Search

A common phenomenon associated with the alluvial river is its meandering action which leads to lateral migration and thus geomorphic hazards. Predicting and preventing this migration is both difficult and necessary. Barak river is one of the highly meandering rivers flowing through the alluvial plains of Assam, India. It is observed that there is a regular shifting of the river banks and development of cutoffs which create uncertainty to the people residing nearby. Therefore, in this study planform geometry and migration behaviour of the Barak river is examined considering 12 representative meandering reaches using multiperiod Landsat remote sensing images, field investigations of channel bed and bank properties and riparian vegetation cover. An attempt is made to describe and evaluate the empirical approach and time sequence extrapolation method to predict channel migration. Channel wavelength to channel width ratio ranges between 5.53 to 12.9, and the bend curvature ranges between 1.1 to 3.93. Rate of river migration varies between 0.54to 85.69 m/year. Sinuosity in most of the meandering reaches is greater than 1.5. Results show that lack of significant riparian vegetative cover, high precipitation and presence of fine sands with very low clay content are probably the main elements responsible for the planform changes. The results of the prediction of meander migration obtained from selected empirical methods show that only Nanson and Hickin method shows a moderate correlation with R2 =0.50. Time sequence extrapolation method was used to predict the radius of the best-fit circle for the year 2025 and 2030. Results of the time sequence extrapolation method indicate that the maximum and minimum radius of the best circle fit for the year 2025 is 1254m at reach-8 and 462m at reach-7 respectively, and maximum and minimum radius of the best circle fit for the year 2030 is 1319m at reach-8 and 387m at reach-12 respectively. It is believed that the outcomes of this study could form a base in river training works and in understanding and predicting the future dynamics and bank migration of this alluvial river and other river of similar geomorphic setting.

Alluvial rivers commonly are subject to the integrated effects of hydrodynamic and eco-geomorphologic controls and there has been a lack of suitable methods to measure the effects. By taking the alluvial reach of the Yellow River over the Yinchuan Plain as a suitable example, this study evaluates the degree of hydrodynamic control in the channel-form adjustment of different channel patterns in light of the advances in equilibrium theory governing alluvial channel flow. In response to the significant variations in flow regime and channel forms, the non-dimensional number (Formula presented.) that measures the equilibrium state of alluvial channel flow varies in the ranges of 0.23–0.65, 0.047–0.17 and 0.0012–0.0024 respectively in the anabranching, meandering and braided reaches during 1993–2015. The significant differences among the H-ranges are mainly because the individual anabranches have neither very narrow nor very deep cross-sections, while the single-thread channels in the meandering and braided reaches take moderately and significantly wider and shallower cross-sections, respectively. These results demonstrate that the (Formula presented.) number is a good discriminator of river channel patterns, and the relatively small variability of (Formula presented.) within each channel pattern implies that the channels in the study reach are resilient to significant change in flow regime but yet hydrodynamic control is only partial. While the (Formula presented.) number is capable of embodying the outcome of the complex integrated effects of multiple localized eco-geomorphic controls with flow dynamics, more studies are required to define its specific varying ranges for different river channel patterns and differing eco-geomorphic controls.

The paper presents recommendations for using the results obtained in sediment transport simulation and modeling of channel deformations in rivers. This work relates to the issues of empirical modeling of the water flow characteristics in natural riverbeds with a movable bottom (alluvial channels) which are extremely complex. The study shows that in the simulation of sediment transport and calculation of channel deformations in the rivers, it is expedient to use the calculation dependences of Chézy’s coefficient for assessing the roughness of the bottom sediment mixture, or the dependences of the form based on the field investigation data. Three models are most commonly used and based on the original formulas of Meyer-Peter and Müller (1948), Einstein (1950) and van Rijn (1984). This work deals with assessing the hydraulic resistance of the channel and improving the river sediment transport model in a simulation of riverbed transformation on the basis of previous research to verify it based on 296 field measurements on the Central-East European lowland rivers. The performed test calculations show that the modified van Rijn formula gives the best results from all the considered variants.

The lower Tejo river basin is an important crop production area in Portugal. The mild climate, diverse water sources and availability along with flat and fertile land areas provide the condition to a highly productive region. Although the historically rainfed cultures installation since the eighteen century, irrigated cultures, such as rice, started to gradually occupy the region in the beginning of the twentieth century. Nowadays, rice cultivation prevails in the area, which requires reliable sources to fulfil water demand for this culture. Paddy-rice fields installation in Tejo tributaries alluvium, such as Sorraia and Almansor rivers, directly supply those parcels in the vicinity. Nevertheless, the largest production area is installed in Lezirias de Vila Franca de Xira, located in Tejo alluvium plains. Such area requires large amounts of water during crop season, from April to October, mainly withdrawn from Tejo river. The Tejo estuary proximity and its salinity influence along with demand peak during summer compromise water quality which, in a long term, can cause salinization and alkalinization of soils. Rice is a light salinity tolerant culture, which is adapted with the current water quality, but the region faces serious sustainability challenges if a shift for some other crop type occurs, as happened in the past. Multiple Correspondence Analysis was used as a statistical method to assess water quality in the region. Water management, driven with water quality allocation by crop and soils requirements can be an answer to minimize permanent soil damages achieving natural resources sustainability in the long term.

The aim of this research was to produce a new methodology for a special river bottom hazard mapping for the stability purposes of the biggest Polish water power plant: Włocławek. During the operation period of the water power plant, an engineering-geological issue in the form of pothole formation on the Wisła River bed in the gravel-sand alluvium was observed. This was caused by increased fluvial erosion resulting from a reduced water level behind the power plant, along with frequent changes in the water flow rates and water levels caused by the varying technological and economic operation needs of the power plant. Data for the research were obtained by way of a 4-year geodetic/bathymetric monitoring of the river bed implemented using integrated GNSS (Global Navigation Satellite System), RTS (Robotized Total Station) and SBES (Single Beam Echo Sounder) methods. The result is a customized river bottom hazard map which takes into account a high, medium, and low risk levels of the potholes for the water power plant structure. This map was used to redevelop the river bed by filling. The findings show that high hazard is related to 5% of potholes (capacity of 4308 m3), medium with 38% of potholes (capacity of 36,455 m3), and low hazard with 57% of potholes (capacity of 54,396 m3). Since the construction of the dam, changes due to erosion identified by the monitoring have concerned approximately 405,252 m3 of the bottom, which corresponds to 130 Olympic-size pools. This implies enormous changes, while a possible solution could be the construction of additional cascades on the Wisła River.

