within the fans accumulated during the Holocene. Fan volumes represent an estimated 18 to 53% of the total gorge volume. Allowing also for the volume of relatively small caliber material flushed through the system, 24 to 97% of the gorge volume is accounted for by an estimated minimum long-term Holocene rate of gorge excavation of 0.002 to 0.010 m³ m–1 yr–1 (minimum long-term Holocene gorge incision rate of 0.15–0.39 mm yr–1) Most of the remaining gorge volume is attributed to substantial pre-Holocene subglacial gorge incision by meltwater action. These rates of Holocene periglaciofluvial erosion of bedrock appear to exceed those characteristic of temperate fluvial systems unaffected by tectonic uplift. The implied rates of frost weathering (macrogelivation) are less than those under optimum conditions in arctic-alpine environments but support the efficacy of frost weathering in locations susceptible to the annual freeze-thaw cycle.
... They separate an upland relict landscape from downstream bedrock rivers that have incised the shore platform that surrounded the relict landscape. The upland landscape was used in seminal studies to test the validity of the catchment-integrated detrital 10 Be denudation rate method in river borne quartz [Brown et al., 1995;1998]. These 10 Be measurements were then combined with measurements of dissolved exports in rivers to propose some of the earliest calculations of longterm weathering fluxes and rates of soil and saprolite development [White et al., 1998;Riebe et al., 2003;Ferrier et al., 2010]. ...
... An observation of riverbed morphology along the knickpoint faces provides important insights on the respective contribution of plucking, abrasion, weathering and mass-wasting [e.g. Wohl, 1998;Montgomery, 2004;Richardson and Carling, 2005]. The knickzones are composed of an alternation of bare bedrock and boulder-strewn reaches (Fig. 5a). ...
Expanses of subdued topographies are common at high elevation in mountain ranges. They are often interpreted as relict landscapes, and are expected to be replaced by steeper topography as erosion proceeds. Preservation of such relict fragments can merely reflect the fact that it takes time to remove any pre-existing topography. However, relict fragments could also possess intrinsic characteristics that make them resilient to dissection. We document here the propagation of a wave of dissection across an uplifted relict landscape in Puerto Rico. Using 10Be-26Al burial dating on cave sediments we show that uplift started four million years ago, and that river knickpoints have since migrated very slowly across the landscape. Modern detrital 10Be erosion rates are consistent with these long-term rates of knickpoint retreat. Analysis of knickpoint distribution, combined with visual observations along the stream beds indicate that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks. Earlier studies have shown that vegetation over the relict topography efficiently limits erosion, allowing for the formation of a thick saprolite underneath. Such slow erosion reduces stream bedload fluxes delivered to the knickpoints, as well as bedload grain size. Both processes limit abrasion. Compounding the effect of slow abrasion, wide joint spacing in the bedrock makes plucking infrequent. Thus, the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream, reducing the celerity at which the relict, subdued landscape is dissected. We conclude that similar top-down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies.
... References: Wohl (1998) ...
... Bed configuration unit sub-type: rock glide (a, b). (a) Modified from Wohl (1998). ...
This part provides a detailed description of the Geomorphic Units survey and classification System (GUS). This method is used to identify, characterise and analyse the assemblage of geomorphic units within a given reach. The system is suitable for integrating the MQI and is also aimed at allowing the establishment of links between hydromorphological conditions at reach scale, characteristic geomorphic units, and related biological conditions.
The document is organised in two parts. Part A provides the general background and describes characteristics, analysis, testing, and typical applications of the method. Part B is an Illustrated Guidebook to the identification and classification of geomorphic units. A series of Forms for the application of the GUS are reported in Appendix 1. The list of gemorphic units included in the GUS is reported in Appendix 2. A brief glossary of significant terms is reported in Appendix 3.
... Potholes are features of abrasion (Wohl and Ikeda, 1997;Whipple et al., 2000a) that are commonly found in bedrock rivers and in various lithologies and channel types (see references in Wohl, 1998). In the geomorphological literature, the record of potholes in basalts is very limited. ...
... In other words, the strength is a consequence of vesicularity, which is a primary variable that influences pothole formation (see Section 5.3). Wohl (1998) suggested that potholes would take longer to form at high rock strength, which was also observed by Ortega et al. (2014) in rivers on granites. The data presented here indicate a low occurrence of potholes in massive basalts, which have a statistically higher mean strength than vesicular-amygdaloidal basalts. ...
Variations in rock properties control geomorphic processes and thus landscape evolution. Potholes developed on basaltic riverbeds are generally associated with vesicular-amygdaloidal zones, although they also occur in massive basalts. Until now, this relationship has not been quantified, nor have the parameters controlling the development of these features in basalts been evaluated. Based on field data collected from 71 sites distributed in three rivers in the Paraná Volcanic Province (PVP), southern Brazil, we investigated the relationship between the occurrence of potholes and features of basalt flows. Reaches were analyzed both in areas with potholes and in areas without these features. The data collected refer to the joint density, the intact rock strength measured with a Schmidt hammer and the typology of basaltic units in terms of vesicularity. It was found that potholes preferentially occur in vesicular-amygdaloidal units (86%). This predominance is not associated with the joint density, which is the same in massive basalts (≈5 m/m2); moreover, potholes occur in basalts with very different joint densities. The intact rock strength is lower in vesicular-amygdaloidal basalts (58) than in massive basalts (61) and does not explain fully the preferential abrasion in vesicular-amygdaloidal units because potholes occur with varying resistances. The basalt strength is a secondary variable. The controlling parameter seems to be vesicularity, which by producing irregularities in the bed and flow triggers the formation of potholes. In the massive basalts, irregularities are produced primarily by joints. In massive basalts there seems to be an upper threshold of stream power beyond which the formation of potholes is restricted (drainage area ≈700 km2; slope ≈0.06). A lower threshold in stream power could also exist to massive basalts of the study area and it is suggested by the inexistence of potholes in sites with drainage area less than 100 km2.
... Knickpoints are the zones of greatest energy dissipation along a channel (Wohl, 1998). The development of substantial knickpoints can lead to rapid erosion of basalts and cataract migration, as colonnade blocks near steep faces lose downstream support and readily fall or slide under the strong shear from fast or deep flows. ...
Round mesas up to 500 m high occur in Martian outflow channels. Mesas in Ravi and Elaver Valles occur in deepest parts of the channels where the most intense megaflood erosion occurred. I theorize that Noachian basalts poured into craters which acted as lava traps, similar to Kilauean lava lakes. Subsequent flood basalts buried the infilled craters. Hesperian megafloods stripped away hundreds of meters of basalts, exhuming erosion‐resistant strata. Lava solidification theory is explored to assess the role of cooling in governing the structure and strength of the Martian basalts. The postulated lava lakes that formed the Ravi Vallis mesas would have needed millennia to cool. The larger Elaver Vallis mesa may have needed up to 20,000 years to solidify. Long cooling times led to coarse, doleritic mineral textures and reduced numbers of cooling joints, greatly increasing bulk rock strength and resistance to hydrodynamic erosion. Using rock mechanics theory, cores of exhumed lava‐filled craters would have bulk rock strengths at least 5 to 30 times greater than more highly fractured basalts. Discharge hydrographs for the Morella Crater breach flood that carved Elaver Vallis peak at ∼2.1 × 10⁷ m³ s⁻¹. Stream power per unit streambed area ranged up to >150,000 W m⁻² as the breach grew in size. The calculations provide an envelope of power sufficient to erode hundreds of meters of basalts, but not great enough to remove the large mesa. Thus the legacy of an ancient, lava‐filled crater is a high mesa on the floor of Elaver Vallis.
... Following the proposal launched by Hayakawa and Oguchi (2009), for this work, I define these geomorphological attention points as knickzones: "locally steep riverbed segment". This definition also encompasses the term knickpoint, generally defined as referred to the exact location where there is a sharp change in channel slope (downstream increases) (Bierman and Montgomery 2013;Tinkler 2004). In geometric terms, a knickpoint is a single record represented by a point while a knickzone refers to a linear entity. ...
The gradient characteristics of Courel Mountains Geopark bedrock rivers were examined. Unlike alluvial rivers, bedrock rivers have been the great forgotten of fluvial geomorphology globally. Based on the decreasing rate of gradient with increasing measurement length, a relative steepness was obtained as indicator of knickzone. Supported by GIS techniques and DEMs, the changes in slope along the longitudinal profile of the rivers were detected. The number of the extracted knickzones rises to 325, which means a frequency of knickzones of 0.467 km⁻¹. The total length of the knickzones is 285 km, representing about half of the drainage network as knickzone (47%). The mean height, the length, and the gradient of all the knickzones were ~ 110 m, ~ 880 m, and 0.178 m·m⁻¹, respectively. There is no distribution pattern and the knickzones are everywhere, although they are more present in reaches with NW–SE direction and order 1. Several environmental factors were crossed to know more about the occurrence and knickzones characteristics, suggesting that density and direction of fractures regulate the number and the trajectory of the knickzones, while the lithology controls the singularity of the forms. The geomorphological and the topographical characteristics of the bedrock rivers make them high ecological, scenic, landscape, and recreational value. Findings from this study can be also used by managers to develop and/or improve strategies for conservation, valorisation, and how to approach the tourist who visits the Geopark. Scientific tourism can offer a unique and educational travel experience, allowing participants to learn about bedrock rivers and knickzones.
... We explored whether there is any difference in the mean water surface slope, canyon width, mean channel depth, or maximum channel depth for fault-controlled and epigenetic canyons. Only mean water surface slope shows significant differences between the two canyon types (Figure 10 For example, bedding has been shown to set erosive patterns at the metre scale (Wohl, 1998) and where the strike of bedding is consistent over the tens to hundreds of metre scale, its role in setting river orientation may correspond with or subsume the effect of fault structures. ...
Many rivers worldwide follow faults because they exploit bedrock damage that occurs in fault zones. However, there has been no quantitative investigation of how closely rivers follow specific faults within a fault zone. Analysis of the Fraser River in southwestern British Columbia reveals the degree of alignment of individual bedrock‐bound canyon reaches with lineaments of the Fraser River Fault. The alignment of the river at the regional scale is controlled by the Fraser River Fault zone, but individual canyons align with subsidiary faults that are visible in elevation data or predicted by the geometry and kinematics of the main fault. We hypothesize canyon reaches that deviate from faults were shifted by large episodic sediment input, creating epigenetic canyons.
