SPATIAL PATTERNS OF SEDIMENT DEPOSITION AND THE EFFECTS ON SALMONID SURVIVAL

Ecohydraulic research in large alluvial rivers indicates that salmon are well adapted to surviving in flood prone ecosystems. Extensive data on spatial patterns of scour and fill in the Trinity River of northern California reinforces that deep scour is a convergent process that affects a few areas of the streambed, whereas fill is a divergent process that affects the bed more uniformly. Furthermore, site selection preferences for spawning fish place egg nests in areas of the river least likely to be scoured during floods, ensuring reproductive success. However, our data indicate that sediment deposition may pose a substantial risk to the survival of these gravel spawning fish. A critical uncertainty in predicting the ecological consequences of high sediment loads is quantifying the depth of sediment that can be penetrated by juvenile fish emerging from subsurface incubation habitat. We used an experimental approach to test the effect of varying depths of sediment deposition atop fertilized egg masses of steelhead trout (Oncorhynchus mykiss). Control nests were buried to a depth of 20cm, equivalent to natural burial depths observed in many rivers. Egg survival and emergence in control nests were compared to treatments of two, three and four times the initial egg burial depth. Results of this experiment indicate an exponential decrease in egg survival with an increase in burial depth. Results were then related to fill depths observed during large flood events in the Trinity River. The spatial distribution of areas susceptible to deep deposits of sediment overlaps with preferred spawning sites for salmon. The combined experimental results and field observations indicate that salmon populations may be poorly adapted to the impacts of increased sediment loads due to diminished reproductive success.