Regional and local controls on the spatial distribution of bedrock reaches in the Upper Guadalupe River, Texas

While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types. We find that local disturbances, interaction of the channel and valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars, and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river.     

High-Resolution Analysis of Debris Flow–Induced Channel Changes in a Headwater Stream,Ashio Mountains,Japan∗

Coupled hillslope and channel processes in headwater streams (HWS) lead to rapid changes in channel dimensions. Changes in channel size and shape caused by a debris flow event along the length of a headwater stream in the Ashio Mountains, Japan, were captured with the aid of repeat high-definition surveys using terrestrial laser scanning (TLS) techniques. The HWS was classified into three distinct reaches below the debris flow initiation zone. A large knickpoint separated an upper bedrock reach from a colluvial reach along the midsection of the drainage. The colluvial reach transitioned to a lower bedrock reach that terminated at the master stream. Cross-sectional and morphometric analyses revealed no statistically significant changes in channel size or shape along the upper bedrock reach. Debris flow erosion generated significant differences in channel size and shape along a colluvial reach. Sediment bulking associated with erosion along the colluvial reach led to increases in channel size along the lower bedrock reach, but no statistical differences in channel shape. Morphometric analyses from the TLS point cloud revealed that debris flow erosion produced a distinct nonlinear change in channel dimensions in the downstream direction within the HWS. Variations in channel substrate along the length of HWS contributed directly to this nonlinear response. The episodic nature and nonlinearity of erosion associated with the current debris flow event highlights the importance of debris flows in general in understanding the transport of sediment, coarse to fine particulate organic material, and large woody debris, which are critical to the long-term management of riverine environments. TLS sampling methods show promise as one component of a multianalytical approach needed to continuously monitor and manage the dynamics of HWS.     

Insights on the kinematics of deep-seated gravitational slope deformations along the 1915 Avezzano earthquake fault (Central Italy), from time-series DInSAR

Small meandering channels of about 1 m wide on an intertidal mudflat in the Westerschelde estuary the Netherlands) were studied with the aim to improve understanding of the effect of highly cohesive bed and bank sediment on channel inception and meander geometry and dynamics. The study is supported by experiments and modelling. The estuarine meandering channels are less dynamical than alluvial meandering rivers, and the dynamics are more localised. Moreover, the high thresholds for bed sediment erosion and for bank failure lead to two processes, uncommon in larger rivers, that cause most of the morphological change. First, the beds of the channels are eroded by backward migrating steps under hydraulic jumps, while the remainder of the bed surface along the channel is hardly eroded. Second, channel banks erode i) where eroding steps locally cause undercutting of otherwise stable channel banks and ii) in very sharp bends where the flow separates from the inner-bend channel boundary and impinges directly on the bank on the opposite side of the channel. Further morphological change is probably induced by rainfall splash erosion and by storm waves that weaken the mud, and by large mud fluxes from the estuary. The steps were successfully reproduced in laboratory flume experiments. An existing model for step migration predicted celerities consistent with field and laboratory observations and demonstrated a strong dependence on the threshold for erosion. Bank stability models confirm that banks and steps only fail when undercut and weakened by waves, rain or excess pore pressure in agreement with observations. The effects of a high threshold for bank erosion was implemented in an existing meander simulation model that reproduced the observed locations of bank erosion somewhat better than without the threshold, but flow separation and its effect on meander bends remains poorly understood.     

Woody vegetation and channel morphogenesis in low-gradient, gravel-bed streams in the Ozark Plateaus, Missouri and Arkansas

Woody vegetation affects channel morphogenesis in Ozark streams of Missouri and Arkansas by increasing local roughness, increasing bank strength, providing sedimentation sites, and creating obstructions to flow. Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area. In upstream reaches, streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions. At drainage areas of greater than 100–200 kM², Ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates, referred to as disturbance reaches and stable reaches, respectively. Whereas stable reaches can develop stable forested floodplains (if they are not farmed), disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes.Disturbance reaches can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectional lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload-transporting events. The same floods caused up to 20 m of erosion of adjacent cutbanks, scoured non-vegetated areas between vegetation bands, and increased thalweg depth and definition. In high-energy (or riffle) zones, channel movement is dominantly by avulsion. In these zones, vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them. Woody vegetation on islands creates steep, root-defended banks that contribute to narrow channels with high velocities.Calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age, mainly through decreasing stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age.     

