Eco-geomorphic flows: Modification of wet-season dam operations to support downstream salmonid habitat

Most large rivers in California and much of the world have dams impeding their flow. These dams provide important benefits to society but have negatively affected river ecosystems and the aquatic organisms that live in them by disrupting the natural patterns of movement of water and sediment. To mitigate some of these harmful effects, river managers often turn to physical habitat restoration and environmental flow manipulation. All dams have designed reservoir release operations and many rivers downstream of them have habitat restoration projects, but rarely do managers design these to work together to yield the best combined outcome. This study investigates the potential to modify flood risk management releases to better support downstream habitat enhancement projects designed to provide endangered salmonid rearing habitat. This goal is accomplished using an hourly reservoir operations model, a one-dimensional sediment transport model, and a two-dimensional steady flow model combined with habitat suitability analysis. Results show that lower magnitude, longer duration flood releases have the potential to increase provision of rearing habitat during fry and juvenile rearing periods while also reducing operations and maintenance costs for the constructed restoration projects and providing ancillary benefits to water supply. Furthermore, because of the changed river corridor and sediment regime, the results indicate that pre-dam flows would not be ideal for meeting current management objectives of providing rearing habitat below the dam, and that flows reconciled to the current physical, ecological, and operational contexts would be more appropriate