Flume tests on fluvial erosion mechanisms in till‐bed channels

Semi‐alluvial stream channels eroded into till and other glacial sediments are common in areas of extensive glacial deposition such as the Great Lakes region and northern interior plains of North America. The mechanics of erosion and erosional weakness of till results in the dominance of fluvial scour and mass erosion due to spontaneous fracture at planes of weakness under shearing flow. There have been few controlled tests looking at erosional mechanisms and resistance of till in river channels. We subjected small blocks of till to unidirectional flows in a laboratory flume to measure the threshold shear stress for erosion and observed the erosion mechanics. Critical shear stress for erosion varied from 7 to 8 Pa for samples with initial saturated moisture content in which a combination of fluvial scour and mass cracking/block erosion dominated. When dried, micro‐fissures occurred in the sample and erosional resistance of the till was extremely low at <1 Pa with erosion appearing to be by fluvial scour. When mobile gravel was added to the test conditions, the gravel reduced the erosion threshold slightly because of the enhanced scour around and below the gravel particles and the tendency for the gravel to aid in crack enlargement. Thus a partial or thin gravel cover over the till may provide no protection from erosion. The erosion processes and effects reflect the complex and contingent mechanics and properties of till, and suggest that the erosion characteristics of till bed semi‐alluvial channels differ from abrasion or plucking dominated processes in more resistant bedrock. Copyright © 2017 John Wiley & Sons, Ltd.     

Energy Expenditure and Geomorphic Work of the Cataclysmic Missoula Flooding in the Columbia River GGorge,USA

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.     

Evaluating the geomorphic channel response to beaver dam analog installation using unoccupied aerial vehicles

Beaver dam analogs (BDAs) are a stream restoration technique that is rapidly gaining popularity in the western United States. These low-cost, stream-spanning structures, designed after natural beaver dams, are being installed to confer the ecologic, hydrologic, and geomorphic benefits of beaver dams in streams that are often too degraded to provide suitable beaver habitat. BDAs are intended to slow streamflow, reduce the erosive power of the stream, and promote aggradation, making them attractive restoration tools in incised channels. Despite increasing adoption of BDAs, few studies to date have monitored the impacts of BDAs on channel form. Here, we examine the geomorphic changes that occurred within the first year of restoration efforts in Wyoming using high-resolution visible light orthomosaics and elevation data collected with unoccupied aerial vehicles (UAVs). By leveraging the advantages of rapidly acquired images from UAV surveys with recent advancements in structure-from-motion photogrammetry, we constructed centimeter-scale digital elevation models (DEMs) of the restoration reach and an upstream control reach. Through DEM differencing, we identified areas of enhanced erosion and deposition near the BDAs, suggesting BDA installation initiated a unique geomorphic response in the channel. Both reaches were characterized by net erosion during the first year of restoration efforts. While erosion around the BDAs may seem counter to the long-term goal of BDA-induced aggradation, short-term net erosion is consistent with high precipitation during the study and with theoretical channel evolution models of beaver-related stream restoration that predict initial channel widening and erosion before net deposition. To better understand the impacts of BDAs on channel morphology and restoration efforts in the western United States, it is imperative that we consistently assess the effects of beaver-inspired restoration projects across a range of hydrologic and geomorphic settings and that we continue this monitoring in the future.     

Flood hydrology and geomorphic effectiveness in the central Appalachians

This paper compares hydrologic records and geomorphic effects of several historic floods in the central Appalachian region of the eastern United States. The most recent of these, occurring in November 1985, was the largest ever recorded in West Virginia, with peak discharges exceeding the estimated 500-year discharge at eight of eleven stations in the South Branch Potomac River and Cheat River basins. Geomorphic effects on valley floors included some of the most severe and widespread floodplain erosion ever documented and exceeded anything seen in previous floods, even though comparable or greater rainfall and unit discharge have been observed several times in the region over the past 50 years. Comparison of discharge-drainage area plots suggests that the intensity and spatial scale of the November 1985 flood were optimal for erosion of valley floors along the three forks of the South Branch Potomac River. However, when a larger geographic area is considered, rainfall totals and discharge-drainage area relationships are insufficient predictors of geomorphic effectiveness for valley floors at drainage areas of 250 to 2500 km². Unit stream power was calculated for the largest recorded flood discharge at 46 stations in the central Appalachians. Maximum values of unit stream power are developed in bedrock canyons, where the boundaries are resistant to erosion and the flow cross-section cannot adjust its width to accommodate extreme discharges. The largest value was 2570 W m^?2; record discharge at most stations was associated with unit stream power values less than 300 W m^?2, but more stations exceeded this value in the November 1985 flood than in the other floods that were analysed. Unit stream power at indirect discharge measurement sites near areas experiencing severe erosion in this and other central Appalachian floods generally exceeded 300 W m^?2; reach-average values of 200-500 W m^?2 were calculated for valleys where erosion damage was most widespread. Despite these general trends, unit stream power is not a reliable predictor of geomorphic change for individual sites. Improved understanding of flood impacts will require more detailed investigation of interactions between local site characteristics and patterns of flood flow over the valley floor.     