Systematic variation of bedrock channel gradients in the central Oregon Coast Range: implications for rock uplift and shallow landsliding

In tectonically active regions, bedrock channels play a critical role in dictating the pace of landscape evolution. Models of fluvial incision into bedrock provide a means of investigating relationships between gradients of bedrock channels and patterns of active deformation. Variations in lithology, orographic precipitation, sediment supply, and erosional processes serve to complicate tectonic inferences derived from morphologic data, yet most tectonically active landscapes are characterized by these complexities. In contrast, the central Oregon Coast Range (OCR), which is situated above the Cascadia subduction zone, has experienced rock uplift for several million years, did not experience Pleistocene glaciation, boasts a relatively uniform lithology, and exhibits minor variations in precipitation. Although numerous process-based geomorphic studies suggest that rates of erosion across the OCR are relatively constant, it has not been demonstrated that bedrock channel gradients in the region exhibit spatially consistent values. Analysis of broadly distributed, small drainage basins (5–20 km) in the central OCR enables us to explore regional variability in bedrock channel gradients resulting from differential rock uplift or other sources. Consistent with previous studies that have documented local structural control of deformed fluvial terraces in the western portion of our study area, our data reveal a roughly 20-km-wide band of systematically elevated channel slopes (roughly twice the background value), roughly coincident with the strike of N–S-trending mapped folds. Although many factors could feasibly generate this pattern, including variable rock strength, precipitation gradients, or temporal or spatial variations in forearc deformation, the elevated bedrock channel slopes likely reflect differential rock uplift related to activity of local structures. Importantly, our analysis suggests that rock uplift and erosion rates may vary systematically across the OCR. Although our calculations were focused on the fluvial-dominated portion of study basins, our results have implications for upstream areas, including unchanneled valleys that often serve as source areas for long-runout debris flows. Zero-order basins (or topographic hollows) within the N–S-trending band of elevated channel slopes tend to be steeper than adjacent areas and may experience more frequent evacuation by shallow landsliding. Thus, this region of the OCR may be highly sensitive to land use practices and high-intensity rainstorms.     

The influence of river training on mountain channel changes (Polish Carpathian Mountains)

The purpose of this paper is to explain the influence of river training on channel changes in mountain rivers. Also considered are the causes of failure of different training schemes. The research was conducted on the regulated Mszanka and Porębianka Rivers, belonging to the Raba River drainage basin in the Polish Flysh Carpathian Mountains. Channel mapping carried out in 2004 drew attention to the contemporary morphology of the channels and the development of their dynamic typology. General changes in channel morphometry and land cover were identified by comparing cartographic sources from various years. Archive material from Cracow's Regional Water Management Authority (RZGW) was used to analyse the detailed channel changes caused by each regulation structure. The material consisted of technical designs of individual training works, as well as plans, longitudinal profiles and cross-sections of trained channel reaches. A series of minimum annual water stages at the Mszana Dolna gauging station was used to determine the tendency of channel bed degradation over 53 years. During the first half of the 20th century, the middle and lower courses of the Mszanka and Porębianka Rivers had braided patterns. The slopes, mostly covered with crops, were an important source of sediment delivery to the river channels. Today, both channels are single-threaded, narrow and sinuous. Downcutting is the leading process transforming the channels. They cut down to bedrock along about 60% of their lengths. The main type of channel is an erosion channel, which occurs also in the middle and lower courses of the rivers. The channel sediment deficit is an important cause for river incision. Sediment supply to the channels was reduced after a replacement of crops on the slopes by meadows or forests. Gravel mining has also caused channel downcutting. The rapid channel changes began after 1959, as systematic training was introduced. Channel regulation seems therefore to be a major factor determining channel adjustment. Debris dams and groynes were built before 1980 and these caused the greatest change of channel pattern, increase of channel gradient and magnitude of river incision. After that date the measures mostly involved drop structures. From then on, the rate of downcutting decreased considerably, but has not ceased. The rivers continued to incise until bedrock was exposed or training structures were destroyed. After that, a tendency to lateral migration and local braiding were observed in the deepened channel. The channels displayed a tendency to return to their morphology and dynamic from before the training. The results demonstrate that river training distorts the equilibrium of channel systems. A channel becomes divided into artificial reaches, which later follow different evolutionary patterns. Most training schemes on mountain channels are ineffective in the long term, as river managers seem to consider a channel at a reach scale only. Individual channel reaches, however, are not independent but rather form a system that must be managed at the entire channel scale.     