Dunes are critical for understanding riverine sediment transport, deposition, flow resistance and channel flow processes. Although previous studies have examined the riverbed micromorphology of the Lower Mississippi River in the USA, our knowledge of detailed quantification of dune morphology in this and other large alluvial rivers is still limited. It is also not well understood how dunes in a straight channel reach and a meander bend differ in their characteristics, as well as how dune morphology may have been affected by human activities (i.e., river engineering). In this study, we utilized multi-beam bathymetric measurements over four 1.6–8.0 km long reaches in the Lowermost Mississippi River (LmMR) to analyze riverbed micromorphologic features. Three of the four reaches were located in the upper part of the LmMR between river kilometers (RK) 474–477, 483.6–485.2 and 491.7–493.3, and the other reach was located in the lower part, between RK 120 and RK 128. We analyzed a total of 3258 dunes in these river reaches and found that large dunes were dominant in the LmMR. These dunes were characterized by low mean leeside slope angle (10.8°), indicating that flow resistance caused by dunes should be relatively small. When compared with dunes in the straight reaches, dunes in the meander bends were much larger (1.06 m vs. 0.81 m) and had a higher bed roughness (0.91 vs. 0.68), which may be related to the varied flow velocity. Dune size increased with increasing water depth across the river channel of a straight reach, while it decreased with increasing water depth across the river channel of a meander bend. When compared with the dunes in the lower-river reach, the dunes in the upper-river reach were significantly higher in height and shorter in wavelength, and showed much higher bed roughness (1 vs. 0.68), which may be closely related to the greater riverbed slope and grain size of bed sediment occurred in the upper-river reach, as well as a combined effect of the Old River Control Structure (RK 500) and backwater. These findings indicate the strong impact of turbulent flow, slope and sand source on dune formation and riverbed deposition.

The presence of bare or partially-covered rock in an otherwise alluvial river implies a downstream change in transport capacity relative to supply. Field investigations of this change and what causes it are lacking. We used two sets of magnet-tagged tracer clasts to investigate bedload transport during the same sequence of floods in fully alluvial, bare rock, and partial-cover reaches of an upland stream. High-flow shear stresses in different reaches were calculated using stage loggers. Tracers seeded in the upstream alluvial channel moved more slowly than elsewhere until the frontrunners reached bare rock and sped up. Tracers seeded on bare rock moved rapidly off it and accumulated just upstream from, and later in, a partial-cover zone with many boulders. The backwater effect of the boulder-rich zone is significant in reducing tracer mobility. Tracer movement over full or partial sediment cover was size selective but dispersion over bare rock was not. Along-channel changes in tracer mobility are interpreted in terms of measured differences in shear stress and estimated differences in threshold stress.

Understanding the spatial organization of river systems in light of natural and anthropogenic change is extremely important because it can provide information to assess, manage, and restore them to ameliorate worldwide freshwater fauna declines. For gravel- and cobble-bedded alluvial rivers studies spanning analytical, empirical and numerical domains suggest that at channel-forming flows there is a tendency towards covarying bankfull bed and width undulations amongst morphologic units such as pools and riffles, whereby relatively wide areas have relatively higher minimum bed elevations and relatively narrow areas have relatively lower minimum bed elevations. The goal of this study was to determine whether minimum bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel–cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges. A key result is that the test river exhibited covarying oscillations of minimum bed elevation and channel top width across all flows analyzed. These covarying oscillations were found to be quasiperiodic at channel-forming flows, scaling with the length scales of bars, pools and riffles. Thus, it appears that alluvial rivers organize their topography to have quasiperiodic, shallow and wide or narrow and deep cross section geometry, even despite ongoing, centennial-scale incision. Presumably these covarying oscillations are linked to hydrogeomorphic mechanisms associated with alluvial river channel maintenance. The biggest conclusion from this study is that alluvial rivers are defined more so by variability in topography and flow than mean conditions. Broader impacts of this study are that the methods provide a framework for characterizing longitudinal and flow-dependent variability in rivers for assessing geomorphic structure and aquatic habitat in space, and if repeated, through time.

In this chapter, we discuss recent developments in paleoflood hydrology that are specific to the collection of high-resolution records from alluvial settings. We describe how to develop and analyze alluvial paleoflood records, including (i) the identification of suitable depositional niches in valley environments and the mechanisms of overbank deposition that lead to flood deposit accumulation at those locations, (ii) approaches for sample material collection and methods to measure the coarseness of individual flood units, (iii) data assessment and reconstruction of absolute and relative flood magnitudes from sedimentary information, and (iv) developing flood chronologies with a discussion of available dating techniques. We argue that alluvial paleoflood archives hold enormous potential for flood hazard assessments in densely populated low-lying areas despite the challenge of quantitative discharge reconstructions in dynamic floodplain settings.

Modelling stable alluvial river profile is one of the most important and challenging issues in river engineering that several studies have been dedicated to it. The main objective of this study is to evaluate the back propagation-based multilayer neural network (BP-MLNN) performance in predicting stable alluvial river profile. We used eighty-five observational datasets to train and test, three separate models to predict each of the channel width (w), flow depth (h) and longitudinal slope (s) of stable channels. The network input parameters are the flow discharge (Q), mean sediment size (d50) and affecting Shields parameter (τ*) and w, h and s parameters are the output. It is concluded from the results that the proposed models to predict the width, depth, and slope of stable channels, with a correlation coefficient (R) of 0.96, 0.886, and 0.870 respectively, perform well. The mean absolute relative error (MARE) value of 0.063 related to the width estimation model in comparison with the depth and slope estimation model with MARE value of 0.077 and 0.518 shows the superior accuracy of the BP-MLNN model. The presented BP-MLNN models in this study are therefore recommended in river engineering projects to estimate the cross-sectional dimensions of stable alluvial channels as simple and robust design tools.

Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift rate, and steepness increases with an increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: to match observed channel concavities, gravel-sized sediments must weather and fine – typically rapidly – and valleys typically should widen gradually. To match the empirical square-root width–discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 680

The profile of a river that conveys sediment without net deposition and net erosion is referred to as ‘graded’ with respect to vertical aggradation of the river segment. Three experimental series, designed in terms of the autostratigraphic view of alluvial grade, were conducted to clarify the diagnostic spatial behaviour of graded alluvial?deltaic rivers: an ‘R series’, which utilized a moving boundary setting with a stationary base level; an ‘F series’ in a fixed boundary setting with a stationary base level to produce ‘forced grade’; and an ‘M series’ in a moving boundary setting with constant base-level fall to produce ‘autogenic grade.’ The results of the three experimental series, combined with geometrical modelling of the effects of basin water depth and other experimental data, suggest the following: (i) in a graded alluvial?deltaic system, lateral shifting and avulsing of active distributary channels are suppressed regardless of whether the downstream boundary of the deltaic system is fixed; (ii) in a delta with a downstream-fixed boundary, the graded streams are stabilized within a valley that is incised in the axial part of the delta plain, whereby the alluvial plain outside the valley is abandoned and terraced; (iii) in moving boundary settings, the graded river simply extends basinward as a linearly elongated channel and lobe system without cutting a valley; and (iv) a modern forced-graded alluvial river is most likely to be found in a valley incised into a fan delta in front of very deep water, and the stratigraphic signal of fossil autogenic-graded rivers will be found in deltaic successions that accumulated in the outer to marginal areas of deltaic continental shelves during sea-level falls. This renewed autostratigraphic view of alluvial grade suggests a thorough reconsideration of the conventional understanding that an alluvial river feeding a progradational delta is graded with a stationary base level., Sedimentology, 63(7), pp.2158-2189; 2016