... ASGH considers changes at a single cross section or averaged from two or three riffle cross sections (Navratil and Albert 2010). A reach is that portion along a stream channel with short-scale variability in terms of the substrate (Olusola et al. 2020a, b), natural stream morphology (Navratil and Albert 2010), and whose length is six times the average width of the stream (Wohl 1998;Fashae et al. 2017). ...
Hydraulic geometry characteristics can influence stream ecosystem and biogeochemical processes at different spatial scales. Globally, studies are emerging on the relationship between channel geometry and water quality. However, these studies are few and far between most humid tropical basins. In this study, we assessed the relationship between hydraulic geometry and some water quality parameters [temperature, salinity, pH, electrical conductivity (EC), total dissolved solids (TDS) and total suspended solids (TSS)] across stream orders of a tropical headwater basin in southwestern Nigeria. Bankfull width, depth, velocity and discharge were measured at twenty-six (26) reaches across first-, second-and third-stream orders within the basin. Water samples were collected from these reaches and were analyzed. Bankfull width, velocity and discharge showed significant variation across stream orders, while the water quality parameters vary slightly in response to fluvial and geogenic interactions. We obtained r = − 0.417, p < 0.05 for TDS and discharge, r = − 0.399, p < 0.05 for depth and TDS, r = − 0.39, p < 0.05 for width and EC, and r = − 0.42, p < 0.05 for width and pH. The headwater basin is underdeveloped and it is still undergoing fluvial adjustment. We recommend the need for future studies to investigate the intrinsic interaction between channel geometry and stream water contamination for natural and disturbed headwater basins.
... Structural analyses, particularly in neotectonics, are not anything out of the ordinary in geomorphology but frequently related to issues of civil engineering and sister drainage systems of the SSS [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. A broader view has been taken on the evolution of fluvial drainage system as a function of tectonics in some comprehensive studies [17,[26][27][28][29][30][31][32][33][34]. Similar comprehensive studies dealing with the mineralogy and petrography of straight to low-sinuosity drainage systems cannot be quoted for the Variscan basement in Central Europe. ...
A holistic-modular approach has been taken to study the evolution of three straight to low-sinuosity drainage systems (=SSS) in an uplifted basement block of the Central European Variscides. The development of the SSS is described by means of a quadripartite model. (1) The geological framework of the SSS: Forming the lithological and structural features in the bedrock as a result of different temperature, pressure and dynamic-metamorphic processes. (2) Prestage of SSS: Forming the paleo-landscape with a stable fluvial regime as a starting point for the SSS. (3) Proto-SSS: Transition into the metastable fluvial regime of the SSS. (4) Modern SSS: Operation of the met-astable fluvial regime Tectonics plays a dual role. Late Paleozoic fold tectonic creates the basis for the studied SSS and has a guiding effect on the development of morphotectonic units during the Neogene and Quaternary. Late Cenozoic fault tectonics triggered the SSS to incise into the Paleozoic basement. The change in the bedrock lithology has an impact on the fluvial and colluvial sediments as well as their landforms. The latter reflects a conspicuous modification: straight drainage system ⇒ higher sinuosity and paired terraces ⇒ hillwash plains. Climate change has an indirect effect controlling via the bedrock the intensity of mechanical and chemical weathering. The impact on the development of the SSS can be assessed as follows: Tectonics >> climate ≅ bedrock lithology. The three parameters cause a facies zonation: (1) wide-and-shallow valley (Miocene), (2) wide-angle V-shaped valley (Plio-Pleistocene), (3) acute-angle V-shaped valley (Pleistocene), (4) V-shaped to U-shaped valleys (Pleistocene-Holocene). Numerical data relevant for the hydrographic studies of the SSS are determined in each reference area: (1) Quantification of fluvial and colluvial deposits along the drainage system, (2) slope angles, (3) degree of sinuosity as a function of river facies, (4) grain size distribution, (5) grain morphological categorization, (6) grain orientation ("situmetry"), (7) channel density, (8) channel/floodplain ratios. Thermodynamic computations (Eh, pH, concentration of solubles) are made to constrain the paleoclimatic regime during formation of the SSS. The current model of the SSS is restricted in its application to the basement of the Variscan-Type oro-gens, to an intermediate crustal maturity state.
... Step-pool bedrock channels are one of many bedrock morphologies that can occur (Wohl, 1998). With bedrock channels, when channel gradients exceed 4%, step-pools are often the dominant morphology (Wohl and Merritt, 2001). ...
Step-pool channel units are common in steep terrain with channel gradients greater than a few percent. Such features are dominated by boulders and cobbles that interact to help lock the bed in place. The interlocking of grains results in the formation of steps over which water plunges, forming downstream pools. Step-pools often display a characteristic spacing and size that depends on channel slope, channel width and grain size. Multiple mechanisms are responsible for step formation and collapse, related to entrainment and deposition patterns, channel geometries (e.g., channel slopes and widths), sediment supply regimes and grain sizes. The complex flow conditions associated with step-pools make the prediction of water depths, velocities, and sediment transport difficult; these questions remain an active area of research.
... 271 [ Figure 3 near here] 272 273 Over the measurement period, discharge generally decreased (Table 1) below the knickpoint was mainly subcritical (Fr < 1) and was predominantly turbulent on the 285 6 th and 7 th July (high Re), with transitional flow on the 8 th July ( the greatest channel change occurs at peak discharges in ice-walled channels (Marston 1983;311 Carver et al. 1994), highlighting the need for measurements at increased temporal resolution to 312 better constrain the timing and rates of evolution. 313 As erosion reflects the balance between driving and resisting forces (Wohl 1998; Snowpack depletion reduces potential water storage at the glacier surface, the volume of 465 delayed runoff and the lag-time between peak melt and supraglacial discharge (see Willis et al. 466 2002). Here, the contribution of delayed runoff following peak ablation is likely to be minimal, 467 indicating that the stream did not experience continuous water flow over a 24-hour period. ...
Despite numerous studies of knickpoints in bedrock and alluvial channels, no detailed description of knickpoint change on the ice has been reported to date. This paper presents the first investigation of knickpoint evolution within a supraglacial stream. Repeat longitudinal profile surveys of a knickpoint on Vadrec del Forno, Switzerland reveal a step height increase of 115 mm and upstream migration of 0.26 m over three days during the 2017 ablation season. Rates and magnitudes of erosion vary spatially across the knickpoint in relation to differing discharge regimes. At high discharges (∼0.013 m³ s⁻¹), erosion is focused at the step base; at low discharges (∼0.003 m³ s⁻¹), erosion is focused on the reach upstream of the knickpoint, at the step lip and the step-riser face. This results in replacement of knickpoint morphology, driven by frictional thermal erosion and hydraulic action. Pool formation further influences step morphology, inducing secondary circulation and increased melt at the base of the step-riser, causing steepening. Results highlight the complexities of water flow over knickpoints, demonstrating that the stream power law does not accurately characterise changing knickpoint morphology or predict retreat rates. Although morphological similarities have been reported between supraglacial and bedrock/alluvial channels, knickpoints in non-ice-walled channels will not necessarily respond to discharge similarly to those in ice due to the different erosion processes involved.
... Geomorphic changes strongly associated with fluvial abrasion (Wohl, 1998;Whipple et al., 2013) may be represented by various features such as the lowering of bedrock channel floors (Seidl et al., 1994;Hancock et al., 1998;Johnson et al., 2010;Inoue et al., 2014), the widening of bedrock channels (Finnegan et al., 2007;Turowski et al., 2008;Fuller et al., 2016), the development of potholes (Springer et al., 2005(Springer et al., , 2006Johnson and Whipple, 2007;Pelletier et al., 2015), the growth of plunge pools (Lamb et al., 2007;Scheingross and Lamb, 2017), knickpoint retreat (Haviv et al., 2010;Valla et al., 2010), and the formation of hanging valleys (Crosby et al., 2007). Following Howard and Kerby (1983) who postulated that the bedrock incision rate can be approximated by the bottom shear stress of streams, many models to describe the erosion rate, collectively called the stream-power model (Whipple, 2004), have been proposed for the study of bedrock river evolution (overviews by Sklar and Dietrich, 2006;Whipple et al., 2013). ...
A major physical process shaping bedrock landforms in fluvial, coastal, and aeolian environments is abrasion by sediment‐carrying fluid flows. A unifying formula to describe the rate of abrasion occurring in these environments has not been presented, the exploration of which is the purpose of this study. Considering the threshold concept the formulation is made including erosivity of fluid flows and erodibility of bedrock. The formula is described as dΓ/dt = C [(FA/FR) – 1], where Γ is the amount of erosion, i.e. eroded length (distance), volume, mass, or weight, t is the time, dΓ/dt is the erosion rate, FA (= A x [fluid force]) is the assailing force of sediment‐laden fluid flows used as an index of the flow erosivity, FR (= B x [bedrock strength]) is the resisting force representing the bedrock erodibility, C is a coefficient with the same unit as that of dΓ/dt, and A and B are dimensionless coefficients. The equation is confirmed by existing laboratory abrasion data and its applicability is examined using existing laboratory data of fluvial and aeolian abrasion experiments and field data from a coastal area. Examinations applying fluvial and aeolian abrasion data indicate that the coefficient C is found to represent the amount of sediment working as abrasives in fluid flows and the hardness of sediment relative to bedrock; and A is presented to reflect the particle size of the sediment. The coefficient B is a conversion factor from conventional mechanical strength of rocks to the resisting force. Selecting appropriate physical quantities for FA and FR enables the application of this equation to abrasion studies on various landforms in these environments. © 2018 John Wiley & Sons, Ltd.
... Channels incised in bedrock range in size from small rills at the scale of centimeters to canyons at the scale of kilometers. They form subaerially and subaqueously in materials ranging from erodible silt-clay alluvium to extremely resistant bedrock (Wohl 1998(Wohl , 1999Whipple et al. 2000aWhipple et al. , 2000b. Incision is a common response of channels that have been disturbed by external forcing, such that they contain excess amounts of flow energy or stream power relative to the supplied sediment load (Schumm et al. 1984;Bowman et al. 2010). ...
... Knickzones in the form of waterfalls often oc-*Corresponding Author: Tuba Zahra: Dept. of Natural Environmental Studies, The University of Tokyo, Kashiwa, Chiba, Japan; Email: tuba.zahra@gmail.com Uttam Paudel: Dept. of Natural Environmental Studies, The University of Tokyo, Kashiwa, Chiba, Japan Yuichi S. Hayakawa, Takashi Oguchi: Dept. of Natural Environmental Studies, The University of Tokyo, Kashiwa, Chiba, Japan cur in bedrock rivers [5][6][7] and are an indicator of channel adjustments to local or regional perturbations [8][9][10]. The steepening of a river reach occurs as a result of a number of factors and processes including resistant lithology and differential erosion [2,11,12], transient response to base level change [13,14], change in shear stress leading to gully head propagation [15], and changes in sediment loads from tributaries [16]. ...