Three-dimensional analogue modelling of an alluvial basin margin affected by hydrological cycles: processes and resulting depositional sequences

The dynamics between sediment erosion and accumulation at an alluvial basin margin affected by changes in the surface hydrology are explored using scaled analogue models produced in a flume. The presented results differ from previous counterparts in that accumulation or erosion has not been forced at a spreading outlet, but occurred at a slope change produced by previously accumulated sediment. Cyclical upstream incision produced by increased stream discharge generated incised valleys, and these were subsequently filled by sediment carried by less efficient streams generated during the low discharge period. High resolution mapping using 2.5 mm contour maps allowed the study of sediment accumulation and terrain modelling. The results of three selected experiments are analysed. The only variable explored was discharge. The basin margin was simulated by a ramp inserted in a low sloping flume, consisting of two segments of different slopes selected to emulate high and low efficiency flume fans produced elsewhere. Water and fine‐medium sand entered the ramp along a narrow (0.1 m) channel and flow expanded but without occupying the complete 1.2 m flume width. Flows were highly concentrated and noncohesive. Fan‐like accumulation (slope: 0.11) began during low discharge (LD) periods at the ramp slope break, and proceeded upstream, onlapping quickly at first, but shifting to mostly progradation at the end of the period. High discharges (HD) usually generated two or three incised channels at the beginning of the period, but one of them prevailed and rapidly eroded parts of the LD fan and moved the sediment to a more distal low‐sloping fan (slope: 0.045). Both LD and HD fans passed downstream into a system of small parallel channels resembling a braided alluvial plain ending in sediment lobes. The mapping of the accumulated sediment during the various periods allowed calculation of sediment budgets for the entire flume. The stratal architecture of the deposits was investigated along five parallel trenches cut after experiment termination. The regression analysis of depositional profiles at fan‐like features (expanding flow) and at braided plains (parallel flow) indicated that these fan‐like systems are linear and dependent on applied discharge, while the latter showed an exponential decrease of slope downstream, with a starting value set up by the fan slope. Two main types of stratigraphic units were generated, the LDST and HDST (system tracts). The LDST has a nonerosive base over ‘bedrock’ and the previous HDST, filling proximal erosional topography and prograding as well, generating an onlap–downlap array. Its geometry is highly variable and dependent on pre‐existing topography. The HDST base is an important erosive surface comparable to sequence boundaries. However, there are places without erosion due to a marginal position with respect to the main stream. Indeed, the results suggest that the three‐dimensional variability of erosion and depositional processes might produce very different architectures along the same basin margin.     

Anabranching in mixed bedrock-alluvial rivers: the example of the Orange River above Augrabies Falls, Northern Cape Province, South Africa

