The Problem with Bed Scour: Improving Characterization of Bed Scour Impacts in a Life Cycle-Based Salmon Habitat Model

Stream bed scour has played an important role in salmonid evolution, influencing adaptation through processes of habitat selection and spawn timing in order to optimize reproductive success. Scour depth is a function of several factors, including watershed hydrology, sediment transport dynamics, and local hydraulic complexity. Each of these parameters is influenced by human interventions on the environment and they are all likely to change in response to future climate conditions. The ability to characterize bed scour is therefore important for understanding trends in salmonid habitat potential and making informed restoration investments. Unfortunately the time and effort required to directly measure depth and timing of scour events can make it impractical to systematically monitor this parameter at the watershed level. This suggests the need for a reliable method for estimating scour depth using more easily measured landscape and reach-specific proxies. We describe a predictive model that uses a Monte Carlo simulation of boundary and critical shear stress to generate probabilistic estimates of bed scour depth in salmonid spawning-sized substrates from empirical channel measurements, modeled flow parameters, and documented relationships between substrate characteristics and spawning preferences.