Observations from field investigations showed that flow velocity greater than 3 m/s rarely occurs in nature, and high flow velocity stresses the bio-community and causes instability to the channel. For alluvial rivers without strong human disturbance, the flow velocity varies within a limited range, generally below 3 m/s, while the discharge and wet area may vary in a range of several orders. This phenomenon was studied by analyzing hydrological data, including daily average discharge, stage, cross sections, and sediment concentration, collected from 25 stations on 20 rivers in China, including the Yangtze, Yellow, Songhua, Yalu, Daling, and Liaohe Rivers. The cross-sectional average velocity was calculated from the discharge and wet area using the continuity equation. For alluvial rivers, the wet cross section may self-adjust in accordance with the varying flow discharge so that the flow velocity does not exceed a limit value. In general, the average velocity increases with the discharge increase at low discharge. As the discharge exceeds the discharge capacity of the banks, any further increase in discharge does not result in a great increase in velocity. The average velocity approaches an upper limit as the discharge increases. This limit velocity, in most cases, is less than 3 m/s. Human activities, especially levee construction, disturb the limit velocity law for alluvial rivers. In these cases, the average velocity may be approximately equal to or higher than the limit velocity. The limit velocity law has profound morphological and ecological implications on alluvial rivers and requires further study. Rivers should be trained and managed by mimicking natural processes and meeting the limit velocity law, so as to maintain ecologically-sound and morphological stability.

There has been quite some debate on the relative importance of particle abrasion and grain size selective transport regarding the river profile form and the associated grain size trends in a graded alluvial stream. Here we present new theoretical equations for the graded alluvial river profile that account for the effects of particle abrasion and grain size selective transport in the absence of subsidence, uplift, and sea level change. Under graded conditions we find that abrasion results in a mild profile concavity and downstream fining, whereas under aggradational conditions grain size selective transport can lead to large spatial changes in channel slope and bed surface mean grain size.

The indeterminate channel problem arises from uncertainty in finding a closure relation for alluvial channels created by self-organizing erosional and depositional processes. Extremal hypotheses have been proposed as one potential approach to closing the system of governing equations for alluvial channels. Many different extremal hypotheses have been presented, but no substantive evidence has been developed to select which hypothesis may be most appropriate for natural alluvial river systems. This paper evaluates the ability of ten extremal hypotheses to identify dynamic equilibrium across a geomorphic gradient in the remote and undeveloped mid-latitude watershed of Rio Murta, Chile. This study (a) introduces extremal hypotheses, (b) describes the field site and geomorphic conditions, and (c) examines which extremal hypotheses are supported by the field data in identifying the evolutionary trend toward dynamic-equilibrium. The extremal hypotheses that identified dynamic equilibrium within the geomorphic gradient in the field are: (1) minimum kinetic energy, (2) minimum specific stream power, (3) maximum friction factor, and (4) maximum total friction factor, which collectively support minimizing kinetic energy of the system.

Synthetic Aperture Radar (SAR) amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength. These backscatter signals are often exploited to assess the roughness of plowed agricultural fields and water surfaces, and less so to complex, heterogeneous geological surfaces. The bedload of mixed sand- and gravel-bed rivers can be considered a mixture of smooth (compacted sand) and rough (gravel) surfaces. Here, we assess backscatter gradients over a large high-mountain alluvial river in the eastern Central Andes with aerially exposed sand and gravel bedload using X-band TerraSAR-X/TanDEM-X, C-band Sentinel-1, and L-band ALOS-2 PALSAR-2 radar scenes. In a first step, we present theory and hypotheses regarding radar response to an alluvial channel bed. We test our hypotheses by comparing backscatter responses over vegetation-free endmember surfaces from inside and outside of the active channel-bed area. We then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates. In a final step, the local variability of sand-dominated patches is analyzed using Fourier frequency analysis, by fitting stretched-exponential and power-law regression models to the 2-D power spectrum of backscatter amplitude. We find a large range in backscatter depending on the heterogeneity of contiguous smooth- and rough-patches of bedload material. The SAR amplitude signal responds primarily to the fraction of smooth-sand bedload, but is further modified by gravel elements. The sensitivity to gravel is more apparent in longer wavelength L-band radar, whereas C- and X-band is sensitive only to sand variability. Because the spatial extent of smooth sand patches in our study area is typically< 50 m, only higher resolution sensors (e.g., TerraSAR-X/TanDEM-X) are useful for power spectrum analysis. Our results show the potential for mapping sand-gravel transitions and local geomorphic complexity in alluvial rivers with aerially exposed bedload using SAR amplitude.

The 1-D saltation–abrasion model of channel bedrock incision of Sklar and Dietrich (2004), in which the erosion rate is buffered by the surface area fraction of bedrock covered by alluvium, was a major advance over models that treat river erosion as a function of bed slope and drainage area. Their model is, however, limited because it calculates bed cover in terms of bedload sediment supply rather than local bedload transport. It implicitly assumes that as sediment supply from upstream changes, the transport rate adjusts instantaneously everywhere downstream to match. This assumption is not valid in general, and thus can give rise to unphysical consequences. Here we present a unified morphodynamic formulation of both channel incision and alluviation that specifically tracks the spatiotemporal variation in both bedload transport and alluvial thickness. It does so by relating the bedrock cover fraction to the ratio of alluvium thickness to bedrock macro-roughness, rather than to the ratio of bedload supply rate to capacity bedload transport. The new formulation (MRSAA) predicts waves of alluviation and rarification, in addition to bedrock erosion. Embedded in it are three physical processes: alluvial diffusion, fast downstream advection of alluvial disturbances, and slow upstream migration of incisional disturbances. Solutions of this formulation over a fixed bed are used to demonstrate the stripping of an initial alluvial cover, the emplacement of alluvial cover over an initially bare bed and the advection–diffusion of a sediment pulse over an alluvial bed. A solution for alluvial–incisional interaction in a channel with a basement undergoing net rock uplift shows how an impulsive increase in sediment supply can quickly and completely bury the bedrock under thick alluvium, thus blocking bedrock erosion. As the river responds to rock uplift or base level fall, the transition point separating an alluvial reach upstream from an alluvial–bedrock reach downstream migrates upstream in the form of a "hidden knickpoint". A tectonically more complex case of rock uplift subject to a localized zone of subsidence (graben) yields a steady-state solution that is not attainable with the original saltation–abrasion model. A solution for the case of bedrock–alluvial coevolution upstream of an alluviated river mouth illustrates how the bedrock surface can be progressively buried not far below the alluvium. Because the model tracks the spatiotemporal variation in both bedload transport and alluvial thickness, it is applicable to the study of the incisional response of a river subject to temporally varying sediment supply. It thus has the potential to capture the response of an alluvial–bedrock river to massive impulsive sediment inputs associated with landslides or debris flows.