Extraction of knickpoints or knickzones from a Digital Elevation Model (DEM) has gained immense significance owing to the increasing implications of knickzones on landform development. However, existing methods for knickzone extraction tend to be subjective or require time-intensive data processing. This paper describes the proposed Knickzone Extraction Tool (KET), a new raster-based Python script deployed in the form of an ArcGIS toolset that automates the process of knickzone extraction and is both fast and more user-friendly. The KET is based on multi-scale analysis of slope gradients along a river course, where any locally steep segment (knickzone) can be extracted as an anomalously high local gradient. We also conducted a comparative analysis of the KET and other contemporary knickzone identification techniques. The relationship between knickzone distribution and its morphometric characteristics are also examined through a case study of a mountainous watershed in Japan.
... Classifications that focus on channel substrate commonly emphasize how differences in the channel bed reflect differences in sediment supply and influence cross-sectional geometry and channel stability. Categories include clay-bed channels with cohesive sediment that require relatively large hydraulic force to erode the channel; sand-bed channels that respond quickly to changes in flow energy and are relatively resilient; gravel-bed channels with beds composed primarily of pebble-to boulder-size sediment, which have higher resistance to erosion and less resilience; and bedrock channels formed primarily in lithified material, which have the largest erosional thresholds and the lowest resilience (Baker 1988;Wohl 1998Wohl , 2014cWhipple et al. 2013). ...
For more than 100 years, the ordinary high water mark (OHWM) has been used to define water boundaries in a number of contexts in the United States. This Special Report summarizes the scientific literature pertaining to the indicators used to identify the OHWM in fluvial systems, building on more than a decade of research and publications related to the OHWM in the ongoing process to implement the Clean Water Act and the Rivers and Harbors Act of 1899. This report does not change or redefine the indicators used to identify the OHWM, nor is it a manual for how to delineate the OHWM. This report first reviews established concepts in river science that relate to the OHWM then reviews various sources of information that can be used to delineate the OHWM, discusses geographic variations in OHWM indicators among river segments, reviews human activities that can affect the OHWM, and finally presents examples of the OHWM in diverse channel types and regions.
... The process of establishing equilibrium in longitudinal profiles in nature can take up to several million years (Stock & montgomery, 1999). The interpretation of a historical development of longitudinal profiles of headwater torrential streams is based on the understanding of the governing processes under which a torrential channel incises (woHl, 1998). The bedrock and channel incision are particularly important because they can regulate how fast erosion changes expand along the torrential channel (tinkler & woHl, 1998). ...
From the national digital elevation model DMV 5 of the Western Karavanke longitudinal profiles of fifty-three torrents were extracted. Longitudinal profiles of torrential channels in study area have generally convex sections and do not correspond to equilibrium state. In this paper, changes in lithology across faults and other potential influences are discussed as possible cause for observed convex longitudinal profiles. Some typical cases where it has been found, that convex sections could be result of faults, are graphically showed. Also lithology of torrential stream bed could be one of main factors for convexity. This applies in particular if the torrent crosses from soft to solid rock.
Iz državnega digitalnega modela višin DMV 5 območja Zahodnih Karavank so bili določeni vzdolžni profili 53 hudourniških strug. Vzdolžni profili hudournikov imajo večinoma odsekoma konveksno obliko in ne ustrezajo pogojem ravnovesnega stanja. V članku so kot možni vzroki za konveksnost vzdolžnih profilov obravnavane spremembe v litologiji preko prelomov in nekateri drugi potencialni dejavniki. Grafično so prikazani posamezni značilni primeri, kjer je bilo ugotovljeno, da so lahko konveksni odseki vzdolžnih profilov hudourniških strug posledica prelomov. Prav tako se je ugotovilo, da je lahko litologija podlage dna hudourniške struge eden izmed glavnih razlogov za pojavljanje konveksnih odsekov. To velja še posebej za primere, kjer hudournik preide iz mehkih v trdne kamnine.
... Alluvial and bedrock channels are different in nature and processes. Wohl (1998) has mentioned that three types of erosional processes are dominant on a bedrock river based on various spatial scales. These are: (i) abrasion, plucking, or flacking of individual grains at the microscale (mm to cm); (ii) erosion of a portion of the channel boundaries or along a cross section that produces potholes, longitudinal grooves, knick points, inner channels, step-pool sequences at the mesoscale (cm to m); and (iii) reach to basin scale channel morphologies in planform and gradient at macrolevel (m to km). ...
The rivers that bear the properties of both alluvial and bedrock character are considered as bedrock and alluvial mixed character rivers. They are characterised by stable planform, erosion resistant bank, rock outcrop, local variability in channel bed gradient and variable flow regime. The Barakar River in the Chotanagpur plateau and its fringe region of India has the distinctive features of bedrock and gravel mixed river with sinuous and braided nature in the upper and the lower sections of the river, respectively. To investigate and extract the geomorphic features of the bedrock and gravel mixed character of the Barakar River, planform view and behaviour of the sediments were considered. The sinuous bedrock and braided reaches have been analysed using the Sinuosity Index and the Braid-channel Ratio, respectively. The upper sinuous bedrock section is marked by a steep-sided, narrow valley with the presence of incised channel. In the downstream reach of the Maithon Dam, the river channel is a box-shaped bedrock channel with a low form ratio. Potholes and grooves of varying size and shape and inner channel are the main features at the mouth zone. While, in the lower section of the river, the wide and straight braided reach is characterised by higher form ratio and higher gradient than the sinuous reach. The riffle-pool sequence in the braided section is not developed significantly like in the alluvial channel. In the braided reach, the sediment grain size is continuously decreased towards the Maithon Dam and the sediment grains are well sorted. The lithology and hydraulic processes of the river determine the mixed character of the river that favours the fluvial system to maintain its lateral stability.
... The last two decades have seen a surge of interest in quantifying bedrock river incision processes to establish relationships between climate, tectonics, sediment flux, drainage area, and channel geometry. Because bedrock channels determine much of the relief structure of mountainous terrains, understanding their evolution is crucial to understanding landscape response to tectonic and climatic forcing [Howard et al., 1994;Wohl, 1998;Snyder et al., 2000;Whipple, 2004a;Larue, 2008]. The majority of bedrock channel studies have focused on active orogens, where uplift and tectonism are ongoing [e.g., Kirby, 2003;Clark et al., 2004;Duvall et al., 2004;Crosby and Whipple, 2006;Harkins et al., 2007;Cowie et al., 2008;Turowski et al., 2008;Ouimet et al., 2009;Hu et al., 2010;Yanites and Tucker, 2010;DiBiase and Whipple, 2011;Allen et al., 2013]. ...
Bedrock channel profile analysis typically assumes that channels evolve toward a condition of topographic steady state where channel morphology is adjusted to rock erodibility, uplift rates, and stream power. Here we use the integral method of channel profile analysis to quantify channel steepness within a large set of tributary channels that incise through layered rocks in the Buffalo National River Basin in northern Arkansas. Statistical analysis of these channels demonstrates that normalized channel steepness is not a function of local bedrock lithology but is influenced by coarse sediment supply. Specifically, normalized steepness is greatest in reaches of the basin where an interval of Pennsylvanian sandstone forms a caprock on the ridges. Block detachment of the sandstone causes large boulders to be stranded in the upper tributaries where stream power is too low to mobilize or effectively erode the boulders. Within these channels, normalized steepness is correlated with sandstone boulder size and percent boulder coverage rather than local lithology, despite strong contrasts in the mechanical strength of the lithologies incised. This analysis suggests that removal of caprock material is rate-limiting within the landscape and may be responsible for the long-term persistence of topography within this tectonically passive setting.
... Reaches that are significantly steeper than adjacent reaches, comprising both steep, critical and gentle stream segments, or a cataract of multiple knickpoints, may be termed knickzones (e.g. Hayakawa and Oguchi, 2006;Wohl, 2013) (Fig. 1). Series of knickpoints or knickzones along a channel often appear as "wave trains" that may exhibit regular frequency or density (e.g. ...
Despite an interest in the hydraulic functioning of supraglacial and englacial
channels over the last 4 decades, the processes and forms of such
ice-bounded streams have remained poorly documented. Recent glaciological
research has demonstrated the potential significance of so-called
"cut-and-closure" streams, where englacial or subglacial flow paths are
created from the long-term incision of supraglacial channels. These flow paths
are reported to exhibit step-pool morphology, comprising knickpoints and/or
knickzones, exaggerated in dimensions in comparison to supraglacial channels.
However, little is known of the development of such channels' morphology.
Here, we examine the spatial organisation of step pools and the upstream
migration of steps, many of which form knickzones, with repeated surveys over
a 10-year period in an englacial conduit in cold-based Austre
Brøggerbreen, Svalbard. The observations show upstream step recession to
be the dominant process for channel evolution. This is paralleled by an
increase in average step height and conduit gradient over time.
Characteristic channel-reach types and step-riser forms are consistently
observed in each of the morphological surveys reported. We suggest that the
formation of steps has a hydrodynamic origin, where step-pool geometry is
more efficient for energy dissipation than meanders. The englacial channel
system is one in rapid transition towards a quasi-equilibrium form within a
decadal timescale. The evolution and recession of knickzones reported here
result in the formation of a 37 m deep moulin shaft, suggesting that over
time an incising supraglacial channel may evolve towards an englacial channel
form exhibiting a stable end-point characterised by a singular vertical
descent, which potentially can reach the glacier bed. This challenges the
prevailing notions that crevasses or hydrofractures are needed to form deep
moulins. Our observations highlight the need to further examine the
adjustment processes in cut-and-closure channels to better understand their
coupling to supraglacial meltwater sources and potential significance in
cold-based glacier hydrology and ice dynamics.
... Hence, bedrock exerts vertical and/or lateral constraints on river forms and processes. Unlike fully alluvial rivers, channel morphology of bedrock rivers reflects interactions between erosive processes and the resistance of the confining substrate (Wohl, 1998). Montgomery and Buffington (1998) differentiated amongst colluvial valleys with no fluvial channel, bedrock valleys (confined perFigure 1), and alluvial valleys that transport and sort sediment loads supplied from upslope but lack the transport capacity to routinelyFigure 1. Various definitions of valley confinement. ...
