Application of watershed-scale habitat modeling and decision-support tools for reservoir reoperations in coastal northern California

Water managers must often balance the needs of both aquatic habitat and human water supply. However, they frequently only have the tools to manage water delivery alone. Existing modeling tools for habitat have gaps in providing detailed biological estimates at a watershed scale and in simulating water supply operations and habitat suitability simultaneously. A new modeling platform and calculation framework, Aquatic Habitat Assessment, was applied in a case study to quantify habitat suitability and fish passage at a watershed scale for local species of Chinook salmon ( Oncorhynchus tshawytscha ) and steelhead trout ( Oncorhynchus mykiss ). Aquatic Habitat Assessment was coupled with a suite of tools, including HEC-RAS used for hydraulics, WEAP for water allocations, and Tableau for visualization. The tools ensemble was used to simulate the operations of a water utility system near San Francisco Bay in California to evaluate the effects of reservoir reoperations on both human water supply and aquatic habitat. The suite of tools was successful in bringing a range of conflicting parties to coalesce around a common solution for reservoir operations. Two sets of alternative reservoir operation schemes were developed, which largely involve higher winter and lower summer releases, aligning more closely with natural Mediterranean patterns and functional flows connected to the biological needs of local aquatic species. Results quantify tradeoffs across reservoir operation schemes, with modeled fish passage habitat suitability increased under the reservoir reoperations, but human water supply delivery decreased. The modeled spawning, incubation, and rearing habitat suitability increased during the winter release period under reoperations, but decreased during the summer release period.