The Padma river is widely known for its dynamic and disastrous behaviour, and the river has been experiencing intense and frequent bank erosion and deposition leading to the changes and shifting of bank line. In this paper, a time series of Landsat satellite imagery MSS, TM and OLI and TIRS images and are used to detect river bank erosion-accretion and bank line shifting during the study period 1975–2015. This study exhibits a drastic increase of erosion and accretion of land along the Padma river. The results show that from 1975 to 2015, the total amount of river bank erosion is 49,951 ha of land, at a rate of 1,249 ha a−1 and the total amount of accretion is 83,333 ha of land, at a rate of 2,083 ha a−1. Throughout the monitoring period, erosion-accretion was more pronounced in the right part of the river and bank line had been shifting towards the southern direction. The paper also reveals that the total area of islands had been increased significantly, in 2015 there was about 50,967 ha of island area increased from 20,533 ha of island area in 1975, and the results evidence consistency of sedimentation in the river bed.

Dam construction in the 1960s to 1980s significantly modified sediment supply from the Kenyan uplands to the lower Tana River. To assess the effect on suspended sediment fluxes of the Tana River, we monitored the sediment load at high temporal resolution for 1 year and complemented our data with historical information. The relationship between sediment concentration and water discharge was complex: at the onset of the wet season, discharge peaks resulted in high sediment concentrations and counterclockwise hysteresis, while towards the end of the wet season, a sediment exhaustion effect led to low concentrations despite the high discharge. The total sediment flux at Garissa (c. 250 km downstream of the lowermost dam) between June 2012 and June 2013 was 8.8 Mt yr-1. Comparison of current with historical fluxes indicated that dam construction had not greatly affected the annual sediment flux. We suggest that autogenic processes, namely river bed dynamics and bank erosion, mobilized large quantities of sediments stored in the alluvial plain downstream of the dams. Observations supporting the importance of autogenic processes included the absence of measurable activities of the fall-out radionuclides 7Be and 137Cs in the suspended sediment, the rapid lateral migration of the river course, and the seasonal changes in river cross-section. Given the large stock of sediment in the alluvial valley of the Tana River, it may take centuries before the effect of damming shows up as a quantitative reduction in the sediment flux at Garissa. Many models relate the sediment load of rivers to catchment characteristics, thereby implicitly assuming that alterations in the catchment induce changes in the sediment load. Our research confirms that the response of an alluvial river to external disturbances such as land use or climate change is often indirect or non-existent as autogenic processes overwhelm the changes in the input signal. Copyright © 2015 John Wiley & Sons, Ltd.

Steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) near the southern limit of the species’ range commonly use shallow alluvial rivers for migration, spawning, and rearing. These rivers have been widely modified for water management, and an enduring question is whether their rehabilitation would create summer nursery habitat for steelhead. We used process-based models to evaluate the thermal potential for steelhead nursery habitat in the Santa Ynez River, California, a regulated alluvial river that currently supports few steelhead. We assessed (1) how well a calibrated model of river heat fluxes predicted summer temperature patterns for a warm year and an average year; (2) whether those patterns created thermal potential for the rapid growth that is characteristic of steelhead nursery habitat; and (3) whether manipulation of flows from an upstream dam significantly altered thermal potential. In the heat flux model, the root mean square error for 15-min temperatures was 1.51°C, about three tim...

We have investigated the geologic controls on hydraulic geometry of bedrock rivers using a single large catchment, the New River, from a stable tectonic setting with variable, resistant lithology but spatially stable climate. Our survey of channel width at 0.5 km spacing along 572 km of the river shows major variation that only roughly fits the expected scaling relationships between width, drainage area, and slope. Considerable variations in width, including steps in trends and large spikes, relate to physiogeologic boundaries that the river passes through. A large fraction (15%) of the river's length classifies as bedrock reach, showing that it behaves more like a bedrock river than an alluvial river. Unlike established trends, the channel is wider in bedrock than in alluvium. Field observations show that aspect ratio (width to depth) is not constant, but fluctuates systematically with width from wide, shallow reaches to narrower, deeper reaches. Our observations of bedrock properties suggest that susceptibility to fluvial plucking versus abrasion may control this anomalous channel morphology. One end member form with aspect ratio as high as 500, which we term the incision plain , is associated with very closely spaced discontinuities (~ 10 cm) in otherwise hard rock. We propose that the closely spaced discontinuities enable efficient plucking that leads to widening by lateral erosion. This morphology locally occurs in other passive margin rivers and may be a fundamental fluvial form that is similar to, but the inverse of, slot canyons. The other end member, which we term channel neck , is narrower and deeper with complex flow paths through blocky bedrock. This form occurs where discontinuity spacing is longer (> 0.5 m) and erosion is abrasion dominated. These results imply that changes in channel width do not necessarily reflect variations in uplift rate, but instead may result from complex response to bedrock properties.

The paper presents a 0-D model of an alluvial watercourse schematized in two connected reaches, evolving at the long time-scale and under the hypothesis of Local Uniform Flow. Each reach is defined by its geometry (constant length and width, time-changing slope) and grain-size composition of the bed, while the sediment transport is computed using a sediment rating curve. The slope evolution is provided by a 0-D mass balance and the evolution of the bed composition is computed by a 0-D Hirano equation. A system of differential equations, solved with a predictor-corrector scheme, is derived and applied to the schematic watercourse to simulate the morphological response to changing initial conditions, and the evolution towards long-term equilibrium conditions. Differently from a single-reach 0-D schematization with uniform grain-size, besides the simplifications adopted, the model proposed here simulates the behaviour of alluvial rivers in a physically-based way, showing a grain-size fining in the downstream direction accompanied by milder slopes, and a tendency to develop concave longitudinal profiles.

Bedsa is one of the districts in Dahshour that lays south west of Cairo and suffered from liquefaction during October 1992 earthquake, Egypt. The soil profile consists of alluvial river Nile deposits mainly sandy mud with low plasticity; the ground water is shallow. The earthquake hypocenter was 18 km far away with local magnitude 5.8; the fault length was 13.8 km, as recorded by the Egyptian national seismological network (ENSN) at Helwan. The analysis used the empirical method introduced by the national center for earthquake engineering research (NCEER) based on field standard penetration of soil. It is found that the studied area can liquefy since there are saturated loose sandy silt layers at depth ranges from 7 to 14 m. The settlement is about 26 cm. The probability of liquefaction ranges between 40% and 100%. The presence of impermeable surface from medium cohesive silty clay acts as a plug resisting and trapping the upward flow of water during liquefaction, so fountain and spouts at weak points occurs. It is wise to use point bearing piles with foundation level deeper than 14 m beyond the liquefiable depth away from ground slopes, otherwise liquefaction improving techniques have to be applied in the area.

