Relations between interrill erosion processes and sediment particle size distribution in a semiarid Mediterranean area of SE of Spain

Erosion and sediment characteristics were measured using simulated rainfall on two cultivated soils of contrasting lithology (Quaternary calcareous colluvium and Tertiary marls) in a semiarid Mediterranean area of SE Spain. Two rainfall intensities, high (56.0±2.4 mm h⁻¹) and medium (31.4±1.4 mm h⁻¹), were used in order to know the mechanisms involved in each selected condition. For each simulated event, runoff and sediment were sampled at 1-min intervals on a 1-m wide by 2-m long erosion plot. The erosion rate was calculated as the total amount of soil lost divided by the time period of the test. The duration of the test was that needed to reach steady state runoff, an average time of 24.5 min for Quaternary calcareous colluvium and 17.7 min for Tertiary marls. The size distribution of the transported sediment in the field (effective size distribution) was compared with equivalent measurements of the same samples after chemical and mechanical dispersion (ultimate size distribution) to investigate the detachment and transport mechanisms involved in sediment mobilisation. The results showed that the soil type determined the hydrological response, regardless of the rainfall intensity. The erosional response was, however, determined by the rainfall and soil surface characteristics.In Quaternary calcareous colluvial soils, the predominant erosion process depended on the rainfall intensity, with a prevailing detachment-limited condition in high-intensity events and prevailing transport-limited conditions in those events of medium intensity. These differences in the main erosion processes were reflected in the size of the transported material and in the change in sediment size within the storm. Thus, a time-dependent size distribution of the eroded material (decreasing coarse fractions and increasing fine fractions with runoff time) was observed for high-intensity events. In medium-intensity events, on the other hand, the time-independent size distribution of the eroded material indicated transport-limited erosion.Due to the rapid surface crusting on the Tertiary marl soil, no differences in the main erosion processes or in the sediment size distribution occurred for the different rainfall intensities tested. The erosion of marl soils was determined mainly by the limited quantity of available sediment.The effective size of material was a more sensitive parameter than the ultimate size of the sediment to study the way in which the sediment was transported.     

Geomorphological impacts of a flood event on ephemeral channels in SE Spain

A flood event occurred on 30 September 1997, in three catchments, Torrealvilla, Salada and Nogalte, in SE Spain which were being monitored for morphological change. Peak discharges were high in the Torrealvilla catchment, moderate in the Salada and low in the Nogalte. Detailed topographic surveys were made before and after the flood and from these DEMs of difference were constructed. The two sites with highest flow both underwent significant incision of the channel. At other sites, both scour and aggradation of the channel took place, with scour holes of up to 1 m depth eroded. Elsewhere, within the channels major erosion took place downstream of tracks and structures such as check dams that impeded sediment movement. The pattern and amounts of change show little relationship to independent factors, other than a scale effect of peak discharge. Behaviour of channels is closely related to local sediment supply. Sediment flux calculated from changes in volumes at the site is comparable with the other figures produced for ephemeral channels and much higher than for perennially flowing streams.     

Double-thresholds in scour–fill processes and some implications in channel adjustment

Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again.Controlled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous.     

Spatial variability, mechanisms and propagation of change in an active meandering river

This paper addresses questions of the spatial pattern of instability and the mechanisms of change in an active meandering river, particularly whether and how change is propagated. More than 20 years of monitoring of a sequence of nearly 100 bends on one dynamic meandering river, combined with historical data and previous analyses of processes of change, provide a unique insight into the link between annual changes produced by erosion and deposition and the longer-term changes in planform. The study reach of the River Dane in NW England exhibits stable and unstable sections adjacent to one another. Rates of movement range up to 3 m a^− 1, with maxima occurring in high curvature, free bends. Stable reaches are due to factors of gradient, curvature and bank resistance. Analysis of the large amount of data on occurrence of erosion and deposition in each bend each year reveals no definite association of changes in one bend with another. The detailed evidence of the morphological features in the bends shows that changes do not take place by bars moving progressively through reaches. Case studies of bends upstream of constrained, stable reaches indicate an oscillation of widening and narrowing of the channel, over a period of a few years, producing a net rotation of the bend. These areas are zones of stalling of sediment and change takes place by absorption and lateral movement. Overall, changes tend to be localised and fit the bend theory of meanders, but with low sensitivity reaches pinning the planform for longer periods in certain locations.     