Erosional effects of cattle on streambanks in Tennessee,U.S.A.

The geomorphological effects of cattle on streambanks in a humid region, which have consequent potential effects on water quality, are examined. Field observations suggest that cattle are important agents in causing streambanks to erode, but so many variables are involved that it is difficult to isolate the role of cattle. Instead, an empirical approach based on long-term controlled experiment was adopted along a small perennial stream in the Central Basin of Tennessee. The results showed that uncontrolled grazing caused about six times as much gross bank erosion as occurred on the protected control stretch. However, most of this difference was due to breakdown of banks by trampling and consequent erosion, rather than by bank scour caused by removal of bank vegetation by grazing. That is, bank vegetation alone did not appear to be a primary control. A relatively inexpensive grade-control structure reduced the gross bank erosion by about 50 per cent. The rapid destruction of streambanks observed in this study suggests that reduction of geomorphic resistance by uncontrolled stock access to streambanks has been an important factor in the stream widening that has taken place during historical time in the eastern United States.     

Braiding process and bank erosion in the Brahmaputra River

The present work explores relations between stream power,braiding intensities and bank erosion in certain stretches of the Brahmaputra River.In this paper,an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure.The braiding index is compared for discrete years to understand the morphological behaviour.Subsequently,a real time estimation of stream power for certain stretches of Brahmaputra River is done in order to analyse its variability in braiding intensity and bank erosion.The paper presents the dynamic behaviour of the channel pattern of the Brahmaputra River System in Assam valley of India over a time span of 18 years.The procedure addresses the selection of input parameters from digital satellite images,comprising scenes for the years 1990,1997 and 2007 with specific dates,from Dhubri near Indo-Bangladesh Border to Upper Assam.Deployment of GIS technique has been made to extract the required parameters to derive Plan Form Indices for the entire study reach.Stream power estimation is done for corresponding latest floods and for corresponding dates of image scenes.The study indicated that due to consistent aggradation of riverbed inducing temporal declination of stream power,there is an occurrence of wide spread braiding.This in turn incurs substantial yearly land loss due to bank erosion,caused by flow concentrations due to temporal evolution of multiple channels in the Brahmaputra River.     

OVERVIEW OF DAM GULLY EROSION RESEARCH

Traditionally gully erosion has been identified with the dissection of the landscape in agricultural settings but it is also recognized as a prevalent erosion feature in earthen dam auxiliary spillways and embankments. Flows through earthen spillways and over dam embankments, due to large rainfall events, have the potential to erode and breach the dam or spillway and result in catastrophic releases from the reservoir. The gully erosion process in an earthen spillway or on an embankment can be characterized by stages of initiation, development, and migration of a headcut. A headcut is defmed as a near vertical drop at the upstream end of a gully. The rate of headcut migration is important in determining the breach potential of an earthen spillway and dam embankment. A research program is being conducted to examine the gully erosion processes of earthen dam auxiliary spillways and embankments. This paper describes： l ） the unique test facilities constructed to examine the dominant factors affecting the erosion of earthen spillways and embankments; 2） the observations of the erosion processes and results to date; and 3） the predictive relationships that have been developed for dam gully erosion research at the ARS Hydraulic Engineering Research Unit laboratory in Stillwater, OK.     