Anabranching is characteristic of a number of rivers in diverse environmental settings worldwide, but has only infrequently been described from bedrock-influenced rivers. A prime example of a mixed bedrock-alluvial anabranching river is provided by a 150-km long reach of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Here, the perennial Orange flows through arid terrain consisting mainly of Precambrian granites and gneisses, and the river has preferentially eroded bedrock joints, fractures and foliations to form multiple channels which divide around numerous, large (up to 15 km long and 2 km wide), stable islands formed of alluvium and/or bedrock. Significant local variations in channel-bed gradient occur along the river, which strongly control anabranching style through an influence on local sediment budgets. In relatively long (>10 km), lower gradient reaches (<0.0013) within the anabranching reach, sediment supply exceeds local transport capacity, bedrock usually only crops out in channel beds, and channels divide around alluvial islands which are formed by accretion in the lee of bedrock outcrop or at the junction with ephemeral tributaries. Riparian vegetation probably plays a key role in the survival and growth of these islands by increasing flow roughness, inducing deposition, and stabilising the sediments. Less commonly, channels may form by eroding into once-continuous island or floodplain surfaces. In shorter (<10 km), higher gradient reaches (>0.0013) within the anabranching reach, local transport capacity exceeds sediment supply, bedrock crops out extensively, and channels flow over an irregular bedrock pavement or divide around rocky islands. Channel incision into bedrock probably occurs mainly by abrasion, with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting. Mixed bedrock-alluvial anabranching also occurs in a number of other rivers worldwide, and appears to be a stable and often long-lived river pattern adjusted to a number of factors commonly acting in combination: (1) jointed/fractured granitoid rock outcrop; (2) erosion-resistant banks and islands; (3) locally variable channel-bed gradients; (4) variable flow regimes.     

Impact of hillslope-derived sediment supply on drainage basin development in small watersheds at the northern border of the central Alps of Switzerland

This paper explores the effects of hillslope mobility on the evolution of a 10-km² drainage basin located at the northern border of the Swiss Alps. It uses geomorphologic maps and the results of numerical models that are based on the shear stress formulation for fluvial erosion and linear diffusion for hillslope processes. The geomorphic data suggest the presence of landscapes with specific cross-sectional geometries reflecting variations in the relationships between processes in channels and on hillslopes. In the headwaters, the landscape displays parabolic cross-sectional geometries indicating that mass delivered to channels by hillslope processes is efficiently removed. In the trunk stream portion, the landscape is (i) V-shaped if the downslope flux of mass is balanced by erosion in channels (i.e. if mass delivered to channels by hillslope processes is efficiently removed) and (ii) U-shaped if in-channel accumulation of hillslope-derived material occurs. This latter situation indicates a non-balanced mass flux between processes in channels and on hillslopes.Information about the spatial pattern of the postglacial depth of erosion allows comparative estimates to be made about the erosional efficiency for the various landscapes that were mapped in the study area. The data suggest that the erosional potential and sediment discharge are reduced for the situation of a non-balanced mass flux between processes in channels and on hillslopes. These findings are also supported by the numerical model. Indeed, the model results show that high hillslope mobility tends to reduce the hillslope relief and to inhibit dissection and formation of channels. In contrast, stable hillslopes tend to promote fluvial incision, and the hillslope relief increases. The model results also show that very low erosional resistance of bedrock promotes backward erosion and steepening of channel profiles in headwaters. Beyond that, the model reveals that sediment discharge generally increases with decreasing erosional resistance of bedrock, but that this increase decays exponentially with increasing magnitudes of fluvial and hillslope mobilities. Very high hillslope diffusivities even tend to reduce the erosional potential of the whole watershed. It appears that besides rates of base-level lowering, factors limiting sediment discharge might be the nonlinear relationships between processes in channels and on hillslopes.     

Rivers turned to rock: Late Quaternary alluvial induration influencing the behaviour and morphology of an anabranching river in the Australian monsoon tropics