Alluvial river channels often exhibit a relatively abrupt transition from gravel- to sand-bedded conditions. The phenomenon is well documented, but few prior studies have analyzed the spatial variability through reaches where transitions occur. The downstream fining pattern observed in the Fraser River is cited as a classic example of an abrupt gravel-sand transition in a large alluvial channel. However, important questions regarding the exact location of the transition, its sedimentology and morphology, and what controls its location remain unanswered. Here we present observations of the downstream change in bed material grain size, river bed topography, and channel hydraulics through the reach within which the transition occurs. These observations indicate that the gravel-sand transition is characterized by a terminating gravel wedge, but there are patches of gravel downstream of the wedge forming a diffuse extension. We show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility. We argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed. We also find that sand in suspension declines rapidly at the downstream end of the wedge, suggesting that sand is delivered to the bed, completing the sedimentary conditions for a gravel-sand transition. We propose that the break in river slope associated with the transition is a consequential feature of the transition.

Alluvial reaches in the source region of the Yangtze and Yellow Rivers, located atop the Qinghai–Tibetan Plateau and mostly at elevations that are typically higher than 3500 m, have similar slope and valley setting conditions. However, alluvial reaches in the source region of the Yangtze River exhibit predominantly braided channel planforms, while equivalent reaches in the source region of the Yellow River have more diverse planform morphologies including anastomosing/braided, anabranching/braided, and anabranching/anastomosing reaches. The regional gradient of variation in riparian vegetation and interactions between vegetation development and hydrologic process are shown to be major factors influencing this difference in channel planforms. A short period above freezing, coincident with recurrent yet pulsed flows in the source region of the Yangtze River, suppress riparian vegetation growth, promoting the development of braided channel morphologies. In contrast, a milder climate, coincident with less variable flows in the source region of the Yellow River, permit the establishment of different forms of riparian vegetation, thereby supporting the development of a wider range of channel planform morphologies.

Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation; the knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that under conditions of Froude-supercritical flow over an alluvial bed, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base-level fall can generate a single knickpoint, or multiple knickpoints. Multiple knickpoints take the form of cyclic steps, that is, trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream. In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state., Earth Surface Dynamics, 2(1), pp.271-278; 2014

The temporal evolution of the Zagros Simply Folded Belt is constrained by a magnetostratigraphic sequence containing a progressive unconformity on the southern limb of the Kuh-e Ghol Ghol anticline, in the Central Fars. The investigated similar to 1400m thick sequence exposes a regressive megacycle containing, from bottom to top, open and shallow marine marls and sandy limestones, fine- to coarse-grained fluvial deposits and alluvial conglomerates. Correlating the magnetostratigraphic section with the geomagnetic polarity time scale constrains the transition from marine to fluvial sediment deposition at similar to 6Ma. This transition was accompanied by a change in the accumulation rate from similar to 15cm/ka to similar to 40cm/ka, as measured on lithified sediments. Alluvial river deposits first occurred at similar to 3.2Ma. The Kuh-e Ghol Ghol anticline began to grow at similar to 3.8Ma, witnessing fastest limb rotation rates of 40 degrees/Ma at similar to 3.3Ma. Reporting magnetostratigraphic sections and ages of growth strata on a map of NE Fars reveal an similar to 1cm/a, southwestward migration rate of the deformation front during the middle and late Miocene.

The rehabilitation of degraded river channels is often guided by assumptions of continuity, yet in response to spatial and temporal variations in controlling conditions rivers typically display discontinuous response in space and time. This study examines the development of a 5 km reach of the Drome River, S.E. France, characterised by alternating alluvial and bedrock zones that are separated by abrupt downstream transitions. This reach is representative of the Drome River as a whole, and other rivers in the European Alps where braided channel planforms have been replaced by more complex, discontinuous morphologies. The primary aims are to understand how this spatial complexity has developed on the Drome; evaluate how temporal channel changes have been affected by local factors, particularly bedrock exposures, and by long-term, catchment-scale changes in sediment supply and the flood activity; and consider the implications of this discontinuous geomorphology for reach management. The development of geomorphological zonation is examined by documenting sequential changes in channel planform between seven periods, using aerial photography (1948–2006) and by analysing change in bed elevation from profiles surveyed in 1928, 2003 and 2005. Between 1948 and 2001 bedrock exposed in the channel bed and along the floodplain margins defined discontinuities in sediment connectivity that were largely responsible for the configuration of channel zones. The impact of floods on this system was not proportional to flood magnitude. A modest flood in 1978 was an important event that, by incision and avulsion at key locations, defined a pattern of zonation that persisted until the end of the study in 2006. During the final 5 years of the study, alluvial zones that previously responded to large floods by widening underwent narrowing, despite the occurrence of a large flood, and led to an overall reduction in width variance. This resulted from progressive incision beneath and disconnection from formerly active channel areas, in response to long-term, catchment-scale reductions in sediment supply and flood frequency. In 2006 the pattern of zonation remains distinct, disguising this recent change in channel response and underlining the need for long-term and sequential perspectives of channel development to fully understand the processes in operation; contemporary snapshots of channel form may be misleading. Understanding interactions between inherent channel complexity and prevailing flow and sediment conditions, and how this shapes channel response to individual floods, is essential when interpreting future trajectories of channel change and likely response to management intervention.

One of the most relevant features of alluvial rivers concerns flow resistance, which depends on many factors including, mainly grain resistance and form drag. For natural sand-bed rivers, dunes furnish the most significant contribution and this paper provides an insight on it. To achieve this aim, momentum balance equations and energy balance equations are applied to free flow in alluvial channels, assuming hydrostatic pressure distribution over the cross sections confining the control volume, which includes a reference bed form pattern. The resulting equation in terms of energy grade accounts for an empirical bed form drag coefficient resulting from the actual flow pattern and bed form geometry. The model has been validated using a large selection of field data and it seems somewhat sensitive to the dune geometry and to the Nikuradse equivalent roughness, whereas it is shows greater sensitivity to the adopted grain surface resistance formula (e.g., Manning&ndash, Strickler formula).

We developed a sampling protocol for pike (Esox lucius L., 1758) in large alluvial rivers in order to estimate adult population density and structure. This species is known as an indicator of lowland river biotic integrity in Europe evidencing the interest to determine population densities to a reliable degree of accuracy. A two occasion mark-recapture experiment based on a combination of net fishing and boat electro-fishing was specifically designed for monitoring of backwaters and channel sectors during the annual high flow level of an alluvial river, the Allier (France). Estimated densities calculated by using Chapman-Petersen and Loglinear models were weak, ranging from 0.68 (±0.16) fish.ha−1 to 0.71 (±0.20) fish.ha−1. However, when the densities are divided by the area of potential riverine habitats, pike abundance was calculated as being 15.3 (±3.7) fish.ha−1 to 15.9 (±3.8) fish.ha−1. Pike growth was high and 8 to 9 year-old females were captured. Age and sex distribution emerged as unbalanced, highlighting the precarious status of pike in the studied stretch of the river due to very limited recruitment. Pike were mainly captured in parapotamic side arms which underlines the importance of lateral habitats in semi regulated large alluvial rivers with wide riffle-pool-run sequences dominating geomorphological units.

