Assessing management options to reduce water temperatures in regulated rivers of California

Abstract:

In many regulated rivers throughout the western United States, deliberate management of water temperature below reservoirs is crucial to the survival of endangered salmonids. However, management options at reservoirs are often limited to adjusting either the flow volume and/or release temperature to alter the downstream water temperature. Here we investigate the efficacy of this approach in three major regulated rivers in California: the Sacramento River, the Klamath River, and the St. Ynez River. We use a 1-dimensional water temperature model (RAFT) to compute heat fluxes to/from the river based on meteorological conditions, channel bathymetry, and reservoir releases. A sensitivity analysis evaluates the thermal response of the river to reservoir releases during summertime flow conditions. Results indicate that the water temperature dynamics downstream of reservoirs vary between rivers. Although both the Sacramento and Klamath Rivers exhibit cooling in response to increased flows, the Sacramento River is cooled more effectively by decreasing the release temperature. In contrast, increased flow rate in the St. Ynez River does not as strongly influence downstream temperatures due to shallower flows combined with significant thermal buffering already provided by deep alluvial streambed.