Late Quaternary alluvial induration has greatly influenced contemporary channel morphology on the anabranching Gilbert River in the monsoon tropics of the Gulf of Carpentaria. The Gilbert, one of a number of rivers in this region, has contributed to an extensive system of coalescing low-gradient and partly indurated riverine plains. Extensive channel sands were deposited by enhanced flow conditions during marine oxygen isotope (OI) Stage 5. Subsequent flow declined, probably associated with increased aridity, however, enhanced runoff recurred again in OI Stages 4–3 (65–50 ka). Aridity then capped these plains with 4–7 m of mud. A widespread network of sandy distributary channels was incised into this muddy surface from sometime after the Last Glacial Maximum (LGM) to the mid Holocene during a fluvial episode more active than the present but less so than those of OI Stages 5 and 3. This network is still partly active but with channel avulsion and abandonment now occurring largely proximal to the main Gilbert flow path.A tropical climate and reactive catchment lithology have enhanced chemical weathering and lithification of alluvium along the river resulting in the formation of small rapids, waterfalls and inset gorges, features characteristic more of bedrock than alluvial systems. Thermoluminescence (TL) and comparative optically stimulated luminescence (OSL) ages of the sediments are presented along with U/Th ages of pedogenic calcrete and Fe/Mn oxyhydroxide/ oxide accumulations. They show that calcrete precipitated during the Late Quaternary at times similar to those that favoured ferricrete formation, possibly because of an alternating wet–dry climate. Intense chemical alteration of the alluvium leading to induration appears to have prevailed for much of the Late Quaternary but, probably due to exceptional dryness, not during the LGM. The result has been restricted channel migration and a reduced capacity for the channel to adjust and accommodate sudden changes in bedload. Consequent avulsions have caused local stream powers to increase by an order of magnitude, inducing knickpoint erosion, local incision and the sudden influx of additional bedload that has triggered further avulsions. The Gilbert River, while less energetic than its Pleistocene ancestors, is clearly an avulsive system, and emphasizes the importance in some tropical rivers of alluvial induration for reinforcing the banks, generating nickpoints, reworking sediment and thereby developing and maintaining an indurated and anabranching river style.     

Geomorphology and dynamics of the Mfolozi River floodplain, KwaZulu-Natal, South Africa

The geomorphology and dynamics of the Mfolozi River floodplain and estuary, located in the subtropical region of northern KwaZulu-Natal, South Africa, were considered with respect to existing models of avulsion and alluvial stratigraphy. The Mfolozi River floodplain may be divided into regions based on longitudinal slope and dominant geomorphic processes. Confinement of the Mfolozi River above the floodplain has led to the development of an alluvial fan at the floodplain head, characterized by a relatively high sedimentation rate and avulsion frequency, at a gradient of 0.10%. The lower floodplain is controlled by sea level, with an average gradient of 0.05%. Between the two lies an extremely flat region with an average gradient of 0.02%, which may be controlled by faulting of the underlying bedrock.Avulsion occurrences on the Mfolozi floodplain are linked to the two main zones of aggradation, the alluvial fan at the floodplain head, and toward the river mouth in the lower floodplain. On the alluvial fan, normal flow conditions result in scour from local steepening. During infrequent, large flood events, the channel becomes overwhelmed with sediment and stream flow, and avulses. The resulting avulsion is regional, and affects the location of the channel from the floodplain head to the river mouth. Deposits resulting from such avulsions contribute significantly to the total volume of sediment stored in the floodplain, and tend to persist for long periods after the avulsion. Contrastingly, on the lower floodplain, reaching of the avulsion threshold is not necessarily linked to large flood events, but rather to long-term aggradation on the channel that decreases the existing channels gradient while increasing its elevation above the surrounding floodplain. Resultant avulsions tend to be local and do not contribute significantly to the overall volume of floodplain alluvium.     

Recent large-magnitude floods and their impact on valley-floor environments of northeastern Yellowstone

The Lamar River watershed of northeastern Yellowstone contains some of the most diverse and important habitat in the national park. Broad glacial valley floors feature grassland winter range for ungulates, riparian vegetation that provides food and cover for a variety of species, and alluvial channels that are requisite habitat for native fish. Rapid Neogene uplift and Quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine–riparian ecosystems. Uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock. As a result, steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt. Recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley-floor meadows. Tree-ring dating identifies major floods in 1918, ca. 1873, and possibly ca. 1790. In 1996 and 1997, discharge during snowmelt runoff on Soda Butte Creek approached the 100-year flood estimated by regional techniques, with substantial local bank erosion and channel widening. Indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996, with similar duration and much greater flood plain impact. Nonetheless, 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper Yellowstone region. The 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of Soda Butte Creek. Channels may still be locally enlarged from flood erosion, and net downcutting has occurred in some reaches, leaving the pre-1790 flood plain abandoned as a terrace. Gravelly overbank deposits raise flood-plain surfaces above levels of frequent inundation and are well drained, therefore flood-plain soils are drier. Noncohesive gravels also reduce bank stability and may have persistent effects on channel form. Overall, floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley-floor ecosystems in northeastern Yellowstone.     