Development of predictors for frictional resistance in an alluvial channel is a challenging task because of the complex interaction of fluid, flow and sediment in the channel bed. In the present study, an effort has been made to develop a functional relationship for friction factor using relevant fluid, flow and sediment parameters. Using field data (years 1984–2002) and the said functional relationship, a predictor for friction factor is proposed for mobile boundary channels. Proposed predictors for friction factor and cross-sectional properties of a gauging station (Sarangkheda, Tapi River, India), in turn, have been used to develop stage–discharge relationship for the same station. The proposed relationship for friction factor and rating curve developed at the Sarangkheda gauging station has been validated using available independent data for years 2003–2007.

Knickpoints are fascinating and common geomorphic features whose dynamics influences the development of landscapes and source-to-sink systems – in particular the upstream propagation of erosion. Here, we study river profiles and associated knickpoints experimentally in a micro flume filled with a cohesive substrate made of silica, water and kaolinite. We focus on the effect on knickpoint dynamics of varying the distribution of base-level fall (rate, increment, and period) and substrate strength, i.e. kaolinite content. Such simple cases are directly comparable to both bedrock and alluvial river systems. Under a constant rate of base-level fall, knickpoints of similar shape are periodically generated, highlighting a self-organized dynamics in which steady forcing leads to multiple knickpoint events. Temporary shielding of the bed by alluvium controls the spacing between these unit knickpoints. Shielding is however not effective when base-level drops exceed alluvium thickness. While the base-level fall rate controls the overall slope of experiments, it is not instrumental in dictating the major characteristics of unit knickpoints. Instead the velocity, face slope and associated plunge pool depth of these knickpoints are all strongly influenced by lithology. The period between knickpoints is set by both alluvium thickness and base-level fall rate, allowing use of knickpoint spacing along rivers as an indicator of base-level fall rate.

Fluvial erosion of peat deposits occurs in many environmental settings; however, the erosion and transport of large peat blocks by river channels has received relatively little attention. This paper describes the sedimentary significance and potential palaeoenvironmental interpretation of peat blocks in alluvial river systems. Evidence is presented from a number of field studies of upland river systems in northern England that illustrate a range of peat block forms and sedimentary features that are briefly compared with examples of peat blocks preserved in gravel stratigraphy. We show that peat blocks are an important geomorphological and sedimentological component of upland rivers draining eroding peatland catchments. They are of widespread occurrence and contribute significantly to river channel roughness and channel sedimentation. A variety of common sedimentary features can be observed including, shadow, crescent, perched, armoured, drape, embedded (part buried), cluster, and step forms. Peat blocks tend to be deposited on channel margins and bar tops and can be used as “maximum” stage indicator for major floods. The role of peat blocks in controlling sedimentation varies with channel width. In narrow channels where the size of the peat block approximates the channel width, blocks become jammed in the channel and exert a primary control on channel sedimentation. In wider channels, blocks tend to occur in isolation or in small clusters and are of only secondary importance in controlling sedimentation. Residence times of peat blocks varies from short periods of temporary deposition (days to months) to much longer timescales (months to years) with some blocks becoming permanently incorporated into the sedimentary record. The sedimentary characteristics of contemporary buried peat blocks have much in common with blocks preserved in alluvial gravel stratigraphy. This offers the potential for using these features for palaeoenvironmental reconstruction. Peat blocks in many ways are analogous to other low density geological materials (ice blocks, woody debris, and some volcanic sediments), and recognising the special sedimentological characteristics of this suite of materials is important as they are not always adequately characterised using conventional hydraulic relationships.

Basinwide hiatal discontinuity has been generally accepted as firm evidence of a distinctive allogenic event such as tectonic movement or a complete eustatic cycle. Here we show through theoretical and experimental modeling that a largescale, hiatal discontinuous surface can be produced autogenically in fluviodeltaic successions under no changes in external dynamic forcing, i.e., constant relative sea-level rise (rate rslr) and constant sediment supply (rate qS per unit width). This “autogenic hiatus” occurs where (1) the hinterland slope is steeper than the subaqueous slope of the delta, and (2) the initial downstream length of the feeding alluvial river exceeds a critical magnitude Lcrt that is specified primarily by qSlrslr. In this topographic setting, the existing depositional system becomes transgressive and nondeltaic as soon as sea level starts to rise. The existing subaqueous surface is starved of sediment and progressively extends landward until alluvial length is reduced to Lcrt. After the retreating river has attained this critical length, the shoreline still retreats but the depositional system is restored to deltaic sedimentation. Because of this, the subaqueous surface, which was previously starved of sediment, becomes overlain by delta foreset deposits after a significant time gap. Thus a steady sea-level rise can produce all of the following: a) the strata underlying the hiatus, b) the hiatus itself, and c) the strata overlying the hiatus. Changes in rslr or qS are not required to account for the presence of a hiatus of this particular type. Numerical simulations suggest that autogenic hiatuses are likely to be produced in most natural river deltas if a sea-level rise such as occurred during latest Pleistocene to Holocene (i.e., 0.01 km/kyr for 10 kyr) is available. This further implies that autogenic hiatuses may well exist in stratigraphic records of river deltas, especially of Quaternary age. An understanding of autogenic hiatuses, when combined with the theory of shoreline autoretreat, provides an alternative view of the origin of some stratigraphic breaks.

Concrete walkways are relatively low cost alternatives to bridges and culverts. They provide good and dependable ways across streams and rivers. This article discusses failures of concrete walkways in selected locations in Nigeria. The selected locations in Katsina state were visited before raining season started and during raining season. Concrete walkways across selected rivers (streams) were examined and their failures were assessed. Soil samples at the location of the walkway were collected and classified. The study revealed that earlier failures of culverts/bridges led to the construction of these concrete walkways in 1985 because the walkways were cheaper and required lower technology than construction of bridges. The soils at the site were mainly sandy–loamy soil with an average of 75.5% sand, 9.85% clay, and 14.65% silt. Failures of the concrete walkways were due to rapid and varied flow in the middle of the river channel, the presence of a hydraulic jump, the lack of resistance to turbulent flow, the presence of impulse forces, high Froude number (F r), and the lack of cohesion between the soil and concrete materials. It was then concluded that provision must be made against turbulent flow and cohesive materials must be provided in the construction of walkways to prevent failure of walkways.

Braiding pattern in sixteen reaches of the Brahmaputra river in Assam, India is discussed. A new braiding index has been introduced incorporating a fraction of area covered by sandbars, number of midchannel bars and maximum width of alluvial reach. Braiding parameters calculated using different formulae showed similar trend but higher values in 2014 compared to 1973. Increase of braiding in different reaches in 2014 was due to development of more sandbars and distributaries. The new index has shown comparable result with other approaches and better correlation with sinuosity.

