Exchange of fine sediments with gravel riverbeds

An excess of fine sediment is the leading cause of impairment of rivers in the United States. Fine sediment poses particular problems for salmon, which bury their eggs within gravel riverbeds. Sand infiltrates the salmon nest and creates a sand seal in the upper pore spaces near the surface. Silt and clay then deposit in the sand seal, reducing bed permeability and cutting o the supply of dissolved oxygen. Large stormflows can mobilize the riverbed and flush out the silt and clay, increasing permeability. Reservoir releases on dammed rivers can simulate this effect; it may also be possible to engineer releases to both increase reservoir flood storage and improve the quality of spawning habitat. Understanding the fraction of fines as a function of depth would allow estimation of bed permeability and ideal flushing depths, which would aid in the design of flushing flows. This dissertation improvesunderstanding of the sand fraction as a function of depth in the sand seal by analyzing infiltration data, testing an existing model, and developing new models. It is found that a simple exponential model performs well at predicting the sand fraction as a function of depth but there is evidence of a more complex structure driven by a change in trapping efficiency as the bed clogs. This study allows for direct calculation of the effect of sand seals on bed permeability and can inform future studies of silt and clay deposition in the sand seal, but it is limited to uniform-flow, plane-bed conditions. The location and structure of salmon nests takes advantage of three-dimensional flowfields to enhance flow over the eggs. Theseforcing mechanisms should affect infiltration into nests. This relationship is explored in an experiment on sand infiltration into a gravel bed with alternate-bar topography. This study demonstrates that sand seals are created throughout the riverbed, even where sand transport is too small to measure. There is spatial variability in the thickness of a sand seal but not in the mean sand fraction in the sand seal. Local infiltration is increased by bed topography that promotes downwelling and is limited by very high sand transport and by thick sand deposits on the surface. The structure of a sand seal will affect flushing depth, but theduration of a flushing flow depends on the entrainment rate of silt and clay from within the sand seal. The entrainment rate of subsurface fines into suspension is not generally known. A case study on the Russian River in California is performed to estimated the entrainment rate during dam releases large enough to mobilize the riverbed. High-frequency turbidity and discharge data from two USGS gauging stations are used to create a timeseries of cumulative entrainment from the reach. Estimates of bed mobility are used to develop rough estimates of local entrainment. Entrainment decays exponentially at a timescale on the order of days,and this timescale decreases as the discharge event increases. Total estimated entrainment is similar to the supply of suspendable sediment calculated to be in the bed