Influence of late Holocene hillslope processes and landforms on modern channel dynamics in upland watersheds of central Nevada

Stratigraphic, geomorphic, and paleoecological data were collected from upland watersheds in the Great Basin of central Nevada to assess the relationships between late Holocene climate change, hillslope processes and landforms, and modern channel dynamics. These data indicate that a shift to drier, warmer climatic conditions from approximately 2500 to 1300 YPB led to a complex set of geomorphic responses. The initial response was massive hillslope erosion and the simultaneous aggradation of both side-valley alluvial fans and the axial valley system. The final response was fan stabilization and axial channel incision as fine-grained sediments were winnowed from the hillslope sediment reservoirs, and sediment yield and runoff processes were altered. The primary geomorphic response to disturbance for approximately the past 1900 years has been channel entrenchment, suggesting that the evolutionary history of hillslopes has produced watersheds that are prone to incision. The magnitude of the most recent phase of channel entrenchment varies along the valley floor as a function of geomorphic position relative to side-valley alluvial fans. Radial fan profiles suggest that during fan building, fan deposits temporarily blocked the flow of sediment down the main stem of the valley, commonly creating a stepped longitudinal valley profile. Stream reaches located immediately upvalley of these fans are characterized by low gradients and alternating episodes of erosion and deposition. In contrast, reaches coincident with or immediately downstream of the fans exhibit higher gradients and limited valley floor deposition. Thus, modern channel dynamics and associated riparian ecosystems are strongly influenced by landforms created by depositional events that occurred approximately 2000 years ago.     

The kinematics and pattern of escarpment retreat across the rifted continental margin of SE Australia

Rifted continental margins generally display an interior, low-relief, highly weathered upland area and a deeply incised, high-relief coastal area. The boundary between the two zones is commonly demarcated by an abrupt, seaward-facing escarpment. We investigate the rate and pattern of escarpment erosion and landscape evolution along the passive margin of south-east Australia, in the region of the New England Tableland. The process of rifting is shown to initiate an escarpment across which rivers flow, resulting in an escarpment that takes the form of dramatic, elongated gorges. Using a mass balance approach, we estimate the volume/unit length of continental material eroded seaward of the escarpment to be between 41 and 68 km², approximately an order of magnitude less than the 339 km² of terrigenous sediments calculated to have been deposited offshore, but consistent with earlier denudation estimates based on apatite fission track data. On the bedrock rivers draining the New England Tableland region, the escarpment is manifested as a series of sharp knickpoints punctuating the river longitudinal profiles. The knickpoints are situated the same distance upstream along the different channels and uniform escarpment retreat rates on the order of 2 km Myr⁻¹ are estimated, despite some differences in bedrock lithologies. Gorge head migration appears to be very important as a bedrock incision mechanism. Field observations indicate a coupling between escarpment retreat and knickpoint propagation, bedrock channel incision, and hillslope development.     

Differential recovery from the effects of a 100-year storm: Significance of long-term hillslope–channel coupling; Howgill Fells, northwest England

A 100-year storm that occurred in 1982 caused major geomorphic changes in the main valleys of the northern Howgill Fells, northwest England. Those changes, which were documented at that time, involved extensive hillslope gully erosion, alluvial fan sedimentation, and substantial sediment input to the stream systems. The streams channels, which had hitherto been dominantly single-thread, relatively stable channels, responded in many reaches by switching to wide shallow unstable locally braided channels. Over the 20 years since the event there has been a partial recovery to channel geometries similar to the pre-flood conditions, however the degree of recovery contrasts between two neighbouring valleys, Bowderdale and Langdale. The channel of Bowderdale Beck has largely recovered. Flood sedimentation zones have largely stabilised and new single-thread channels have cut through most of the former braided reaches. In some places channel widths remain higher than the pre-flood values, and locally recovery has been modified by a lagged complex response. In Langdale, recovery is only partial with many reaches demonstrating sustained instability over the 20-year post-flood period. Furthermore, the overall spatial patterns suggest some reach-to-reach transfer of coarse sediment, shifting zones of instability downstream. The contrasts between the two valleys appear to relate to different hillslope-to-channel coupling characteristics, themselves inherited from late Pleistocene conditions. These contrasts are also evident in the longer-term (post-1949) history of channel change and stability in these two streams, indicative of the higher intrinsic instability of the Langdale system.     