Response of transient rock uplift and base level knickpoints to erosional efficiency contrasts in bedrock streams

Knickpoints in bedrock streams are often interpreted as transient features generated by a change in boundary conditions. It is typically assumed that knickpoints propagate upstream with constant vertical velocities, though this relies on a stream being in erosional steady state (erosion rate equals rock uplift rate) prior to the knickpoint's formation. Recent modeling and field studies suggest that along-stream contrasts in rock erodibility perturb streams from erosional steady state. To evaluate how contrasts in rock erodibility might impact knickpoint interpretations, we test parameter space (rock erodibility, rock contact dip angle, change in rock uplift rate) in a one-dimensional (1D) bedrock stream model that has variable rock erodibility and produces a knickpoint with a sudden change in rock uplift rate. Upstream of a rock contact, the vertical velocity of a knickpoint generated by a change in rock uplift rate is strongly correlated with how the rock contact has previously perturbed erosion rates. These knickpoints increase vertical velocity upon propagating upstream of a hard over soft contact and decrease vertical velocity upon propagating upstream of a soft over hard contact. However, interactions with other transient perturbations produced by rock contacts make for nuances in knickpoint behavior. Rock contacts also influence the geometry of knickpoints, which can become particularly difficult to identify upstream of soft over hard rock contacts. Using our simulations, we demonstrate how a contact's along-stream horizontal migration rate and cross-contact change in rock strength control how much an upstream reach is perturbed from erosional steady state. When simulations include multiple contacts, the knickpoint is particularly prone to colliding with other transient perturbations and can even disappear altogether if rock contact dips are sufficiently shallow. Caution should be taken when analyzing stream profiles in areas with significant changes in rock strength, especially when rock contact dip angles are near the stream's slope.     

Characterisation of erosional features associated with tsunami terrains on rocky coasts of the Maltese islands

In recent years there has been a growing body of literature on depositional signatures associated with historic extreme waves on rocky coasts. Here, in the context of the Maltese islands, we place an innovative focus on evidence of erosional forms. The field evidence is concentrated along the NE flank of the islands at a topographically varied range of sites and up to an altitude of 13 m asl. A range of forms is broadly classified in terms of their morphologies and the forces responsible for their formation. Sockets, eroded scarps, scoured terrains, clifftop erosion scars, swept terrains and spillways are interpreted as consequences of overwashing of the landscape by an extreme wave or waves. These forms are shown to be controlled by flow intensity, topography and lithology, and especially rock bedding and jointing. They comprise the source areas for associated depositional evidence allowing transport paths to be estimated, and may significantly enhance the reconstruction of extreme wave events. It is likely that similar (and additional) erosional forms are present elsewhere in the Mediterranean domain, where comparable lithological and topographic situations are exposed to extreme waves. Copyright © 2015 John Wiley & Sons, Ltd.     

10Be‐derived assessment of accelerated erosion in a glacially conditioned inner gorge,Entlebuch, Central Alps of Switzerland

Inner gorges often result from the propagation of erosional waves related to glacial/interglacial climate shifts. However, only few studies have quantified the modern erosional response to this glacial conditioning. Here, we report in situ ¹⁰Be data from the 64 km² Entlen catchment (Swiss Alps). This basin hosts a 7 km long central inner gorge with two tributaries that are >100 m‐deeply incised into thick glacial till and bedrock. The ¹⁰Be concentrations measured at the downstream end of the gorge yield a catchment‐wide erosion rate of 0.42 ± 0.04 mm yr^‐1, while erosion rates are consistently lower upstream of the inner gorge, ranging from 0.14 ± 0.01 mm yr^‐1 to 0.23 ± 0.02 mm yr^‐1. However, ¹⁰Be‐based sediment budget calculations yield rates of ~1.3 mm yr^‐1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are ~2.0 mm yr^‐1 and ~1.9 mm yr^‐1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from ~2.5 mm yr^‐1 to ~3.8 mm yr^‐1. These rates, however, are too low to infer a post‐glacial age (15–20 ka) for the gorge initiation. This would require erosion rates that are between 2 and 6 times higher than present‐day estimates. However, the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the LGM glaciers, and subsequent hillslope relaxation led to a progressive decrease in erosion rates. This hypothesis of a two‐ to sixfold decrease in erosion rates does not conflict with the ¹⁰Be‐based erosion rate budgets, because the modern erosional time scale recorded by ¹⁰Be cover the past 2–3 ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimization strategies for sediment reduction practices on roads in steep,forested terrain