Uncertainty in discharge data must be critically assessed before data can be used in, e. g. water resources estimation or hydrological modelling. In the alluvial Choluteca River in Honduras, the ri ...

Longitudinal flow variation is an emerging field of study in river ecohydrology. Longitudinal changes in the frequencies, magnitudes, durations and timing of floods, low-flows and intermittence create a dynamic environment for flow-dependent species and ecological processes. Analyses of flow variation and flow–ecosystem relationships in the longitudinal dimension require synoptic flow time-series at multiple sites along a river. Complex channel geomorphology and a scarcity of rivers equipped with multiple flow gauges have slowed progress in longitudinal ecohydrology. The empirical longitudinal flow model (ELFMOD) is a new statistical tool for estimating flows at multiple points along rivers; which circumvents the requirement for multiple gauges. In this study, we used ELFMOD to explore longitudinal flow variation in four alluvial rivers from different geological terrains and climate zones: the Albarine (France), Methow (USA) and Selwyn and Orari (New Zealand). Differences among rivers in longitudinal and temporal flow variation were evident on date × location flow matrices and longitudinal profiles. There were notable differences among longitudinal gradients in flow magnitude and flow percentiles, and in the recurrence, extent, and rate of expansion and contraction of dry river reaches. There were also large differences in longitudinal flow-permanence patterns. Temporal variation in the lengths of dry channel and numbers of dry reaches was predominately seasonal in the Albarine and Methow, and interannual in the Orari and Selwyn. Broad differences among the rivers in longitudinal flow patterns correspond to differences in the configuration of hydrogeomorphic discontinuities such as tributary confluences, channel divergences and convergences, and lithological contacts. Copyright © 2010 John Wiley & Sons, Ltd.

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.

The changing form of developing alluvial river bars has rarely been studied in the field, especially in the context of the fixed, compound, mainly alternate gravel bars that are the major morphological feature of the wandering style. Century scale patterns of three-dimensional growth and development, and the consequent scaling relations of such bars, are examined along the gravel-bed reach of lower Fraser River, British Columbia, Canada. A retrospective view based on maps and aerial photographs obtained through the twentieth century shows that individual bars have a life history of about 100 years, except in certain, protected positions. A newly formed gravel bar quickly assumes its ultimate thickness and relatively quickly approaches its equilibrium length. Growth continues mainly by lateral accretion of unit bars, consistent with the lateral style of instability of the river. Bar growth is therefore allometric. Mature bars approach equilibrium dimensions and volume that scale with the overall size of the channel. Accordingly, the bars conform with several published criteria for the ultimate dimensions of alternate barforms. Sand bars, observed farther downstream, have notably different morphology. Fraser River presents a typical wandering channel planform, exhibiting elements of both meandered and low-order braided channels. Hydraulic criteria to which the Fraser bars conform illustrate why this planform develops and persists. The modest rate of bed material transfer along the channel – typical of the wandering type – determines a century-length time scale for bar development. This time scale is consistent with estimates that have been made for change of the macroform elements that determine the overall geometry of alluvial channels. Copyright © 2009 John Wiley & Sons, Ltd.

In the northern part of the Indian sub-continent, the Gomati River (a tributary of the Ganga River) was selected to study the dynamics of Arsenic (As) mobilization in fluvial environment of the Ganga Plain. It is a 900-km-long, groundwater-fed, low-gradient, alluvial river characterized by monsoon-controlled peaked discharge. Thirty-six water samples were collected from the river and its tributaries at low discharge during winter and summer seasons and were analysed by ICP-MS. Dissolved As and Fe concentrations were found in the range of 1.29–9.62 and 47.84–431.92 μg/L, respectively. Arsenic concentration in the Gomati River water has been detected higher than in its tributaries water and characteristically increases in downstream, attributed to the downstream increasing of Fe2O3 content, sedimentary organic carbon and silt-clay content in the river sediments. Significant correlation of determination (r 2 = 0.68) was also observed between As and Fe concentrations in the river water. Arsenic concentrations in the river water are likely to follow the seasonal temperature variation and reach the level of World Health Organization’s permissible limit (10 μg/L) for drinking water in summer season. The Gomati River longitudinally develops reducing conditions after the monsoon season that mobilize As into the river water. First, dissolved As enters into pore-water of the river bed sediments by the reductive dissolution of Fe-oxides/hydroxides due to microbial degradation of sedimentary organic matter. Thereafter, it moves upward as well as down slope into the river water column. Anthropogenically induced biogeochemical processes and tropical climatic condition have been considered the responsible factors that favour the release of As in the fluvial environment of the Ganga Plain. The present study can be considered as an environmental alarm for future as groundwater resources of the Ganga–Brahmaputra Delta are seriously affecting the human–environment relationship at present.

Summary Complex spatial and temporal flow patterns in alluvial plain rivers can be caused by variation in groundwater surface-water exchange, channel planform, and climatic variation. Such complexity poses a challenge for developing relationships between runoff, groundwater recharge, and river flow, for predicting effects of water resource developments, and for understanding hydrologic effects on ecological processes. The Selwyn River of New Zealand is one such hydrologically complex river. To begin to understand the spatio-temporal flow patterns of the Selwyn River, we used linear and logistic models to reconstruct a 22-yr record of river flow across the alluvial Canterbury Plains. The 60 km ‐ long model domain encompasses perennial, ephemeral, and intermittent reaches embedded within larger effluent and influent sections. Flow at 18 cross-sections distributed along the river mainstem was modelled as a function of flow at stage recorders located at each end of the domain with flow interpolated between cross-sections to generate a flow record continuous in space. Data from 38-months of flow-gauging at the crosssections was used to calibrate model parameters. The reconstructed record indicates that while the central section of the river was dry during most of the 38-month observation period, it was not uncommon during the preceding two decades for the river to flow along its entire course for several months consecutively. The recent dry period may be part of a longer trend: the mean annual simulated length of dry river channel has increased by 0.6 km yr � 1 over the last two decades. Output metrics from the model can increase our understanding of hydrologic control of ecological processes in alluvial rivers. We provide several examples of potential model applications to ecological studies. a 2008 Elsevier B.V. All rights reserved.