Sources of debris flow material in burned areas

The vulnerability of recently burned areas to debris flows has been well established. Likewise, it has been shown that many, if not most, post-fire debris flows are initiated by runoff and erosion and grow in size through erosion and scour by the moving debris flow, as opposed to landslide-initiated flows with little growth. To better understand the development and character of these flows, a study has been completed encompassing 46 debris flows in California, Utah, and Colorado, in nine different recently burned areas. For each debris flow, progressive debris production was measured at intervals along the length of the channel, and from these measurements graphs were developed showing cumulative volume of debris as a function of channel length. All 46 debris flows showed significant bulking by scour and erosion, with average yield rates for each channel ranging from 0.3 to 9.9 m³ of debris produced for every meter of channel length, with an overall average value of 2.5 m³/m. Significant increases in yield rate partway down the channel were identified in 87% of the channels, with an average of a three-fold increase in yield rate. Yield rates for short reaches of channels (up to several hundred meters) ranged as high as 22.3 m³/m. Debris was contributed from side channels into the main channels for 54% of the flows, with an average of 23% of the total debris coming from those side channels. Rill erosion was identified for 30% of the flows, with rills contributing between 0.1 and 10.5% of the total debris, with an average of 3%. Debris was deposited as levees in 87% of the flows, with most of the deposition occurring in the lower part of the basin. A median value of 10% of the total debris flow was deposited as levees for these cases, with a range from near zero to nearly 100%. These results show that channel erosion and scour are the dominant sources of debris in burned areas, with yield rates increasing significantly partway down the channel. Side channels are much more important sources of debris than rills. Levees are very common, but the size and effect on the amount of debris that reaches a canyon mouth is highly variable.     

How does alluvial sedimentation at range fronts modify the erosional dynamics of mountain catchments?

At the geological time scale, the way in which the erosion of drainage catchments responds to tectonic uplift and climate changes depends on boundary conditions. In particular, sediment accumulation and erosion occurring at the edge of mountain ranges should influence the base level of mountain catchments, as well as sediment and water discharges. In this paper, we use a landform evolution model (LEM) to investigate how the presence of alluvial sedimentation at range fronts affects catchment responses to climatic or tectonic changes. This approach is applied to a 25 km × 50 km domain, in which the central part is uplifted progressively to simulate the growth of a small mountain range. The LEM includes different slope and river processes that can compete with each other. This competition leads to ‘transport‐limited’, ‘detachment‐limited’ or ‘mixed’ transport conditions in mountains at dynamic equilibrium. In addition, two end‐member algorithms (the channellized‐flow and the sheet‐flow regimes) have been included for the alluvial fan‐flow regime. The three transport conditions and the two flow algorithms represent six different models for which the responses to increase of rock uplift rate and/or cyclic variation of the precipitation rate are investigated. Our results indicate that addition of an alluvial apron increases the long‐term mountain denudation. In response to uplift, mountain rivers adapt their profile in two successive stages; first by propagation of an erosion wave and then by slowly increasing their channel gradients. During the second stage, the erosion rate is almost uniform across the catchment area at any one time, which suggests that dynamic equilibrium has been reached, although the balance between erosion and rock uplift rates has not yet been achieved. This second stage is initiated by the uplift of the mountain river outlets because of sedimentation aggradation at the mountain front. The response time depends on the type of water flow imposed on the alluvial fans domains (× by 1.5 for channelized flow regime and by 10 for the sheet flow one). Cyclic variations of precipitation rate generate cyclic incisions in the alluvial apron. These incision pulses create knick‐points in the river profile in the case of ‘detachment‐limited’ and ‘mixed’ river conditions, which could be mistaken for tectonically induced knick‐points. ‘Transport‐limited’ conditions do not create such knick‐points, but nevertheless trigger erosion in catchments. The feedbacks linked to sedimentation and erosion at range front can therefore control catchment incision or aggradation. In addition, random river captures in the range front trigger auto‐cyclic erosion pulses in the catchment, capable of generating incision–aggradation cycles.     