Valley setting and confinement (or lack thereof) are primary controls on river character and behaviour. Although thereare various proxies for valley confinement, direct measures that quantify the nature and extent of confinement are generally lackingand/or inconsistently described. As such they do not lend themselves to consistent analysis over large spatial scales. Here we clearlydefine forms of confinement to aid in quantification of degrees of confinement. Types of margin that can induce confinement aredifferentiated as a valley margin, valley bottom margin, and/or anthropogenic margin. Such margins sometimes overlap and sharethe same location, and in other situations are separated, giving immediate clues as to the valley setting. We apply this frameworkto examples from Australia, United States and New Zealand, showing how this framework can be applied across the spectrum ofriver diversity. This method can help to inform interpretations of reach-scale river behaviour, highlighting the role of antecedentcontrols on contemporary forms and processes. Clear definitions of confinement are shown to support catchment-scale analysis ofriver patterns along longitudinal profiles, and appraisals of the geomorphic effectiveness of floods and sediment flux in catchments(e.g. process zone distribution, lateral sediment inputs and (dis)connectivity).
... In the relationship used by Whipple and Tucker (2002) f(Q s ) increases from 0 to 1.0 as Q s /Q c increases from 0 to 0.5 (sediment enhances erosion) and f(Q s ) decreases from 1.0 to 0 as Q s /Q c increases from 0.5 to 1.0 (sediment covers the bed). The function used in this study for f(Q s ) is the same as that used by Whipple and Tucker (2002) except we slightly adapt the function so that erosion can still take place when there is there is no sediment load, theoretically through processes other than wear by sediment, such as plucking or solution (e.g., see Wohl, 1993;Hancock et al., 1998;Wohl, 1998;Whipple et al., 2000a, for a discussion of bedrock erosion processes). This assumption is also made partly as a boundary condition to avoid infinite slopes . ...
We use a numerical model to investigate disequilibrium conditions in detachment-limited river networks. Erosion rates are modeled using two different equations that include sediment flux as a variable for determining incision rates into bedrock. A number of numerical simulations are performed to explore erosion patterns, channel profile shape, and network concavity after an increase in uplift rate across the network. In the case where an increase in sediment flux (relative to carrying capacity) is considered only to decrease incision rates, the main channel has a two-part response to a faster uplift rate; initially a knickpoint steepens channel slopes locally, but at later times channel slopes rise throughout the network. However, in the case where an increase in sediment flux can both enhance and suppress incision rates, the transient network response can be much more dynamic; channel slopes (and also elevations) can both rise and fall, all in response to a single increase in uplift rate. The response varies depending on the magnitude of change in uplift rate and the initial ratio of sediment flux to sediment carrying capacity. In all examples, the lower parts of the network respond quickly to an increase in uplift rates by increasing channel slopes, while the response of erosion rates in the upper parts of the network occurs later. As a result, the change in sediment flux delivered to higher order channels lags the initial changes in the slope of these channels and causes a complex response in erosion rates. These findings highlight that erosion rates at any point in the network respond to changes both downstream and upstream, and therefore variables such as sediment flux that integrate the upstream response can play an important role is shaping the transient morphology of river networks.
Geomorphological evidence of incised bedrock channels is widespread in all mountain landscapes worldwide. However, the processes controlling incision and gorge formation in bedrock have not directly been observed in an actualistic way. Here, we show a LiDAR change detection deciphering the erosive power of a 60,000 m3 hyperconcentrated flow (transition between flood and debris flow) in a deeply incised rock gorge in June, 2020. The flow laterally eroded up to 1 m of massive limestone and widened a 4 m narrow section of the gorge by up to 15%. Sinuosity, convergence, and gradient of the channel were proven to not influence erosivity indicating the hyperconcentrated nature of erosion. Furthermore, other than in prior studies no abrasion of thin rock veneer dominates erosion but mechanically excited breakout of rock fragments. Magnitude-frequency relations of eroded volumes mimic subaerial rock wall retreat. We show how single hyperconcentrated flows can erode bedrock channels far more efficient than decades of turbulent flows and hypothesise that repeated hyperconcentrated flows in phases of enhanced precipitation or by elevated material supply could control erosion boosts in gorge formation, e.g. in the Lateglacial or during climatic fluctuations.
The base-level fall signal is transmitted through the drainage system as a transient (migrating upstream) erosional wave in form of a knickpoint or knickzone. Its movement includes: (1) migrating backwards and (2) moving vertically. There is no one-to-one correlation between knickpoints along river profiles and base-level events. Knickpoint retreat scales with stream discharge and gradient. The travel distance and the celerity of knickpoints have been suggested as power functions of the drainage area. All experiments show a delay between the base-level fall and the appearance of erosional features upstream. The greater the drainage area and the higher the rate of the base-level fall, the shorter is the expected delay. Additional competing controls that determine the rate of knickpoint retreat include the stream gradient, weathering processes, abrasion and undermining.KeywordsTransient waveWaterfallPlunge poolComplex responseDegradation indexDiachroneityLag
Potholes are one of the spectacular landforms of rocks, especially sandstones. In this paper, 48 samples of Agha-Jari Sandstones were prepared and thin sections were obtained from them to investigate the sandstone minerals and porosity by Polarizing microscope and also by Bernard calcium meter. For estimating of rock hardness we use Schmidt Hammer Model N and ISRM (1978) standard. Maps were prepared with 1:25000 topographic map, Arc/GIS software and Jmicrovision software was used to obtain the amount and percentage of thin section minerals. The results showed that Potholes are a function of physical conditions of rocks and mineralogy compounds of rocks with the erosion role of water flow in calcium carbonate of stone. Based on the data and results, it was found that the characteristics of Agha-Jari sandstone compounds play an important role in the generation of Potholes in this formation. It was determined that the abundant presence of Feldspar minerals and high percentage of porosity in the oldest layer (A) of Agha-Jari Formation plays a great role in the looseness and creation of Potholes in that layer. Other layers with Potholes such as (B, D, C, and F Layers) show a significant relationship between increasing quartz percentage and decreasing the percentage of feldspar and calcium carbonate. The results also showed that the mean calcium carbonate of Agha-Jari sandstone is about 50% which is one of the reasons for the existence of Potholes in layers despite the hardness of Schmidt Hammer is high.
Bedrock rivers carry large amounts of fine sediment in suspension. We developed a mechanistic model for erosion of bedrock channel banks by impacting bedload and suspended load particles that are advected laterally by turbulent eddies (advection‐abrasion model). The model predicts high lateral erosion rates near the bed, with rates decreasing up to the water surface. The model also predicts greater erosion within the suspended load layer than the bedload layer for many typical sediment supply and transport conditions explored. We compared the advection‐abrasion model with a previously derived model for lateral erosion of bedrock banks by bedload particles deflected by stationary bed alluvium (deflection‐abrasion model). Erosion rates predicted by the deflection‐abrasion model are lower, except within limited conditions where sediment is transported near the threshold of motion and the bed is near fully covered in sediment. Both processes occur in bedrock rivers at the same time, so we combined the advection‐abrasion and deflection‐abrasion models and found that the lateral erosion rate generally increases with increasing transport stage and relative sediment supply for a given grain size. Application of our combined‐abrasion model to a natural bedrock river with a wide distribution of discharge and supply events, and mixed grain sizes, indicates that finer sediment dominates the lateral erosion on channel banks in low sediment supply environments and can be as important as coarser sediment in high sediment supply environments.
Nearly all tributaries of the major rivers in western Carinthia emerging from the mountain fronts show knickpoints. They are preconditioned both by glacial overdeepening of the trunk valley and tectonic/isostatic displacements that led to marked base level change and in consequence to sharp incisions of the lowest tributary reaches. Initiation and development of gorges, however, are controlled by very different parameters as demonstrated by the three selected examples. The Ragga Slot (Kreuzeck Mts.) is outstanding in revealing the shaping of a bedrock channel due to a very recent downcutting of the lowest reach of a hanging valley. The Gaisloch Gorge (Gailtal Alps) illustrates an advanced stage in gorge development accomplished by accelerated retreat of its dolomitic sidewalls, creating a spectacular rock scenery. Finally, the Garnitzen Valley (Carnic Alps) represents a complex of gorges, including slots and gaps that are integrated into a glacial through valley. The three presented gorges can be classified as typical geomorphosites. They were opened for visitors at the beginning of the nineteenth century thanks to local initiatives and by the Alpine Club; the gorges/slots of the Ragga River and the Garnitzen River have been declared as natural monuments.KeywordsGorgesSlotsBedrock channelsScalloped escarpmentsInner gorgesGeomorphositesGeosites
Dam-failure floods typically involve greater peak discharge than the largest meteorological flood at a basin. Determining the geomorphic effectiveness of extreme flooding caused by a breach mechanism provides insight into the role of flood scale on the resulting processes and landforms. Here, we present a geomorphological and hydraulic analysis of the 1959 Vega de Tera (NW Spain) dam-break flood, a worldwide notable dam-failure incident that released a flow of 7.8 106 m3 that caused the death of 144 people at Ribadelago before reaching Lake Sanabria (9 km down valley). This watershed-lake connection provides a comprehensive analysis of an extreme sediment delivery event in the context of a millennial long lake depositional record. One-dimensional unsteady flow computation shows a peak flow hydrograph attenuating from 13,000 m3 s−1 to 5150 m3 s−1, that reached a maximum flow depth of 34 m and velocity of 30 ms−1. Spatial variation of erosional and depositional landforms are related with local flow hydraulics: i) in steep sectors flow regime was supercritical (shear value up to 11,200 Pa) and produced up to 30 m deep bedrock channel erosion; ii) at the boundary of steep and flatten sectors, transition to subcritical regime generated large plunge pools (up to 6000 m2 and 15.2 m in depth); iii) in low-gradient sectors low shear stress gave rise to depositional landforms, namely gravel bars with dam boulders up to 3 m long, and a debris cone with coarse gravel and expansion sand bars. The depositional landforms amount for a total volume of ca 2.11 · 106 m3 in the Tera valley (37% in the gorge and 63% in the floodplain). The dense, energetic sediment-laden flow reached Lake Sanabria forming a debris cone close to the mouth and caused an underwater hyperpycnal current, depositing a ~10 cm-thick sandy-silt layer all over the two distal subbasins. The estimated volume of the deposited fine sediments in the lake ranges between 200,000 and 368,000 m3. The lake record shows that this was the largest flood in the basin during the Holocene. Previous to the dam break, the sediment connectivity between the Sanabria watershed and the lake was limited because of the “staircase” topography and the presence of small glacial depressions filled with sediments since deglaciation. Even during the flood, the great majority of the sediments were deposited along the flood pathway, and only a small percentage (10–20%) reached the lake. Although the hydraulics of the Tera River were not changed after the flood, the newly formed pools in the watershed could diminish the connectivity between the river and the lake in the future, as some new sedimentation areas (pools) were generated acting as natural dams and thus decreasing sediment input to the lake.