Many forested steeplands in the western United States display a legacy of disturbances due to timber harvest, mining or wildfires, for example. Such disturbances have caused accelerated hillslope erosion, leading to increased sedimentation in fish‐bearing streams. Several restoration techniques have been implemented to address these problems in mountain catchments, many of which involve the removal of abandoned roads and re‐establishing drainage networks across road prisms. With limited restoration funds to be applied across large catchments, land managers are faced with deciding which areas and problems should be treated first, and by which technique, in order to design the most effective and cost‐effective sediment reduction strategy. Currently most restoration is conducted on a site‐specific scale according to uniform treatment policies. To create catchment‐scale policies for restoration, we developed two optimization models – dynamic programming and genetic algorithms – to determine the most cost‐effective treatment level for roads and stream crossings in a pilot study basin with approximately 700 road segments and crossings. These models considered the trade‐offs between the cost and effectiveness of different restoration strategies to minimize the predicted erosion from all forest roads within a catchment, while meeting a specified budget constraint. The optimal sediment reduction strategies developed by these models performed much better than two strategies of uniform erosion control which are commonly applied to road erosion problems by land managers, with sediment savings increased by an additional 48 to 80 per cent. These optimization models can be used to formulate the most cost‐effective restoration policy for sediment reduction on a catchment scale. Thus, cost savings can be applied to further restoration work within the catchment. Nevertheless, the models are based on erosion rates measured on past restoration sites, and need to be updated as additional monitoring studies evaluate long‐term basin response to erosion control treatments. Copyright © 2006 John Wiley & Sons, Ltd.     

Vegetation disturbances and flood energy during an extreme flood on a sub-tropical river

The advent of 2D hydraulic modelling has improved our understanding of flood hydraulics, thresholds, and dynamic effects on floodplain geomorphology and riparian vegetation at the morphological-unit scale. Hydraulic concepts of bed shear stress, stream power maxima, and energy (cumulative stream power) have been used to characterize floods and define their geomorphic effectiveness. These hydraulic concepts were developed in the context of reach-averaged, 1D hydraulic analyses, but their application to 2D model results is problematic due to differences in the treatment of energy losses in 1D and 2D analyses. Here we present methods for estimating total and boundary resistance from 2D modelling of an extreme flood on a subtropical river. Hydraulic model results are correlated with observations of the flood impacts on floodplain geomorphology and the riparian vegetation to identify thresholds and compute variants of flood energy. Comparison of LiDAR data in 2011 and 2014 shows that the 2011 flood produced 2–4 m of erosion on floodplain bars that were previously forested or grass-covered. Deposition on flood levees, dunes, and chute bars was up to 3.4 m thick. Various hydraulic metrics were trialled as candidates for thresholds of vegetation disturbance. The accuracy of thresholds using metrics extracted at the flood peak (i.e. boundary resistance and stream power maxima) was similar to that using energy as a threshold. Disturbance to forest and grass on vegetated bars was associated with stream powers of >834 W/m² and unit flows of >26 m²/s, respectively. Correlation of the hydraulic metrics with erosion and deposition depths showed no substantial improvement in using flood energy compared to metrics extracted at the flood peak for describing erosion and deposition. The extent of vegetation disturbances and morphological adjustments was limited for this extreme flood, and further 2D studies are needed to compare disturbance thresholds across different environments.     

Longitudinal distributions of river flood power: the combined automated flood,elevation and stream power (CAFES) methodology

Stream power can be an extremely useful index of fluvial sediment transport, channel pattern, river channel erosion and riparian habitat development. However, most previous studies of downstream changes in stream power have relied on field measurements at selected cross‐sections, which are time consuming, and typically based on limited data, which cannot fully represent important spatial variations in stream power. We present here, therefore, a novel methodology we call CAFES (combined automated flood, elevation and stream power), to quantify downstream change in river flood power, based on integrating in a GIS framework Flood Estimation Handbook systems with the 5 m grid NEXTMap Britain digital elevation model derived from IFSAR (interferometric synthetic aperture radar). This provides a useful modelling platform to quantify at unprecedented resolution longitudinal distributions of flood discharge, elevation, floodplain slope and flood power at reach and basin scales. Values can be resolved to a 50 m grid. CAFES approaches have distinct advantages over current methodologies for reach‐ and basin‐scale stream power assessments and therefore for the interpretation and prediction of fluvial processes. The methodology has significant international applicability for understanding basin‐scale hydraulics, sediment transport, erosion and sedimentation processes and river basin management. Copyright © 2008 John Wiley & Sons, Ltd.     

