Scour, fill, and salmon spawning in a California coastal stream

Streambed scour and fill affecting incubation survival of salmon embryos were investigated in a Northern California coastal stream (Freshwater Creek) for coho (Oncorhynchus kisutch) and chinook (Oncorhynchus tshawytscha) species. Objectives of the study were to: (1) test a reach-scale scour and fill model (Haschenburger 1999) based on Shields stress (dimensionless shear stress), and (2) test two published hypotheses of salmon spawning adaptation to streambed scour. Testing of the model clarified some limitations, revealed potential improvements, and demonstrated sufficient potential for predicting scour at salmon spawning areas (redds) based on a small sample size (n = 9 redds) to warrant additional testing. The model appears best suited for individual floods on reaches that are straight, in equilibrium between sediment supply and transport, and have roughness elements similar to the creeks where the model was developed. Differences in model predictions and measured values were likely due to variable scour and fill patterns in Freshwater Creek that were weakly influenced by Shields stress and highly influenced by sediment supply, location within the channel network, and channel morphology (form roughness).