Differences in hillslope runoff and sediment transport rates within two semi-arid catchments in southeast Spain

This paper focuses on hillslope runoff and sediment transport within two catchments in southeast Spain. Five monitoring sites were established on hillslope concavities throughout the two catchments. The techniques used were mini-crest stage recorders, spray-painted lines, sediment traps and tipping bucket rain gauges (established during previous research). Results show that a storm event in the Rambla Nogalte on 30 June 2002 of 83.0 mm was responsible for a maximum runoff depth of 12 cm and a maximum hillslope sediment transport of 1886 cm³ m⁻¹. The same storm in the Rambla de Torrealvilla produced 53.4 mm of rainfall on the 1 July 2002, had a maximum runoff depth of 26 cm and resulted in 2311 cm³ m⁻¹ of sediment transport. There is evidence to suggest that measured sediment transport is related to runoff and a qualitative estimate of Morphological Runoff Zones (MRZ). Sediment transport and depth of runoff varied dramatically with lithology; marl sites produced most runoff and sediment transport, followed by the sites of mixed red and blue schist, then blue schist. These results are important for understanding the behaviour of slopes and show that for short lived storms with high, but not rare, rainfall intensities and total rainfall amounts, runoff can cause significant hillslope sediment transport in semi-arid areas.     

Quaternary landscape evolution and erosion rates for an intramontane Neogene basin (Guadix–Baza basin, SE Spain)

The landscape evolution in Neogene intramontane basins is a result of the interaction of climatic, lithologic, and tectonic factors. When sedimentation ceases and a basin enters an erosional stage, estimating erosion rates across the entire basin can offer a good view of landscape evolution. In this work, the erosion rates in the Guadix–Baza basin have been calculated based on a volumetric estimate of sediment loss by river erosion since the Late Pleistocene. To do so, the distribution of a glacis surface at ca. 43 kyr, characterised by a calcrete layer that caps the basin infilling, has been reconstructed. To support this age, new radiometric data of the glacis are presented. The volume of sediment loss by water erosion has been calculated for the entire basin by comparing the reconstructed geomorphic surface and the present-day topography. The resulting erosion rates vary between 4.28 and 6.57 m³ ha^− 1 yr^− 1, and are the consequence of the interaction of climatic, lithologic, topographic, and tectonic factors. Individual erosion rates for the Guadix and Baza sub-basins (11.80 m³ ha^− 1 yr^− 1 and 1.77 m³ ha^− 1 yr^− 1 respectively) suggest different stages of drainage pattern evolution in the two sub-basins. We attribute the lower values obtained in the Baza sub-basin to the down-throw of this sub-basin caused by very recent activity along the Baza fault.     

Groundwater control on the suspended sediment load in the Na Borges River, Mallorca, Spain

Groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems. Low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment-free spring water dilute the suspended sediment generated by storms. However, in many Mediterranean rivers, groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes. Such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers. Moreover, urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its ‘flashy’ regime. In this context, we monitored suspended sediment fluxes during a two-year period in the Na Borges River, a lowland agricultural catchment (319 km²) on the island of Mallorca (Balearic Islands). The suspended sediment concentration (SSC) was lower when the base flow index (i.e., relative proportion of baseflow compared to stormflow, BFI) was higher. Therefore, strong seasonal contrasts explain the high SSC coefficient of variation, which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions. A lack of correlation in the Q-SSC rating curves shows that factors other than discharge control sediment transport. As a result, at the event scale, multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events. In the winter, the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables (i.e., maximum discharge) to exert the most important control on events, whereas in the summer, it is necessary to accumulate important volumes of rainfall, creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network. The BFI is also related to sediment delivery processes, as the loads are higher with lower BFI, corroborating the fact that most sediment movement is caused by stormflow and its related factors. Overall, suspended sediment yields were very low (i.e., < 1 t km^− 2 yr^− 1) at all measuring sites. Such values are the consequence of the limited sediment delivery attributable to soil conservation practices, low surface runoff coefficients and specific geomorphic features of groundwater-dominated rivers, such as low drainage density, low gradient, steep valley walls and flat valley floors.     