Nondimensional sediment transport capacity of sand soils and its response to parameter in the Loess Plateau of China

Soil erosion is a major contributor to land degradation in the Loess Plateau in China. To clarify the sediment transport capacity of overland flow influenced by hydraulic parameters, such as shear stress, sand shear stress (hydraulic gradient partition method and hydraulic radius partition method), mean flow velocity, Froude number, stream power, and unit stream power, indoor experiments with eight-unit-width flow discharges from 0.0667 × 10⁻³ to 0.3333 × 10⁻³ m²·s⁻¹, six slope gradients from 3.49 to 20.79%, and two kinds of sand soils (d₅₀ = 0.17 and 0.53 mm) were systematically investigated. A nondimensional method was adopted in data processing. Results showed that there was a partition phenomenon of relation curves because of the different median grain diameters. The correlation between the nondimensional stream power and nondimensional sediment transport capacity was the highest, followed by the correlation between the nondimensional unit stream power and nondimensional sediment transport capacity. However, there was a poor correlation between the flow intensity indices of velocity category and nondimensional sediment transport capacity. Nondimensional stream power, nondimensional unit stream power, and nondimensional shear stress could predict sediment transport capacity well. Ignoring the partition phenomenon of the relation curves, stream power could be used to predict sediment transport capacity, with a coefficient of determination of .85. Furthermore, a general flow intensity index was obtained to predict sediment transport capacity of overland flow. Finally, an empirical formula for predicting sediment transport capacity with a coefficient of determination of .90 was established by multiple regression analyses based on the general flow intensity index. During the analysis between measured sediment transport capacities in present study and predicted values based on Zhang model, Mahmoodabadi model, and Wu model, it was found that these three models could not accurately predict sediment transport capacities of this study because different models are estimated on the basis of different experimental conditions.     

Application of a simple headcut advance model for gullies

Gully erosion begins in streambanks and uplands as a consequence of adjustments in driving forces on the landscape imposed by changes in land use or climate. The deleterious effects of gullies worldwide have led to many site‐specific studies of gully form and function. In the continental United States, gully erosion in agricultural land has destroyed valuable farmland yet, prediction of gully processes remains problematic on a national scale. This research has proposed a simple method to predict gully headcut advance. When combined with SWAT hydrologic flow routines, the model predicted gully headcut advance with reasonable accuracy on a daily time step for time periods exceeding two decades. The model was tested in two distinct land resource areas of the United States with differing climate, soils, cover and drainage. The inputs for the headcut model have been kept simple as the model will be applied over large areas. Model inputs consist of headcut height, headcut resistance (based on soil erodibility and a root‐cover factor), and daily flow. The model is compared with an annual time step model used in assessment of headcut advance and appears to offer a better way to assess gully headcut advance. Copyright © 2017 John Wiley & Sons, Ltd.     

Evaluation of Bias Associated with Capture Maps Derived from Nonlinear Groundwater Flow Models

The impact of groundwater withdrawal on surface water is a concern of water users and water managers, particularly in the arid western United States. Capture maps are useful tools to spatially assess the impact of groundwater pumping on water sources (e.g., streamflow depletion) and are being used more frequently for conjunctive management of surface water and groundwater. Capture maps have been derived using linear groundwater flow models and rely on the principle of superposition to demonstrate the effects of pumping in various locations on resources of interest. However, nonlinear models are often necessary to simulate head‐dependent boundary conditions and unconfined aquifers. Capture maps developed using nonlinear models with the principle of superposition may over‐ or underestimate capture magnitude and spatial extent. This paper presents new methods for generating capture difference maps, which assess spatial effects of model nonlinearity on capture fraction sensitivity to pumping rate, and for calculating the bias associated with capture maps. The sensitivity of capture map bias to selected parameters related to model design and conceptualization for the arid western United States is explored. This study finds that the simulation of stream continuity, pumping rates, stream incision, well proximity to capture sources, aquifer hydraulic conductivity, and groundwater evapotranspiration extinction depth substantially affect capture map bias. Capture difference maps demonstrate that regions with large capture fraction differences are indicative of greater potential capture map bias. Understanding both spatial and temporal bias in capture maps derived from nonlinear groundwater flow models improves their utility and defensibility as conjunctive‐use management tools.     

Availability and performance of sediment detachment and transport functions for overland flow conditions

Abstract

Soil erosion is a global environmental problem. To quantify water erosion rates at the field, hillslope or catchment scale, several spatially-distributed soil erosion models have been developed. The accuracy of those models depends largely on the sediment detachment and transport functions used, many of which were developed from empirical research. In this paper, the physical basis of the available sediment detachment and transport functions is reviewed, and their application boundaries determined. Well-known and widely-used sediment detachment and transport functions are discussed on the basis of composite force predictors, i.e. shear stress, stream power, unit stream power and effective stream power, and their suitability is elucidated based on information in the literature. It was found that only a few sediment detachment functions are available, and those have been poorly tested. Most erosion models ignore direct calculation of sediment detachment, but use the sediment transport capacity deficit approach to estimate detachment rate. Many more sediment transport functions are available that also tested better for overland flow conditions. However, our tests did not result in a single function that appeared to perform best under a range of experimental conditions. The unit stream power-based functions developed by Govers seem to be the most promising ones for water erosion modelling. It is therefore recommended to evaluate the performance of existing sediment transport functions with more detailed field and laboratory datasets.

