Modeling grain size adjustments in the downstream reach following run-of-river development

Disruptions to sediment supply continuity caused by run-of-river (RoR) hydropower development have the potential to cause downstream changes in surface sediment grain size which can influence the productivity of salmon habitat. The most common approach to understanding the impacts of RoR hydropower is to study channel changes in the years following project development, but by then, any impacts are manifest and difficult to reverse. Here we use a more proactive approach, focused on predicting impacts in the project planning stage. We use a one-dimensional morphodynamic model to test the hypothesis that the greatest risk of geomorphic change and impact to salmon habitat from a temporary sediment supply disruption exists where predevelopment sediment supply is high and project design creates substantial sediment storage volume. We focus on the potential impacts in the reach downstream of a powerhouse for a range of development scenarios that are typical of projects developed in the Pacific Northwest and British Columbia. Results indicate that increases in the median bed surface size (D-50) are minor if development occurs on low sediment supply streams (<1 mm for supply rates 1 x 10(-5) m(2) s(-1) or lower), and substantial for development on high sediment supply streams (8-30 mm for supply rates between 5.5 x 10(-4) and 1 x 10(-3) m(2) s(-1)). However, high sediment supply streams recover rapidly to the predevelopment surface D-50 (similar to 1 year) if sediment supply can be reestablished.