Quantifying channel development and sediment transfer following chute cutoff in a wandering gravel-bed river

Three-dimensional morphological adjustment in a chute cutoff (breach) alluvial channel is quantified using Digital Elevation Model (DEM) analysis for a ca. 0.7 km reach of the River Coquet, Northumberland, UK. Following cutoff in January 1999, channel and bar topography was surveyed using a Total Station on five occasions between February 1999 and December 2000. Analysis of planform change coupled with DEM differencing elucidates channel and barform development following cutoff, and enables quantification of sediment transfers associated with morphological adjustment within the reach. This exercise indicates an initial phase of bed scour, followed by a period characterised by extensive bank erosion and lateral channel migration where erosion (including bed scour) totalled some 15,000 m³ of sediment. The channel in the post-cutoff, disequilibrium state is highly sensitive to relatively low-magnitude floods, and provision of accommodation space by bank erosion encouraged extensive lateral bar development. Bar development was further facilitated by infilling of channels abandoned by repeated within-reach avulsion and large-scale aggradation of sediment lobes deposited by higher magnitude floods. Calculations indicate that at least 6600 m³ of sediment was deposited on emerging bars within the reach over the survey period, and >2300 m³ deposited within the channel. Sediment losses from the reach may have exceeded 6500 m³.     

Bank and near-bank processes in an incised channel

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through:

1. increase in soil bulk unit (specific) weight,

2. decrease or complete loss of matric suction, and, therefore, apparent cohesion,

3. generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength,

4. entrainment of in situ and failed material at the bank toe, and

5. loss of confining pressure during recession of stormflow hydrographs.

Relatively small frequent flows during the winter have the ability to erode failed bank materials, maintain oversteepened, unstable bank surfaces and promote prolonged periods of bank retreat, channel migration and high yields of fine-grained sediment. Confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe, which steepens the bank. For example, more than 2 m of bank retreat occurred during the study period at the research site on Goodwin Creek, northern Mississippi. The loss of matric suction (negative pore pressures) due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability. Apparent cohesion, friction angle, soil bulk unit weight and moisture content were measured in situ. Matric suction was measured continuously, in situ with a series of five pressure-transducer tensiometers. A bank-failure algorithm, which combines the Mohr–Coulomb approach, for saturated conditions and the Fredlund modification for unsaturated conditions was developed for layered cohesive streambanks. The resulting equation has been used successfully to investigate the role of matric suction, positive pore-water pressures and confining pressure for layered streambanks composed of cohesive materials.     

黄河头道拐—潼关区间植被恢复及其对水沙过程影响

黄河头道拐—潼关区间是黄河泥沙的主要来源区,也是中国植被恢复最快的地区.植被的快速恢复对径流和输沙过程产生了深远影响.本文基于250 m分辨率的归一化植被指数数据(MOD13Q1 NDVI),使用统计分析和趋势分析技术,分析了头道拐—潼关区间不同景观单元植被恢复特点、影响因素以及其对水沙过程的影响,并对头道拐—潼关区间...     

Frequency and variability of missing tree rings along the stems of Pinus halepensis and Pinus pinea from a semiarid site in SE Spain

We studied the occurrence of missing rings (MR) around and along the stems of three Pinus halepensis and three Pinus pinea trees growing in a semiarid Mediterranean site in SE Spain. Tree-ring analyses were performed along 8 radii on stem discs taken at 1 m distances from the base to the top of the trees. The tree rings analysed showed that 19% were missing in P. halepensis and 10% in P. pinea. MR were recorded in 61% of P. halepensis and 24% of P. pinea in the years analysed. Dry conditions from January to May and high late winter to spring temperatures seem to be the main climatic elements promoting MR in both species. In our research into P. halepensis, the frequency of MR gradually decreased from the bottom upward, but no such pattern was observed in P. pinea. Most tree rings were missing only locally (LMR), indicating that wood formation occurred every year in at least some part of a tree, and the tree rings were often discontinuous around and along the stem. Since the frequency of LMR greatly varies around and along the stem, our results suggest that serial sectioning along the stem provides more information on wood formation and responses of trees to environment than studying the samples at breast height only. The disadvantage of the technique is that, as in our study case, research is based on a limited sample size, since it requires destructive sampling with tree felling, which is not usually possible or desirable on highly vulnerable semiarid forest ecosystems.     