Stream-terrace genesis: implications for soil development

Genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting, base level of erosion, complex response, threshold of critical power, diachronous and synchronous response times, and static and dynamic equilibrium. Climatic and tectonic stream terraces are major terraces below which flights of minor complex-response degradation terraces can form.These three types of terraces can be summarized by describing a downcutting-aggradation-renewed downcutting sequence for streams with gravell bedload. By tectonically induced downcutting, streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion. Lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream-terrace landform. Aggradation events record brief reversals of long-term tectonically induced downcutting because they raise active channels. They may be considered as major (the result of climatic perturbations) or minor (the result of complex-response model types of perturbations). Climatically controlled aggradation followed by degradation leaves an aggradation surface; this type of fill-terrace tread is the fundamental climatic stream-terrace landform. Aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces. Raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form; first in alluvial fill and then in underlying bedrock along tectonically active streams. These are complex-response terraces because they result from interactions of dependent variables within a given fluvial system. Pauses in degradation to a new base level of erosion, and/or minor episodes of backfilling, lead to formation of complex-response fill-cut and strath, or of fill terraces. Fill-cut terraces are formed in alluvium; they are complex-response terraces because they are higher than the base level of erosion. Good exposures and dating are needed to distinguish static equilibrium complex-response minor strath terraces from dynamic equilibrium tectonic (major) straths. Strath terraces may be regarded as complex-response terraces where degradation rates between times terrace-tread formation exceed the long-term uplift rate for the reach based on ages and positions of tectonic terraces.Late Quaternary global climatic changes control aggradation events and even the times of cutting of major (tectonic) straths, because the base level of erosion can not be attained during times of climatically driven aggradation-degradation events.Most terrace soils form on treads of climatic and complex-response terraces. Aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence. Each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads. Multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces. Pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous - variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors. Climatic terraces of adjacent watersheds also should be roughly synchronous (correlatable) - variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds. Such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times, and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations. Soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full-glacial times and interglacial times. Soils on some complex-response terraces may be diachronous within a given fluvial system, and typically are diachronous between watersheds.     

Channel processes on the spawning Kerbi and Nimelen rivers (Lower-Amur Region)

We examine the morphology and dynamics features of the river channels within the basin of the lower reaches of the Amur river on spawning grounds. We report evidence that the spawning grounds tend to occur at definite elements of the river channel as well as data on particle-size composition of alluvial deposits in autumn chum salmon (keta) spawning stretches. Factors having a negative influence on the preservation of spawning grounds have been identified.     

Use of Thermal Infrared Multispectral Scanner (TIMS) imagery to investigate upslope particle size controls on arid piedmont morphology

Geomorphic differences between slopes backing two distinct desert piedmont types provide a proxy indicator for the kind of landform developed at the corresponding mountain base. Here, the term ‘bedrock pediment’ describes subaerial bedrock platforms that emanate from a mountain base while ‘alluvial slope’ describes suballuvial bedrock platforms that extend from the mountain. Mountain slopes backing bedrock pediments have been demonstrated to be mantled by larger clast sizes than corresponding slopes backing alluvial slopes in the Phoenix region, Arizona, USA. The present research focuses on using the disparate particle sizes between slopes backing bedrock pediments and alluvial slopes as an indicator for the piedmont form developed at the mountain base, and uses high-resolution remotely sensed digital data as a medium for quantitative landform assessments. A gravel + bedrock versus soil index developed from airborne midinfrared multispectral imagery acquired by the Thermal Infrared Multispectral Scanner (TIMS) indicates the presence of slopes mantled with larger particle sizes versus slopes mantled with smaller particle sizes and greater soil coverage. Two test areas confirm the applicability of this method and further demonstrate the usefulness of high-resolution midinfrared multispectral imagery as a geomorphic tool in arid regions.