Thomas Dunne

/ The potential for gravel extraction to adversely affect anadromous fish habitat in three gravel-bed rivers of southwestern Washington, U.S.A,, prompted the need to determine sustainable rates of gravel removal. This was accomplished by evaluating the components of a long-term sediment mass balance for the three rivers. Average annual gravel transport was determined by three independent methods. The closely agreeing results indicate that annual bedload supply decreases downstream through deposition and storage in respOnse to declining gradient and from attrition during transport, as confirmed by laboratory experiments. A survey of gravel-bar harvesting operations indicates that the annual replenishment rate has been exceeded for up to three decades, often by more than tenfold. Analysis of data from nine stream gauging stations over a 55-yr period indicates degradation of about 0.03 m/yr in these reaches and suggests that bed degradation has produced the difference between the replenishment rates and the volumes of gravel harvested from the river beds and bars.

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Maasai settlements in Amboseli are distributed in a pattern which reflects various physical and biological characteristics of the landscape. The settlements avoid hillslope gradients exceeding O. 08, and the lower sections of long hillsides, which receive large amounts of runoff Long, relatively high hillslopes are difficult for exhausted eattle to climb at the end of the dry season, and even the well-drained sites at the upper ends of these slopes are not commonly used. Deep, poorly drained, and light-colored soils are avoided because they affect the comfort of humans, and espeeially the milk production of cattle. The settlements are located away from dense tree and bush vegetation because of the danger of predators, but during occupation of the site important changes in the vegetation are wrought by the use of trees for settlement construction and firewood. The reasons for the pattern were elucidated by making a set of systematic measurements of settlement distributions and various environmental factors. The conclusions of this analysis were then checked and extended through conversations with Maasai elders. These latter exhibited a sophisticated knowledge of environmental charaetetqstics and processes which is reflected in their choice of settlement sites. Such knowledge has commonly been overlooked by other writers on the subfect of pastoralism.

Channel stability of gravel bed rivers is enhanced by coarsening and structural modification of the bed, which determines the bed's resistance to entrainment, and hence the mobility of particles on the bed. In this study we propose a grain-scale classification system that qualitatively and quantitatively describes grain-to-grain interactions of fluvially re-worked gravel beds. The quantitative measure takes the form of the ratio of the vertical plucking force (Fv) required to remove a grain from a static, dry bed to the weight of the grain (Fw). Fv/Fw is an empirical measure of the restraining effect of neighboring grains and can range between 1 when the bed is loosely packed to >> 1 when the grains are interlocked. Preliminary results from a large (30 m long and 0.86 m wide) flume channel suggest that individual grains range from being loosely arranged on the bed (mean Fv/Fw approx. = 1) to highly packed or interlocked grains (mean Fv/Fw approx. = 5). Preliminary data fro...

The U.S. EPA estimates that 40 percent of U.S. rivers are ecologically impaired by sedimentation. On dam- controlled rivers that are impaired by sedimentation, flushing flows with the potential to flush the fine sediments are commonly prescribed to improve ecological conditions. One goal of these flushing flows is to mobilize the riverbed's coarse surface layer and thus to create the potential to release fine sediment trapped beneath the surface. Given the economical value of water, releasing flows large enough to generate shear stresses adequate to mobilize the coarse surface can be difficult. Gravel augmentation using fine gravel has been recently demonstrated as a viable alternative to mobilizing coarse surface layers. This occurs because sheets of fine gravel smooth the bed, increasing the local shear stress available to move the grains. Does the mobilization of the coarse surface layer by fine gravel increase the depth to which fines are selectively removed or decrease the ...

Prediction of the frequency and spatial pattern of bed mobility in gravel bed rivers is central to a wide range of theoretical and applied interests ranging from sediment transport to the impacts of natural or managed floods on aquatic organisms. Although bed mobility has been investigated in numerous flume and field studies, accurate predictions of grain entrainment and transport in gravel bed rivers remain elusive. Alluvial rivers typically encompass a much wider range of hydraulic and sedimentological conditions than those that have been recreated in laboratory flume studies upon which many grain entrainment and transport models are based. These flume studies are limited to the examination of processes occurring over the short term, commonly with the absence of slower processes such as fine-grain infilling. On the other hand, in field studies key variables can not be controlled and the spatial complexity of processes and conditions complicate data collection and analysis. A uniqu...

An emergent paradigm within restoration science is that restoration of natural physical processes is the best way to restore habitat for native organisms in degraded rivers. This concept, which underpins many restoration projects, is based on the notion that the establishment of an actively migrating, alluvial river channel-floodplain system will provide a number of desired ecological functions, each related to specific physical processes that occur at the habitat-scale. Here we quantify the rates of morphologic change, channel migration and the development of high-quality habitat, using a recently restored gravel-bed reach of the Merced River, California, USA. DEM-derived differences in bed elevation indicate that sediment storage accelerated processes of bar-building, pool scour, and bank erosion, leading to more asymmetric cross- sectional geometry. The volume of sediment stored on developing point bars was correlated with the migration distance of the outer bank, whereas in bend...

... In this case, too, an understanding of the sediment regime is usu-ally central to ... Method M Monitoring Monitoring was carried out for the study F Field work Field measurements or ... scarp depths, or deposit volumes (M). Estimates have also been made by applying an estimated ...