Assessing Water Temperature Zones in Idealized Holding Pools for Chinook Salmon: A Hypothetical Study Based on the Regulated Lower San Joaquin River

Abstract:

In regulated rivers such as the Lower San Joaquin River (LSJR) in California, environmental policy requires releasing adequate flows to maintain habitat quality for flora and fauna. The prescribed reservoir releases on the LSJR are tied to water year classifications in order to help satisfy competing water demands in dry years. The question remains as to whether relatively low releases will be adequate to maintain habitat quality for key aquatic species under current and projected climate conditions. This work examines one critical determinant of anadromous fish habitat suitability, water temperature, as a function of reservoir release conditions. More specifically, we study idealized pools subject to the conditions of the LSJR using an established 2D (longitudinal and vertical) flow and heat transport model (CE-QUAL-W2). We assessed the releases in the context of the spring run Chinook salmon (Onchorhynchus tshawytscha) life cycle (the key species in an ongoing restoration effort), focusing on summer holding conditions. Our objective was to determine the conditions under which pools provide sufficiently cool havens for the holding salmon under current and projected climate conditions. Using river bathymetry from measured cross-sections and a high-resolution DEM lidar product, we created a range of ideal pool size representative of LSJR conditions. After calibrating hydraulic and heat transfer parameters using available temperature profiles, we simulated temperature profiles in the pools for scheduled flow releases, at different downstream locations from the reservoir. Results include modeled temperature profiles in holding pools, and derived estimates of suitable holding capacity under a range of pool, releases and climate conditions. Potential engineering modifications are explored as potential mitigation strategies, such as modified flow schedules, riparian shading, pool sizes and pool relocations.