Editor Z.W. Kundzewicz     

Tracking sediment provenance and erosional evolution of the western Greater Caucasus

This article investigates landscape characteristics and sediment composition in the western Greater Caucasus by using multiple methods at different timescales. Our ultimate goal is to compare short‐term versus long‐term trends in erosional processes and to reconstruct spatio‐temporal changes in sediment fluxes as controlled by partitioning of crustal shortening and rock uplift in the orogenic belt. Areas of active recent uplift are assessed by quantitative geomorphological techniques [digital elevation model (DEM) analysis of stream profiles and their deviation from equilibrium] and compared with regions of rapid exhumation over longer time intervals as previously determined by fission‐track and cosmogenic‐nuclide analyses. Complementary information from petrographic and heavy‐mineral analyses of modern sands and ancient sandstones is used to evaluate erosion integrated throughout the history of the orogen. River catchments displaying the highest relief, as shown by channel‐steepness indices, correspond with the areas of most rapid exhumation as outlined by thermochronological data. The region of high stream gradients is spatially associated with the highest topography around Mount Elbrus, where sedimentary cover strata have long been completely eroded and river sediments display the highest metamorphic indices and generally high heavy‐mineral concentrations. This study reinforces the suggestion that the bedrock–channel network can reveal much of the evolution of tectonically active landscapes, and implies that the controls on channel gradient ultimately dictate the topography and the relief along the Greater Caucasus. Our integrated datasets, obtained during a decade of continuing research, display a general agreement and regularity of erosion patterns through time, and consistently indicate westward decreasing rates of erosional unroofing from the central part of the range to the Black Sea. Copyright © 2014 John Wiley & Sons, Ltd.     

Estimating overland flow erosion capacity using unit stream power

Soil erosion caused by water flow is a complex problem. Both empirical and physically based approaches were used for the estimation of surface erosion rates. Their applications are mainly limited to experimental areas or laboratory studies. The maximum sediment concentration overland flow can carry is not considered in most of the existing surface erosion models. The lack of erosion capacity limitation may cause over estimations of sediment concentration. A correlation analysis is used in this study to determine significant factors that impact surface erosion capacity. The result shows that the unit stream power is the most dominant factor for overland flow erosion which is consistent with experimental data. A bounded regression formula is used to reflect the limits that sediment concentration cannot be less than zero nor greater than a maximum value. The coefficients used in the model are calibrated using published laboratory data. The computed results agree with laboratory data very well. A one dimensional overland flow diffusive wave model is used in conjunction with the developed soil erosion equation to simulate field experimental results. This study concludes that the non-linear regression method using unit stream power as the dominant factor performs well for estimating overland flow erosion capacity.     

Prediction of episodic acidification in North‐eastern USA: an empirical/mechanistic approach

Observations from the US Environmental Protection Agency's Episodic Response Project (ERP) in the North‐eastern United States are used to develop an empirical/mechanistic scheme for prediction of the minimum values of acid neutralizing capacity (ANC) during episodes. An acidification episode is defined as a hydrological event during which ANC decreases. The pre‐episode ANC is used to index the antecedent condition, and the stream flow increase reflects how much the relative contributions of sources of waters change during the episode. As much as 92% of the total variation in the minimum ANC in individual catchments can be explained (with levels of explanation >70% for nine of the 13 streams) by a multiple linear regression model that includes pre‐episode ANC and change in discharge as independent variables. The predictive scheme is demonstrated to be regionally robust, with the regional variance explained ranging from 77 to 83%. The scheme is not successful for each ERP stream, and reasons are suggested for the individual failures. The potential for applying the predictive scheme to other watersheds is demonstrated by testing the model with data from the Panola Mountain Research Watershed in the South‐eastern United States, where the variance explained by the model was 74%. The model can also be utilized to assess ‘chemically new’ and ‘chemically old’ water sources during acidification episodes. Copyright © 1999 John Wiley & Sons, Ltd.