The effects of vegetative ash on infiltration capacity, sediment transport, and the generation of progressively bulked debris flows

Through the alteration of the physical characteristics of a landscape, such as the destruction of vegetation and the formation of a hydrophobic layer, a fire can dramatically amplify erosion rates. On the basis of field observations, it has been proposed that the deposition of a layer of ash on the ground surface can enhance the erosion of mountainous terrain by surface runoff and might even be a necessary condition for the generation of progressively bulked debris flows. In this study, a flume was constructed to investigate the role of ash in increasing both the volume and the transport capacity of runoff. The experiments demonstrated that the presence of ash on the soil surface reduces the ability of flowing water to infiltrate; this effect is even greater when the ash has been pre-wetted. In addition, the ability of ash slurries to infiltrate decreases with increasing ash concentration. The results also indicate that the transport capacity of runoff is enhanced by the incorporation of ash into the flow because of the increased fluid density. However, the addition of ash reduces the boundary Reynolds number such that, at high ash concentrations and with fine-grained sediment, sediment transport declines as the flow becomes hydraulically smooth. The experimental results were also used to evaluate the ability of steep flow fronts, a common characteristic of debris flows and flash floods, to increase sediment transport rates. Finally, it is proposed that ash slurries may evolve into progressively bulked debris flows through a positive feedback between fluid density, transport capacity, and erosivity.     

Effects of large floods on sediment transport and reach morphology in the cobble-bed Sainte Marguerite River

Sediment transport rates were estimated for two flood events on the cobble-bed Sainte Marguerite River in the Saguenay region, Canada. Morphologic methods were used to derive one set of estimates, and a combination of the Meyer-Peter and Muller equation with a dimensionless sediment transport ratio (after Dietrich et al. [Nature 340 (1989) 215]) was used to derive another set of estimates. Both sets of estimates give consistent results for the first event (which had a decade-scale return period), and for the second event (which was the largest flood on record and had a century-scale return period). The transport occurring during the second event was an order of magnitude greater than that occurring during the first event: despite this disparity in the transport intensity of the two events, the channel morphology remained qualitatively similar. The observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s.     

Effect of vegetation with different evolution degree on soil organic matter in a semi-arid environment (Cabo de Gata-Níjar Natural Park, SE Spain)

The organic matter of the surface horizons of soils developed below scrub vegetation in a Mediterranean semi-arid area of great environmental interest (Cabo de Gata-Níjar Natural Park, SE Spain) has been studied. The study mainly concentrates on examining the influence of two vegetation types, one evolved (according to its successional stage), and the other clearly degraded as a result of prior removal of vegetation. In spite of the homogeneity in the results obtained from the analysis of the organic matter from the soils studied, a relationship may be established between vegetation biotype and characteristics and evolution of the soil organic matter. The evolved vegetation results in the presence in the soil of a somewhat more evolved and stable organic matter (demonstrated by certain chemical and microbiological aspects), resulting in a greater degree of humification, thus favouring the protection of the soil and the ecosystem as a whole. Hence, the presence of degraded vegetation might lead to soil degradation, something that is unsustainable in semi-arid areas that are particularly fragile in nature.     