A study approach is developed to assess the longitudinal channel slope under the equilibrium condition as well as the transient evolution of a mixed alluvial-soft-bedrock stream. Both the historical field data and 2D mobile-bed numerical modeling are adopted. The proposed approach is applied to a 14 km reach downstream of the Ji-Ji Weir, Chuo-Shui River, Taiwan, where continuous maintenance works have been carried out to stabilize this reach. In this study, the temporal evolution of the longitudinal channel profile is assessed numerically with three spatial scales: The large (the entire study reach), the medium (four sub-reaches), and the local (cross-sections) scale. The large scale analysis is the approach for purely alluvial streams and is shown to be difficult to use to characterize mixed alluvial-bedrock streams. The local scale analysis shows that the soft-bedrock incision has a widely fluctuating slope, reflecting the compound environmental forcing and complex riverbed setting. With the medium scale analysis, the longitudinal channel profile is found to follow a predictive trend if the reach is partitioned into four distinctive sub-reaches. Characteristics of the dynamic channel slope evolution in different spatial scales are computed and presented. The study results can be used to select the proper locations and types of the engineering stabilizing structures in a mixed alluvial and soft bedrock stream
Potholes carved into streambeds of the Jurassic and Cretaceous sandstone and mudstone along the Sunxi River (Chongqing, China) have been carefully investigated in order to characterize the geometrical features of the potholes and to explore their implications for river incision into bedrock. The potholes formed by gyratory currents through abrasion are characterized by an average aperture aspect-ratio (a/b) of ∼1.2 and a mean diameter-to-depth ratio (D/h) of ∼1.0. The most parsimonious model for the h-D relationship over the full range of data is a linear equation in which 2 parameters (n is the slope of the line and m is the D-intercept) are essentially controlled by hydrodynamics. Data of the Sunxi River yield that n = 0.71 ± 0.18, indicating that the pothole deepening was faster than the pothole widening by a quasi-constant factor of ∼1.4 during abrasive erosion as the dominant mechanism of pothole development in bedrocks of sandstone interbedded with mudstone. The m value, which varies from 5.9 cm to 11.0 cm, apparently depends on hydrodynamics at each locality, and can be inferred as the dimensional threshold of initial depressions from which potholes grew by deepening and lateral expanding. The potholes with D/h = 1 are prone to entrap and retain coarser sediments which are more effective for pothole growth. The bedrock weakened significantly by coalescence of numerous potholes is easily broken, detached, and removed by flooding (truncation). The cyclic pothole-truncation process is particularly important for incision of bedrock channels and thus landscape evolution of mountain ranges in response to tectonic uplift and climate change.
This chapter reviews the basic principles of sediment transport in streams, channel classification, mountain channel morphology, and associated implications for fish spawning habitats. Local sediment transport processes construct the differing channel morphologies, which are often opportunistically used by spawning fish. At the watershed scale, the various channel morphologies are spatially organized, as represented by channel classifications. Thus, sediment transport, channel morphology, and freshwater fish ecology are critically linked, and appreciation for the characteristic associations between these natural attributes is crucial to our understanding of (i) river evolution, (ii) responses to natural and human-induced disturbances, and (iii) implications for riverine ecosystems and the distribution of fish habitats. This chapter reviews exiting links between physical and biological processes and identifies the needs for future research for channel ecosystems in mountain streams.
Reconstructing the incision history of bedrock rivers can provide evidence of formation processes in knickzones on longitudinal river profiles. In this study, we determined incision rates at the upstream end of a knickzone in the Oshika Gorge, Tottori prefecture, southwest Japan, by exposure dating of a series of granitic strath terraces using terrestrial cosmogenic 10Be. The measured 10Be concentrations increased with the height of the sampling surface above the present-day riverbed. Exposure ages were determined as 50.2 ± 3.9 kyr BP (Before Present) for the highest strath terrace, which was 11.0 m above the present riverbed, and 5.3 ± 0.7 kyr BP for the lowest at 1.6 m. Regression analysis of all data gave an average incision rate of 0.24 mm/yr, which is comparable to the uplift rate in this region. Results from the multiple sampling points suggest that the incision rate may have changed in a stepwise manner over the past 50 kyr BP, as follows: 0.05 mm/yr between 50.2 and 32.4 kyr BP, 0.53 mm/yr between 32.4 and 19.9 kyr BP, and 0.15 mm/yr between 19.9 kyr BP and the present. This pattern of an increasing and then decreasing incision rate is difficult to explain in the context of the retreating knickzone that is located just downstream from the sample site. The timing of the increase in incision rate corresponds to the beginning of decreasing precipitation and temperature toward the Last Glacial Maximum. Although the present study was unable to determine the reason for this accelerated incision, it possibly reflects the response of the channel to enhanced sediment supply from the surrounding hills during the glacial period.
The ravine of Tambúc, is part of the head of one of tributaries of the Júcar river, placed in the eastern part of the Muela de Cortes de Pallás and Macizo del Caroig (Valencia). It occupies a relic Upper Miocene – Qua ternary incised valley. Alhough it has been described previously as generated by fluviokarstic processes, the forms of recent karstification they do not explain the development of the fluvial incision. The ephemeral current river of Tambúc’s Ravine occupies a compound valley, with a meandering pattern. This meandering pattern is controlled by a dense network of two systems of vertical joints. Nowadays these joints determine the liberation of fractured blocks and pebbles. The action of the gravity processes and the flow during floodings they are the mechanisms that more contribute to the transport of sedimentary particles.
This paper examines the erosional forms in a calcareous bedrock channel, in a meander of the Tambuc Rambla (Caroig platform, Valencia). The paper has been written in order to clarify some comments made by Santisteban (2014). Our work aims to solve any possible confusion or controversy on the nature of three forms identified in this bedrock channel, flutes, strath terraces and pseudo-ripples (hummocky forms), and presents some observations on the current hydrogemorphological dynamic of this Mediterranean ephemeral river. © 2014, Asociacion Espanola para el Estudio del Cuaternario (AEQUA). All rights reserved.
Steep headwater channels are the headmost channels in a drainage network, with gradient greater than 0.05 and dimensions comparable with the dimensions of individual clasts, if present, that make up the boundaries of the channel. Hence, the stability of the individual clasts or refractory boundaries determines the stability of the channel. In clast-lined channels, jammed or locked clast structures are pervasive, allowing them to persist in channels much steeper than expected. In forest environments, woody debris also forms part of the material that enters the channel and defines its morphology. Channels with these characteristics commonly are first to third order, exceptionally extending to fourth order. At their headmost limits, channels may begin at spring points or may grade into even steeper colluvial deposits within gullies. They drain mountainside or valley-side slopes and commonly join larger channels at slope base across an alluvial or colluvial fan. They characteristically exhibit cascade or step-pool reach morphologies, and may also exhibit 'forced' alluvial reaches behind logjams or boulder chokes. Flow resistance in steep channels follows a shallow-flow limit equation and is contributed primarily by form resistance. Consequently, only a small fraction of flow energy is effective in sediment transport, and classical hydraulic capacity sediment transport equations grossly overestimate transport. Destabilization of the channel bed in extreme flows or following the influx of large volumes of sediment into the channel commonly issues in a debris flow or debris flood.
Step-pool channel units are common in steep terrain with channel gradients greater than a few percent. Such features are dominated by boulders and cobbles that interact to help lock the bed in place. The interlocking of grains results in the formation of steps over which water plunges, enhancing the formation of downstream pools. Step-pools tend to display a characteristic spacing and size that depends on channel slope, grain size, and channel width. The complex flow conditions associated with step-pools make the prediction of water depths, velocities, and sediment transport difficult; these questions remain an active area of research.
Fluvial bedrock incision constrains the pace of mountainous landscape
evolution. Bedrock erosion processes have been described with incision models
that are widely applied in river-reach and catchment-scale studies. However,
so far no linked field data set at the process scale had been published that
permits the assessment of model plausibility and accuracy. Here, we evaluate
the predictive power of various incision models using independent data on
hydraulics, bedload transport and erosion recorded on an artificial bedrock
slab installed in a steep bedrock stream section for a single bedload
transport event. The influence of transported bedload on the erosion rate
(the "tools effect") is shown to be dominant, while other sediment effects
are of minor importance. Hence, a simple temporally distributed incision
model, in which erosion rate is proportional to bedload transport rate, is
proposed for transient local studies under detachment-limited conditions.
This model can be site-calibrated with temporally lumped bedload and erosion
data and its applicability can be assessed by visual inspection of the study
site. For the event at hand, basic discharge-based models, such as
derivatives of the stream power model family, are adequate to reproduce the
overall trend of the observed erosion rate. This may be relevant for
long-term studies of landscape evolution without specific interest in
transient local behavior. However, it remains to be seen whether the same
model calibration can reliably predict erosion in future events.
Landform-sediment-process assemblages associated with four gorges and their corresponding downstream boulder fans in the alpine periglaciofluvial system of the Storutla river, Jotunheimen, southern Norway, are described. The potential volume of frost-weathered sediment excavated from the gorges is compared using a sediment-budget approach to calculate the volume of angular sediment within the fans accumulated during the Holocene. Fan volumes represent an estimated 18 to 53% of the total gorge volume. Allowing also for the volume of relatively small caliber material flushed through the system, 24 to 97% of the gorge volume is accounted for by an estimated minimum long-term Holocene rate of gorge excavation of 0.002 to 0.010 m3 m-1 yr-1 (minimum long-term Holocene gorge incision rate of 0.15-0.39 mm yr-1). Most of the remaining gorge volume is attributed to substantial pre-Holocene subglacial gorge incision by meltwater action. These rates of Holocene periglaciofluvial erosion of bedrock appear to exceed those characteristic of temperate fluvial systems unaffected by tectonic uplift. The implied rates of frost weathering (macrogelivation) are less than those under optimum conditions in arctic-alpine environments but support the efficacy of frost weathering in locations susceptible to the annual freeze-thaw cycle.
