A Decision Support System for Mitigating Stream Temperature Impacts in the Sacramento River

The management of freshwater resources is one of the greatest challenges currently facing society. With increasing demand, alteration of water systems, and a changing climate, the thermal regimes of freshwater habitats are being substantially impacted. In the Sacramento River Basin (SRB) of California, the long-term decline in salmon populations has made management of the remaining freshwater habitat critical. This is magnified by the increasing demands on water resources and an extended drought that has enveloped the western United States in recent years. The construction of Shasta and Keswick Dams in the SRB headwaters during the mid-20th century provided additional storage facilities to meet regional water needs; however, hundreds of kilometers of spawning habitat above the dams were lost. The declines of the winter run and the late fall runs of Chinook salmon in the SRB have been listed on the endangered and threatened species lists, respectively, by the Environmental Protection Agency. As a result, water temperature thresholds have been established at specific compliance locations downstream of Shasta Dam to ensure suitable thermal conditions for fish.

Water quantity and quality require joint management in multi-purpose basins like the SRB. The effective use of water to protect fish requires water managers to modify operational strategies by incorporating water quality objectives into daily operations and long-term (i.e., seasonal) planning; this typically involves management of flow releases to control water quality parameters, such as temperature. Shasta Dam, however, was retrofitted with a temperature control device in the mid-1990s, which allows selective withdrawals from different depths (and, therefore, different temperatures) in the reservoir based on water temperature requirements downstream; therefore, water managers may modify either the release volume and/or release temperature to meet objectives downstream.

Coupling of multiple modeling techniques have been successfully employed to meet planning objectives related to water quality and quantity. We have developed an integrated decision support system (DSS) for modeling and mitigating stream temperature impacts and demonstrate it on the section of river downstream of Shasta Dam on the Sacramento River. The DSS has four broad components that are integrated to produce the decision tool for stream temperature mitigation: (i) a suite of statistical models for modeling stream temperature attributes using hydrology and climate variables of critical importance to fish habitat; (ii) a hydrodynamic model for modeling reservoir thermal structure and consequently, the water release temperature; (iii) a stochastic weather generator to simulate weather sequences consistent with seasonal outlooks; and (iv) a set of decision rules (i.e., ‘rubric') for reservoir water releases in response to outputs from the above components. The DSS incorporates forecast uncertainties and reservoir operating options to help mitigate stream temperature impacts for fish habitat while efficiently using the reservoir water supply and maintaining cold pool storage for late summer violations. The use of these tools in simulating impacts of future climate on stream temperature variability is also demonstrated. The results indicate that the DSS could substantially reduce the number of violations of thermal criteria, while ensuring maintenance of the cold pool storage throughout the summer.