9000 years of geochemical evolution of lithogenic major and trace elements in the sediment of an alpine lake – the role of climate, vegetation, and land-use history

A 9000cal. year record of geochemistry was analysed in a sediment core obtained from a Swiss alpine hard-water lake (1937 ma.s.l.) that is located at the present-day tree-line. Geochemical stratigraphies are compared to changes in mineralogy, grain-size, pollen, and macrofossil records. This allows the reconstruction of the effects of changes in vegetation and of 3500 years of land-use in the catchment area on sediment geochemistry. Using principal component analysis, two major geochemical groups are distinguished: (i) Changes in concentrations of Rb, Ti, Zr, Fe, As, and Pb are closely related to corresponding changes in the concentrations of quartz and clay. They are thus considered to represent the silicate fraction which shows an increase from the oldest to the youngest core section. (ii) In contrast, Ca and Sr concentrations are positively correlated with changes in silt, sand, and calcite. They are therefore considered to represent the carbonate fraction which gradually decreased. Based on constrained cluster analysis, the core is divided into two major zones. The oldest zone (A; 9000–6400 cal.BP) is characterised by high concentrations of detrital carbonates. The more open catchment vegetation at that time promoted the physical weathering of these carbonates. The second major zone (B, 6400 cal.BP–1996 AD) is divided into four subsections with boundaries at ca. 3500, 2400, and 160cal. BP. The lower part of this zone, B1, is characterized by a gradual decrease in the carbonate-silt fraction and a pronounced increase in the silicate-clay fraction. This is concurrent with the expansion of Picea in the catchment area, which probably stabilized the soil. The middle part, B2 and B3 (3500–160cal. BP), comprises pronounced fluctuations in all elements, especially Ca, Sr, Mn, and Rb, but also in clay and silt. These changes are related to varying intensities of alpine farming. In the same section, Mn/Fe ratios are highly variable, suggesting changes in the mixing regime of the lake with phases of anoxic bottom water. The uppermost section, B4 (since 160cal. BP), is characterized by a steep decline in the silicate fraction and an increase in Ca and Sr. Despite the decrease in the silicate fraction, Pb increases, due to elevated atmospheric input resulting from early metal pollution, are masked by the high natural variability. Generally, changes in vegetation, which correspond to climate changes in the early Holocene and to human activities since ca. 3700cal. BP, are the controlling factor for variations in the geochemical composition of the sediment of Sägistalsee.     

Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.     

Underlying gravel layers in a large sand bed river and their influence on downstream-dam channel adjustment

Due to the changes in environmental factors during a river's historical development, underlying gravel layers are found in many large plain rivers. When the buried depth of this gravel layer is within the reach of down-cutting by clear water scour after reservoir construction, it may be exposed and exert a far-reaching influence on channel adjustment. In Hanjiang River, the longest tributary of the Yangtze River in China, down-cutting has been greatly reduced and even stopped, due to the total change of bed material composition. The exposure of a gravel layer results in a dramatic increase in the bed's hydraulic roughness, so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer. Moreover, the exposed gravel layer increases the erosion resistance of bed material, making a lower relative erosion resistance of bank to bed material. Where the bank's erosional resistance is weak, this may lead to a tendency towards channel widening.     

The influence of landsliding on sediment supply and channel change in a steep mountain catchment

Rates of sediment supply by landsliding to an alluvial channel in a small catchment in central Switzerland were estimated over an 11-month study period. Fluvial sediment transport in the channel is independently monitored at the upstream and downstream ends of the study reach, yielding a unique opportunity to quantitatively compare the volume of sediment supplied to the channel with the volume in fluvial transport. Landslide-derived sediment discharge to the channel was greatest during the winter and spring months, while most of the fluvial sediment transport occurred during short, intense summer storms. Approximately 98 m³ of sediment was delivered directly to the study reach by landsliding,  80 m³ was transported into the reach from upstream, and  70 m³ was transported out of the reach. Thus, there was a net accumulation of  100 m³ of sediment during the 11-month study. Decadal-scale channel aggradation was independently assessed by comparing channel longitudinal profiles in 1993 and 2004. During this 11-year period, a total of  1500 m³ of sediment has accumulated in the study reach. Aggradation has occurred largely in two broad zones that correspond with both the locations of major landslide complexes and reaches of high channel slope, indicating that hillslope sediment input left an imprint on the morphology of the channel bed that appears to be stable over at least decadal time scales.