Analysis of topographic maps of rivers incised into dated Hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power. Field observations suggest that two processes may control Hawaiian channel downcutting: 1) stream power-dependent erosion, including abrasion of the channel bed by transported particles, and 2) step-wise lowering caused by knickpoint propagation. Modeling results indicate that a simple stream power-dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of Kauai channels but is insufficient to predict two other characteristic features: the upslope propagation of knickpoints and the straight 5-8° channel slopes below the knickpoints; thus more than this single transport law is apparently required to model bedrock channel incision. We propose that the boulder mantling of long channel reaches inhibits channel incision, reducing downcutting to a rate set by boulder weathering, breakdown and transport of the material, and perhaps by knickpoint propagation sweeping under the boulder armor. -from Authors
Several erosional forms on bedrock at Cantley, Quebec, differ from well-known glacial abrasion forms. The forms consist of obstacle marks, hollows, depressions, and channels, which are defined by sharp rims, smooth inner surfaces, divergent flow features, and remnant ridges. These forms are found on lee, lateral, and overhung rock surfaces. This assemblage of features is best explained by differential erosion produced by separation eddies along lines of reattachment. Rapid, sediment-laden, turbulent, subglacial meltwater flows likely produced the forms by corrasion and cavitation erosion. Sculpted fluvial forms in terrain subject to flooding in Australia are identical to some of the Cantley forms which confirms their formation by water erosion. Although glacial abrasion may not be eliminated as an explanation for sculpted forms, it is not necessary. Ice-abrasion forms, such as striations, and such plucked forms as gouges and crescentic fractures are also present at the Cantley site. -from Authors
Stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large-magnitude, low-frequency flow events have occurred within the past 500 to 1000 radiocarbon years B.P.-from Authors
Active valley meanders are identified and some of their physical and hydrological characteristics described. A detailed study of a small catchment shows the channel morphology and valley floor to be in equilibrium. The implications of the active valley meanders are considered in terms of previously described fossil forms in high latitudes and in terms of the normal development of a fluvial landscape with incising streams. The appearance of a flood-plain may be independent of the hydrological regime of a basin and the cause of a reduction in flow effectiveness rather than the result of a real reduction in flow. Effective flows in valley meander formation have a recurrence interval from 10 to 50 yrs, and this constitutes a chanel-full flow that shapes the meanders. Identification of such flows with a recurrence interval from 1 to 2 yrs (as for bankfull flow in alluvial streams) may have led to overestimates of the past discharges and climatic parameters.
Debris fans debouching into the bottom of Grand Canyon create rapids and flow separation in the Colorado River. The patterns of flow and the behavior of recirculation zones formed by flow separation are consistent throughout the Canyon's length. Zones of recirculating flow occur along the margin of channel expansions. Recirculation zones are comprised of one primary eddy; secondary eddies and areas of unorganized low velocity may exist upstream from the primary eddy. The longest recirculation zones are formed by channel constrictions of low width-to-depth ratio. Recirculation zones increase in length with increasing discharge. Sand bars form beneath recirculation zones, especially near separation and reattachment points. Reattachment bars project upstream from the reattachment point and underlie primary eddies. Separation bars mantle the downstream parts of the debris fans and form beneath secondary eddies and low-velocity areas. Sediment that forms reattachment bars is dominated by sizes characteristic of suspended load, while sediment that forms separation bars is finer. Reattachment bars are more common than separation bars, and both occur more frequently and are larger in wide reaches. The form and location of these bars is consistent with the location and behavior of stagnation points; however, the locations of these stagnation points change. Although velocity increases in the main channel at high discharges, velocities near the separation and reattachment points remain low. Sedimentation can occur in a bedrock gorge at high discharges and low transport rates, although the location of high-discharge sand bars may differ from those deposited at lower flows.
A flight of Quaternary coral reef terraces in New Guinea well dated by Th230 provides an excellent site for investigation of the evolution of certain landforms. This paper describes small valleys which cut back into the outer zone of broad reef terraces, and which are isolated from runoff from higher levels. Dissection of the youngest reefs shows that valley initiation occurs within 5,000-20,000 years of emergence. Measurements are given for a set of eight valleys from four terraces which emerged 70,000, 160,000, 210,000, and ≃320,000 years ago. The initial profiles are reconstructed from valley interfluves. Regression analysis shows that area removed above the thalweg is closely described by \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \b...
Experiments were conducted in a 4-m-wide flume to simulate recirculating flow and sedimentation in a lateral eddy within a channel expansion. The percentage of main stem sediment that was captured by the eddy decreased from 37% (when the eddy was empty) to 24% (when sand filled approximately 32% of the eddy volume). The reattachment bar within the eddy grew in an upstream direction, and the finest size sediment was deposited in the lee of the obstruction; both observations are consistent with field observations. Measurements of reattachment length during sediment transport (0.5–1.0 kg/s) at constant discharge (0.60 m3/s) show that reattachment length depends not only on characteristics of the expanding jet, but also on the topography of the channel bed downstream; reattachment length decreased when part of the channel expansion was filled by an aggrading midchannel bar. Comparison of these results with measurements in the Colorado River in Grand Canyon suggests that downstream channel irregularities play a large role in controlling the length of eddies in natural rivers.
A flight of marine terraces along the central California coastline
provides a unique setting for the study of topographic evolution.
Wavecut platforms mantled by 2-6 m of marine terrace cover deposits are
separated by 10-50 m tall decaying sea cliffs. Paleoclif edges become
more rounded with age, yet the details of the profiles and frequent
bedrock exposure on the upper slopes imply weathering-limited transport.
Five bedrock stream channels etched through the marine terrace sequence
display one to three distinct convexities in their longitudinal
profiles. Detailed hand level surveys of the hillslopes and of the
stream channel longitudinal profiles constrain hillslope evolution and
channel incision components of a numerical model of landscape evolution.
We account for regolith production as a function of regolith depth. In
accord with the field observation that hillslope processes are presently
dominated by the activities of burrowing rodents, the transport process
is taken to be diffusive. Stream incision is assumed to be controlled by
stream power, for which we use the surrogate of local drainage
area-slope product. Best fits of the numerical model to field data
imply: hillslope diffusivity is 10 sq m/kyr; regolith production rate on
bare bedrock is 0.3 m/kyr, and falls off rapidly with regolith cover;
and the constant controlling the efficiency of stream incision is 5 to 7
x 10(exp -7)/m/kyr.
A method of calculating the location of cavitation damage in spillways is presented. This method, together with experience curves, allows damage to be estimated as a function of duration of operation. Procedures are given to design aeration grooves which can protect spillways from cavitation damage.
Sonic velocities of geologic fluids, such as volcanic magmas and geothermal fluids, can be as low as 1 m/s. Critical velocities in large rivers can be of the order of 1–10 m/s. Because velocities of fluids moving in these settings can exceed these characteristic velocities, sonic and supersonic gas flow and critical and supercritical shallow‐water flow can occur. The importance of the low characteristic velocities of geologic fluids has not been widely recognized, and as a result, the importance of supercritical and supersonic flow in geological processes has generally been underestimated. The lateral blast at Mount St. Helens, Washington, propelled a gas heavily laden with dust into the atmosphere. Because of the low sound speed in this gas (about 100 m/s), the flow was internally supersonic. Old Faithful Geyser, Wyoming, is a converging‐diverging nozzle in which liquid water refilling the conduit during the recharge cycle changes during eruption into a two‐phase liquid‐vapor mixture with a very low sound velocity. The high sound speed of liquid water determines the characteristics of harmonic tremor observed at the gyeser during the recharge interval, whereas the low sound speed of the liquid‐vapor mixture influences the fluid flow characteristics of the eruption. At the rapids of the Colorado River in the Grand Canyon, Arizona, the channel is constricted into the shape of a converging‐diverging nozzle by debris flows that enter from tributary canyons. Both subcritical and supercritical flow occur within the rapids. The transport capacity in the rapids can be so great that the river contours the channel to a characteristic shape. This shape can be used to interpret the flood history of the Colorado River over the past 10³–10⁵ years. The unity of fluid mechanics in these three natural phenomena is provided by the well‐known analogy between gas flow and shallow‐water flow in converging‐diverging nozzles. This paper is not subject to U.S.
Located on the Colorado River, 15 miles upstream of Lee's Ferry, Arizona, the 710 ft high concrete arch dam has an open-channel flow-type spillway in each abutment. Each spillway consists of a 41 ft diameter concrete lined section inclined at 55 degrees, a vertical bend, some 1,000 ft of near horizontal tunnel and a flip bucket at the downstream tunnel portal. Together, the two spillways are designed to handle flows up to 280,000 cfs. When spring runoff rampaged down the Colorado River in 1983, Glen Canyon Dam was one of its victims. The Bureau of Reclamation had only 10 months to make repairs in time for the following spring's flows.
A global biogeochemical cycle is a conceptual model of the pathways and flows of individual chemical elements or their compounds in the surface environment of the Earth. The flows of materials on the Earth’s surface are driven by processes of diverse nature, such as: the major physical and tectonic forces that control the shapes and elevation of the continental plates; the major climatic processes that are responsible for the global water flows and temperature distribution over the Earth’s surface; the chemical and microbiological processes that result in dissolution of some crustal rock minerals and formation of other sedimentary minerals; the transport of the continental crustal materials by wind, rivers and ground-waters, and their ultimate delivery to the oceans.
This steep section of river channel roughly 335m in length, has about 600 potholes with a mean diameter of just over 1m. Relationship between pothole size and depth suggests that their growth follows an allometric law. -K.Clayton
Waterfalls are the sites of the greatest dissipation of energy in streams, and they also provide a key to deciphering the denudation history of river systems. Examples from SE Australia demonstrate that many actively retreating waterfalls are not undercut, but are buttressed outwards at their base. Analysis of these examples directs attention to: 1) the relationship between morphology and stress distribution in waterfalls, 2) structural and lithological controls of mass collapse on the face of the waterfalls, 3) the variety of fluvial and groundwater forces operating on the waterfalls. Field observations of waterfalls are linked here to considerations of rock mechanics and to research in the engineering of large dams. -from Author
Emphasizes the importance of resistance factors as well as dynamic ones in establishing the erosive effects of floods. Especially important is the distinction between alluvial channels and nonalluvial (resistant boundary) channels. -from Editors
Sole markings have been produced by pouring suspensions of plaster-of-paris and water into tanks containing settled clay or gelatine. The observed processes lead to new or modified hypotheses concerning the origin of dendritic ridges, longitudinal furrows, certain tool marks and frondescent marks. In other experiments, load structures were produced by sieving sand and plaster-of-paris mixtures on to settled clay of varying densities.