In landscape ecology, landscapes are viewed as hierarchically-organized groups of interacting components or zones. This perspective is applied in studies of riverscapes with increasing frequency,particularlyin studies ofchannel habitats and channel-floodplain interactions. Stygoscapes, the subsurface portions of riverscapes, have received less attention. Stygoscapes are composed of three saturated zones beneath and lateral to river channels: the hyporheic, parafluvial and riparian zones. These zones are differentiated on the basis of hydrological, biological and chemical conditions, and the degree of interaction with the channel surface. In this paper, we describe lateral and longitudinal patterns in the shallow (30 cm - 10 m depth) stygoscape of the alluvial Selwyn River, New Zealand. We used abutting hyporheic, parafluvial and riparian zones in large-scale downwelling (losing) and upwelling (gaining) sections of the river mainstem to compare physico-chemical and biological conditions across a broad stygoscape. The stygoscape zones had the same arrangement in both sections: hyporheic zones within the wetted channel, riparian zones furthest from the channel, and parafluvial zones between the hyporheic and riparian zones. Nitrate and dissolved reactive phosphorus concentrations and electrical conductivity were significantly higher, and dissolved organic phosphorus concentrations and pH levels lower, in the gaining section than the losing section. In both sections, ammonium, dissolved reactive phosphorus, and dissolved organic carbon concentrations and pH were higher in the hyporheic zone than the riparian zone. Within the losing section, DON and DOP concentrations were higher in the hyporheic zone than the riparian zone. Physico-chemical conditions were intermediate in the parafluvial zones of both river sections. These among-zone differences suggest that hyporheic zones are sinks for dissolved nutrients, and that effects of hyporheic sinks are greatest in losing river sections with unidirectional flow from the surface to the hyporheic zone. There were large differences among stygoscape zones in invertebrate assemblage composition, and smaller differences between the losing and gaining sections. Invertebrate densities, taxon richness and % insect taxa were higher in hyporheic zones than riparian zones. Invertebrate densities were intermediate in the parafluvial zones, and taxon richness and % insect taxa were similar in parafluvial and riparian zones. The spatial patterns we observed in the Selwyn River suggest that hydrological connectivity between zones, and permeability within zones, are among the most important determinants of chemical and biological dynamics in stygoscapes. Differences in large-scale upwelling and downwelling, and differences in water source (runoff and groundwater) appeared to be of secondary importance.

Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant conditions of relative sea-level fall, sediment discharge, and water discharge, demonstrates that "sustained" alluvial aggradation is the inherent stratigraphic response of alluvial rivers that are steeper than the receiving basin. High alluvial gradients are primarily a consequence of high sediment supply relative to available water discharge (low rate of terrestrial diffusion). During the course of the experiments, fluvial grade was never reached, fluvial incision was never observed, and aggradation occurred independently of shoreline position. The experiments revealed detachment of the shoreline from the alluvial river as an autogenic response of certain fluviodeltaic systems to steady relative sea-level fall, creating a "non-deltaic" alluvial-river system that continues to aggrade beyond shoreline detachment. We term this behavior "autodetachment." Autodetachment is a consequence of sustained, extensive alluvial aggradation during relative sea-level fall. Sediment supply was increasingly partitioned into the ever-enlarging alluvial profile to the detriment of the delta front prior to shoreline detachment. Thus, as relative sea level fell, the length of the delta front decreased to zero. Simultaneously, alluvial aggradation also led to decreasing rates of delta progradation. Therefore, as the delta front disappeared, the rate of shoreline regression exceeded the rate of progradation, producing a "bedrock" river between the detached alluvial river and the shoreline. The Canterbury Plains of New Zealand is a natural-world system with alluvial gradient greater than shelf gradient. Geometric modeling suggests that autodetachment could have occurred at the Canterbury Plains within the timescale of Quaternary glacio-eustatic sea-level falls, and therefore, is a realistic stratigraphic response at the scale of natural-world systems. Our model demonstrates that deltas with these geometric parameters become poorly supplied during regression, and large lowstand deltas should not be expected for these types of systems. The results of this study complement existing autostratigraphic theory, and provide an alternative to allogenic drivers for geomorphic and architectural evolution of fluviodeltaic wedges.

This study combines mathematical modelling and supporting flume experiments to address the problem of how coastal plain rivers respond to a steady fall in relative sea-level. The theoretical component of the study focuses on the development of a moving boundary model of fluviodeltaic progradation that treats rigorously the dynamics of the shoreline and alluvial–basement transition (the upstream limit of the alluvial river system). Dimensional analysis and numerical solutions to the model governing equations together suggest that, at first order, coastal plain rivers will remain aggradational on a timescale that varies with allogenic sediment and water supply and the fall rate of relative sea-level. In natural fluviodeltaic systems, this intrinsic timescale is likely to vary by several orders of magnitude, suggesting that the aggradational phase of river response can be geologically long-lived. At second order, the duration of alluvial aggradation is controlled by two dimensionless numbers that embody system geometry and the kinematics of alluvial sediment transport. Model predictions were tested in a series of carefully scaled flume experiments. The level of agreement between predicted and measured trajectories for the shoreline and alluvial–basement transition strongly suggests that the moving boundary theory developed here successfully captures the response of small-scale fluviodeltaic systems to falling sea-level. The results of this study have several sequence-stratigraphic implications: a fall in relative sea-level at the shoreline is not a sufficient condition for river incision; the onset of alluvial degradation and sequence-boundary formation need not coincide with a maximum in the rate of sea-level fall; and the onset of sequence-boundary formation is sensitive to allogenic sediment supply.

Since the end of the post-glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south-eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid-Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid-Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd.

Large-scale patterns of hyporheic exchange are predictable within some river systems, but our understanding of the factors driving hyporheic processes and the magnitude of hyporheic exchange needed to influence biophysical patterns at larger scales remains limited. We investigated the patterns, magnitude, and potential effects on biota of reach-scale hyporheic exchange in an alluvial river of the Pacific Northwest. The river was topographically similar to and in the same geographic region as other systems where large-scale hyporheic exchange and associated biological responses have been observed. We hypothesized that predictable reach-scale patterns of hyporheic exchange would occur in alluvial valley segments of the river and that hyporheic upwelling would be associated with reach-scale patterns of physical and biological characteristics. We used in-channel piezometers and synoptic stream flow measurements to quantify hyporheic exchange. We measured temperature, dissolved O2, pH, specific conduct...

Based on a review of a number of documented case studies from various countries and a detailed analysis of sediment exploitation from five rivers in Italy and Poland, we discuss alluvial river response to extensive sediment mining. A sediment deficit caused by in-stream mining typically induces upstream- and downstream-progressing river incision, lateral channel instability and bed armouring. The resultant incision alters the frequency of floodplain inundation along the river courses, lowers valley-floor water tables and frequently leads to destruction of bridges and channelization structures. Mining also results in the loss or impoverishment of aquatic and riparian habitats. In the rivers of Italy and southern Poland studied, where mining coincided with other human activities that reduce sediment delivery to the channels, deep river downcutting, changes in channel pattern and, in one case, transformation from alluvial to bedrock boundary conditions were recorded over recent decades. The type and magnitude of channel response to sediment mining depend mainly on the ratio between extraction and sediment replenishment rates. The effects of mining will be especially severe and difficult to reverse: (i) where material is extracted at a rate greatly exceeding the replenishment rate; (ii) in single-thread rivers, that are generally associated with relatively low rates of catchment sediment supply; (iii) in channelized reaches; (iv) where rivers are underlain by a thin cover of alluvium over bedrock; and (v) where mining coincides with other human activities that reduce upstream sediment delivery. With a large number of detrimental effects of instream mining, the practice should be prohibited in most rivers except aggrading ones. Copyright © 2005 John Wiley & Sons, Ltd.