Large floods in narrow, steep mountain valleys are more likely to leave
a lasting imprint on the landscape than floods of comparable magnitude
in broad, low-gradient valleys. Longitudinal variations in valley width
and channel orientation are potentially more important than average
width in determining location and severity of flood impacts. In order to
explore the hydraulic implications of locally-varying boundary
conditions, a set of eight finite-element meshes representing
alternative versions of valley topography was designed for use with
numerical flow models. A two-dimensional depth-averaged flow model
(HIVEL2D), based on the conservation form of the shallow-water
equations, was used to simulate flood flows routed through each design
reach. Results, presented in the form of velocity fields, water-surface
elevations, and calculated Froude number and shear stress distributions,
indicate that valley expansions, channel bends along straight valleys,
and flow obstructions may have a profound impact on the pattern of flow
and geomorphic consequences. Maximum calculated shear stress on the
floodplain along the outside bank at a channel bend is comparable to
maximum channel shear stress along a straight, narrow canyon reach.
Maximum shear stress on the floodplain where a valley expansion
coincides with a channel bend is as much as three times greater than the
maximum channel shear stress along the canyon reach and 5-7 times
greater than the maximum floodplain shear stress along a constant-width
valley with a straight channel. Comparison of the predicted spatial
distributions of hydraulic parameters with photointerpretation of flood
impacts suggests that hydraulic modeling may be useful for predicting
spatial patterns of erosion and deposition resulting from a catastrophic
flood.
We present a new compilation of estimates of modern rates of mechanical
and chemical denudation for externally drained basins exceeding 5 x
10(exp 5) sq km in area. These estimates are based on sediment and
solute load data selected in order to represent natural rates as far as
possible. Chemical denudation rates have been calculated by deducting
the nondenudational component of solute load. Mechanical denudation
rates range from 1 mm/kyr for the St. Lawrence and Dnepr basins to 670
mm/kyr for the Brahmaputra basin. Chemical denudation rates vary from 1
mm/kyr (Kolyma, Niger, Nile and Rio Grande basins) to 27 mm/kyr (Chiang
Jiang basin). Relationships between denudation rates and a range of
morphometric, hydrologic, and climatic variables are investigated
through correlation and regression analysis. Morphometric variables,
such as mean local relief, are accurately calculated for large basins
for the first time by using the National Geophysical Data Center
10-minute topographic database. Although direct comparisons cannot be
made, it appears that rates of basin denudation derived from present-day
mass flux estimates are not, overall, significantly different from
estimates of long-trm rates based on sediment volume and
thermochronologic data. It therefore appears that the key factors
identified as controlling denudation rates here are also applicable to
the geological time spans relevant to the interaction between tectonic
and denudational processes.
Analysis of the Sabie River in the Eastern Transvaal has facilitated the development of a hierarchical classification suitable as a framework for fluvial research in a variety of disciplines and at a variety of scales. The system, which has its foundations in geomorphology, is based on existing discipline-specific classifications. It provides for the characterisation of a river in relation to geomorphological units (the smallest fluvial division in the classification), channel types (combinations of geomorphological units), reaches (specific assemblages of channel types), macro-reaches (reaches in a specific association) and zones (with boundaries at major breaks in slope or geological contacts). The classification has the advantage of readily being adaptable to other river systems, particularly those which, like the Sabie, do not conform to rivers which are well-documented in the fluvial research literature.
Potholes preserved along abandoned sections of bedrock-bounded rivers should provide significant paleohydraulic data if the conditions for pothole formation are understood. In a prototype study, we recognize two kinds of potholes along the paleo-Potomac River near Great Falls, Virginia. Vertical potholes, drilled into the strath floor, are circular in plan and are interpreted to form at sites of small stream depths and high flow velocity. Lateral potholes, noncircular with overhanging roofs, are found on flanks of rock obstacles. They are interpreted to be eroded by sediment-laden vertical flow near the air-water interface. The vortices circulate water in the horizontal plane, and begin to form near the free surface in flow separation zones downstream of the obstacles. The geometry of flow separation zones and thus the location of the lateral potholes are scaled by flow Reynolds number. Therefore, lateral potholes could be used to reconstruct paleoflow depths and velocities if parts of the channel bed adjacent to the flow obstruction are preserved.
The 1997 William Bowie Medal, given by AGU for outstanding contributions to fundamental geophysics and for unselfish cooperation in research, was presented to Raymond Hide at the AGU Spring Meeting Honor Ceremony on May 28 in Baltimore. The award citation and Hide's response are given here.
Geologic controls on knickpoint development and the mechanics of channel-bed degradation were elucidated by comparison of downcutting streams in south-central Indiana developed in lithologically "homogeneous' carbonate units and layered siliciclastic strata. Knickpoints formed in a single lithologic unit are stepped; each step consists of a knickpoint face and tread. The height of each step is equal to and controlled by bed thickness. Channel incision occurs by the erosion of bedrock strata along bedding plane discontinuities and typically proceeds in the direction of dip. Geologic controls imposed on degradation lead to distinctive types of channel bed morphology. It is generally agreed that downcutting streams are disequilibrium systems that exhibit rapidly evolving landforms. However, the downcutting channels appear to develop an equilibrium form (determined by bedrock geology) maintained until longer-term (hundreds of years) profile adjustments to base level lowering can occur. -from Author
At Crystal Creek, a debris fan was emplaced in 1966, constricting the channel of the Colorado River to about 0.25 of its upstream width between 1967 and 1983, forming a major rapid. The hydraulics of Crystal Creek rapid are described, and an analysis is presented to support the hypothesis that the major wave in the rapid was a normal wave (one type of hydraulic jump). Hydraulic jumps rarely occur in natural river channels with erodible beds, but one was present at Crystal Rapid because of the unusually severe constriction of the Colorado River by the 1966 debris fan. A quantitative model for river debris fan shapes is proposed and is used to estimate prehistoric flood levels from the observed constrictions: the 0.5 value of river constriction found at the more mature debris fans in the Grand Canyon suggests that peak flood discharges of approximately 11 320 m3/s have occurred. -from Author
1. Potholes develop only in streams that are actively eroding in fairly well consolidated rock. 2. An initial hollow in the bedrock is necessary to permit of the primary collection of the sediment and stones that are to be the tools with which the pothole is ground out. 3. The stream must carry at least a moderate amount of material to be used as grinding tools. Streams heavily laden with sediment tend to deposit rather than erode and too much material chokes the initial depressions. 4. The initial hollow may be originated by any one of the following factors: irregularity in bedding; ripple marks; lenticular concretionary structure; solution irregularities; joint planes. 5. The inception of a given pothole may be due to one or more of these structural weaknesses. 6. The larger holes seem almost invariably to be inherited from preexisting conditions. 7. The later development of a hole is influenced by the following factors, among others: the volume and velocity of the stream; the direction of currents; nature, structure and position of the rock, the hade of joint planes; union of two or more holes. 8. Most of the erosion of the holes is apparently accomplished during flood stages.
The Burdekin Gorge of northeastern Australia lies within the seasonal tropics and is characterized by high discharge variability. Slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1200 yrs. These floods range in magnitude from 11 000 to 30 000 m3s-1 and are characterized by large downstream variations in hydraulics. Downstream fluctuations help to explain the location of boulder bars, high flood levees, small-scale erosional features in the bedrock, and the formation of inner channels. Boulder bars and flood levees form where shear stress and stream power decrease due to channel widening. Small-scale erosional features, such as potholes and troughs, are best developed at sites of channel constriction and associated increases in shear stress and stream power. The development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure, and by flow hydraulics. -from Author
Paleoflood hydrologic analysis of the deposits, employing step-backwater flow modeling, allows quantitative estimates to be made of geomorphologically significant flows.-from Authors
A series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal-profile evolution. Knickpoints are created by successive drops in base level, which simulate intermittent uplift. Channel morphology and flow characteristics vary systematically along a knickpoint reach. Knickpoints are rapidly destroyed through the process of 1) knickpoint replacement and 2) subsequent knickpoint inclination. Within experimental limits, this study supports the notion that knickpoints do not undergo parallel headward retreat in homogeneous bedrock, given intermittent uplift, except in those possible cases of extensive and pervasive bedrock jointing.-from Author
We feel that the statistical procedure employed by Keller & Melhorn (Geo Abstracts 78E/2230) is inappropriate and that consequently their conclusion is not justified by the results. Indeed, when appropriate statistical methods are used, the null hypothesis that the aluvial and bedrock streams have equal mean pool spacings is rejected at the 0.05 level. This is the opposite of their result. -from Authors
Quantitative analysis of the spacing of pools in bedrock and alluvial stream channels in California, Indiana, Virginia, and North Carolina suggest that the tendency for streams to meander in the vertical (or third) dimension, as in the horizontal plane, is a fundamental characteristic of many streams that is independent of material type. Simple linear-regression and correlation models reveal that approximately 70% of the variability of the spacing of pools can be explained by the variability of channel width. Analysis of the spacing of 251 pools in eleven streams, utilizing the Kolmogorov-Smirnov goodness of fit test and one-way analysis of variance suggests that the hypothesis that the data from bedrock and alluvial channels are from the same population cannot be rejected at the 0.05 level of significance. Morphologic maps and field observations of stream channels incised in sandstone, limestone, metavolcanic rock, and syenite suggest that although these streams have much in common with alluvial stream channels, there exist considerable differences in certain aspects of channel morphology. This results because bedrock control of morphology locally may be more significant than the effects of general processes that tend to produce rhythmic channel forms such as pools and riffles. However, local controls tend to mask rather than destroy the effects of more general processes that produce the third dimension of meandering streams.
In a flume study of knickpoint development in stratified sediment, two thin sand beds were intercalated between beds of cohesive material. The sand beds acted as knickpoint-forming horizons on which stepped knickpoints were developed and maintained as they moved upstream, creating an apparently stable form consisting of a channel-in-channel system. The system had four basic elements: (1) an aggraded reach upstream of the knickpoint, terminating on the downstream end at a fill-incision transition zone; (2) an oversteepened reach just above the knickpoint face; (3) the knickpoint face; and (4) an incising reach, often covered by moving sediment between successive knickpoints. Apparently the presence of a knickpoint-forming horizon reduces knickpoint retreat rates, which were much lower in this study than those observed in an experiment with cohesive material but no intercalated sand beds and a similar discharge. Also, the channel-in-channel system that results from stratification is apparently fairly insensitive to the effects of changing discharge.
Experiments in a 60-ft-long tilting, recirculating flume were conducted to study river incision in simulated bedrock, which was a mixture of sand and kaolinite. Slope, sediment feed, and water discharge were controlled during the development of four channels. After an increase of slope at constant discharge, the following sequence of erosion occurred: (1) development of longitudinal lineations, ripples, and potholes; (2) enlargement of the lineations into prominent grooves; (3) coalescence of the grooves into a single, narrow, and deep inner channel. The inner channel was incised below base level and a sequence of bedrock lows and highs formed. Bedrock scour lows had a weakly regular spacing during incision and a randomly clustered spacing following aggradation. Incision around stabilized alternate bars in a sinuous sand-bed channel resulted in destruction of the bars and maximum scour where the flow was locally constricted. In an initially sinuous bedrock channel, scour depth was greater at bends than at crossings. Provided all of the available sediment load was entrained, the bed was eroded more at convex banks of bends than at concave banks. However, after deposition occurred, the maximum erosion shifted to the concave bank. These results indicate that lateral or vertical incision at bends of incised meandering streams is controlled by the amount of available sediment load entrained by channel-forming discharges. The results also suggest that incised meanders superposed from an earlier pattern on a peneplain should rarely occur in nature, if epeirogenic tilting caused the incision. Representing the locus of deepest scour by a bedrock stream, inner channels may be the locations of heavy mineral concentrations as well as gravel deposits. The experimental results help to explain inner channels discovered at damsites, provide an explanation for some paleochannels in California and South Africa, and suggest that, like the Dalles type of river channel, bedrock floors of valleys will be uneven in both transverse and longitudinal sections.
Six flume runs using a substrate of 30% bentonite and 70% fine sand were used to examine channel incision into a cohesive, homogeneous substrate. Gradients for the various runs were 1%, 2%, 5%, 10%, and 20%; other variables remained constant (discharge = 2 L/s, sediment discharge averages 3 g/s, run time = 15). With increasing flume gradient, bed forms change from parallel longitudinal grooves to a broad, shallow channel with weakly undulating bed and walls and then to a progressively deeper and more undulating inner channel. Downstream changes in reach-scale (several channel widths) gradient may thus be responsible for downstream variability of erosional bed forms in the absence of changes in other controlling variables. For bedrock channels, it is not necessary to invoke substrate variability to explain the presence of erosional bed forms and associated variations in cross-sectional area. Erosional bed forms may approximate an equilibrium state in which negligible change occurs in bed-form dimensions or shape with time. Downstream alternations between solely erosion and deposition of an alluvial veneer over incisional features occur along channel reaches of constant gradient; the downstream length of these alternations is inversely proportional to gradient.
A recent debris flow in Monument Creek illustrates the nature of debris
flows in small tributaries and their hydrologic effects on the Colorado
River in Grand Canyon National Park. A debris avalanche originated in
the Permian Esplanade Sandstone of the Supai Group during intense
rainfall on July 27, 1984, and fell 600 m into Monument Creek, forming a
7-m-high barrier across the channel. The subsequent debris flow traveled
4.5 km to the Colorado River and achieved velocities of 3.4 to 4.0 m/s
and a peak discharge of 100 to 120 m3/s. The flow consisted
of a main pulse followed by subsequent pulses of debris flow or
hyperconcentrated flow. The main pulse moved boulders as large as 2.7 m
in diameter, and deposition at the mouth of Monument Creek enlarged the
fan surface and significantly constricted the Colorado River. Most of
the major rapids on the Colorado River in Grand Canyon National Park
appear to be maintained by episodic debris flows.
Three channels on the Boso Peninsula, Japan, were examined to assess how cross-sectional to reach-scale channel morphology relates to substrate variables, and how cross-sectional to reach-scale erosional patterns relate to basinscale longitudinal profile. Two of the channels, Shichiri-gawa and Torii-zawa are deeply incised into interbedded sandstone and mudstone, whereas the third channel, Futama-gawa, is incised into mudstone. Reach-scale channel morphologies include knickpoints, chutes and pools, plane beds, and both transverse and longitudinal bed erosional features. Step-backwater modeling of a September 1996 typhoon-related flood flow along each channel indicated that unit stream power is highly variable downstream. Stream power minima are associated with bends and with coarseclast deposition. Magnitude of erosional bed features correlates with hydraulics and flow energy expenditure along Futama-gawa, and with variability in substrate resistance along Shichiri-gawa and Torii-zawa. Thickness and orientation of the bedrock strata strongly control erosional patterns along Shichiri-gawa and Torii-zawa. Reaches dominated by sandstone have shallower pools or a plane bed. Reaches with a higher proportion of resistant mudstone and with strata that strike across the channel have more knickpoints and a stepped morphology. Comparison of main stem and tributary drainage areas and gradients indicates that a simple stream power-dependent erosion law does not fully account for the evolution of longitudinal profiles on the Boso Peninsula. Ability to incise does not necessarily increase with stream power or basin area for channels on the Boso Peninsula because of substrate influences on bed erosional forms and on coarse clast resistance.
Piccaninny Creek in northwestern Australia drains approximately 60 km2 of high relief sandstone and conglomerate terrain. The drainage basin lies within the seasonal tropics and is characterized by ephemeral, high-magnitude flows. Flood flows of 50 to 100 m3s-1 are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek. These erosional features occur in two basic forms: as parallel longitudinal grooves, and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep, narrow, inner channel. The longitudinal grooves are assumed to result from longitudinal vortices, while the erosional sequences, which closely resemble von Karman vortex streets, are interpreted to result from turbulent vortices shed off irregularities of the channel bed. The longitudinal grooves and the inner-channel sequence alternate with each other and with gravel-floored depositional reaches downstream along the channel. The locations of these three types of channel beds do not correlate with channel-substrate characteristics like rock strength, lithology, or structural variability. The locations do not appear to be related to channel gradient, however. The number and length of channel segments with inner channels are greatest at, and immediately upstream of, a 1 km length of steeper channel interpreted as a knickzone. -Author
The manner in whichh a bed of weakly cohesive mud is eroded by a plain, turbulent flow of water is found experimentally to depend upon the severity of the flow over the bed. The major structures left behind on a weakly cohesive mud bed after a prolonged period of erosion also depend upon the severity of the flow. With ascending severity we obtain: (1) longitudinal rectilinear grooves, (2) longitudinal meandering grooves, (3) flute marks, (4) transverse erosional markings. The more severe flows also give rise to shear wrinkles and to erosional structures of a less well-defined nature. Of the four structures enumerated, all but longitudinally rectilinear grooves have counterparts in the fossil record. Of particular importance is evidence confirming that the flute marks are associated with separated flows and are due to processes of flow separation and reattachment.
The height and spacing of channel bed-steps formed in boulder and bedrock in the ephermeral channels of the Nahal Yael watershed show a strong correlation with the channel slope. The distance between steps is inversely proportional to slope at slopes of 0 to 20%, but then remains fairly constant at higher slopes. Step height increases proportionally with channel slope. These bed-steps are similar to those described on perennial channels elsewhere, where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps. However, the ephemeral flows in the tributary channels of the Nahal Yael system are not sufficient to submerge the clasts forming the boulder steps. Using Rouse's criteria, channel bed-steps are creating maximum flow resistance at slopes of 12 to 50% along the channels of Nahal Yael. Using Davies' criteria, bed-steps are creating maximum flow resistance at slopes of 3 to 10%. The bed-steps also relate to sediment transport; Nahal Yael has a low sediment yield by world standards, and the increase in flow resistance due to step formation may serve to increase sediment movement. Both the boulder and bedrock steps at Nahal Yael are hypothesized to reflect the scales of flow turbulence along the channels. We were unable to evaluate the role of flow volume or boundary roughness in controlling these scales, but channel slope appears to exert some control on the scale of turbulence, based on the correlation between slope and bed-steps. The hypotheses developed from field evidence at Nahal Yael should be testable in flume simulations.
Cataclysmic releases from the glacially dammed Lake Missoula, producing exceptionally large floods, have resulted in significant erosional processes occurring over relatively short time spans. Erosional landforms produced by the cataclysmic Missoula floods appear to follow a temporal sequence in many areas of eastern Washington State. This study has focused on the sequence observed between Celilo and the John Day River, where the erosional features can be physically quantified in terms of stream power and geomorphic work. The step-backwater calculations in conjunction with the geologic evidence of maximum flow stages, indicate a peak discharge for the largest Missoula flood of 10 × 106m3s−1. The analysis of local flow hydraulics and its spatial variation were obtained calculating the hydrodynamic variables within the different segments of a cross-section. The nature and patterns of erosional features left by the floods are controlled by the local hydraulic variations. Therefore, the association of local hydraulic parameters with erosional and depositional flood features was critical in understanding landform development and geomorphic processes. The critical stream power required to initiate erosion varied for the different landforms of the erosional sequence, ranging from 500 W m−2 for the streamlined hills, up to 4500 W m−2 to initiate processes producing inner channels. Erosion is possible only during catastrophic floods exceeding those thresholds of stream power below which no work is expended in erosion. In fact, despite the multiple outbursts which occurred during the late Pleistocene, only a few of them had the required magnitude to overcome the threshold conditions and accomplish significant geomorphic work. © 1997 by John Wiley & Sons, Ltd.
A study of the distribution and geometry of incised meanders in 64 reaches encompassed approximately 600 km of the Green, Colorado, and San Juan Rivers in the central Colorado Plateau. The sinuosity, average planform size, and average cross-sectional symmetry of each reach were determined by map measurements and by spectral analysis of the curvature series for each reach, as determined from interpolations of the digitized traces of the channels. Possible controlling variables examined, including average channel gradient, drainage area, average bedrock erodibility, and bedrock structure, were compiled for each reach, using available maps. Gradients in the studied reaches are significantly correlated with bedrock type. Sinuous incised channels are generally found in low-gradient reaches. In the San Juan River, the channel is sinuous where it flows against the bedrock dip and generally straight in reaches where flow is downdip. This correlation is weak in the Green and Colorado Rivers. The average meander size of the sinuous reaches, as described by the median curvature value for each reach, is generally less in steeper reaches than in low-gradient reaches, although the relation of bend size to controlling variables is much less clear than for sinuosity or cross-section shape. Most meander cross sections in the area are relatively symmetrical, but highly ingrown forms are also present. In general, symmetric bends are associated with resistant bedrock units, whereas ingrown forms develop in massive sandstone and in highly erodible bedrock. Gradient significantly influences the distribution of ingrown bends, with asymmetric meanders concentrated in reaches of low average gradient; this correlation is stronger than that between cross-section shape and lithology itself. Incised meanders of the central Colorado Plateau are probably at least partly inherited from ancestral streams of unknown age that flowed across the area before the present canyons were cut. Correlation between meander distribution and regional structures suggests that the general location of low-gradient sinuous reaches has probably not changed during the incision of the present canyons. Incised meanders, however, are clearly able to modify their geometry in response to changes in bedrock resistance, as indicated by the strong correlation between bedrock type and cross-section symmetry. One mechanism of modification is abandonment of bends, which is documented by 18 cutoff meanders in the study area.