Rapid structured decision making for Hypomesus transpacificus (delta smelt) summer–fall freshwater outflow management

Abstract

Managers of the Central Valley Project (CVP) and State Water Projects (SWP) in California are confronted with difficult tradeoffs between water uses and associated values affected by water management decisions. These decisions involve altering the timing and magnitude of water releases from dams and reservoirs, which can affect habitats for economically important and Federally and State-listed endangered fish species, water deliveries for agriculture or municipalities, and water quality. In this report, we describe the results of a rapid structured decision-making process used to assist management agencies in evaluating tradeoffs while gathering input from cooperating agencies, rightsholders, or interested parties (hereafter participants) through facilitated workshops in spring 2025. Consideration of alternative water management actions was initiated by the continued decline of Hypomesus transpacificus (delta smelt) populations and the issuance of a new biological opinion for the CVP and SWP long-term operations on the effects on delta smelt and other Endangered Species Act-listed species in November 2024. An Executive Order was also issued in January 2025, directing the Bureau of Reclamation to maximize water deliveries. Participants, led by the U.S. Geological Survey and cooperating agencies, identified 8 fundamental values (hereafter objectives) and 11 alternative water management scenarios (or “alternative management actions” based on the PrOACT model). Using multicriteria decision analysis, we evaluated performance (or “consequences” based on a consequence table analysis) and analyzed tradeoffs of alternative water management actions to the fundamental objectives. We ranked the alternative water management actions based on four participants’ objective weights and composite utility scores calculated using a linear value function. The three highest ranking alternative water management actions had the poorest performance for delta smelt but performed best for CVP and SWP water exports and objectives related to coldwater pool operations for salmonids. An optimum strategy that could prevent the extinction of delta smelt was not determined for this study. However, insights gained from our rapid decision analysis suggested nonflow scenarios could benefit the delta smelt population, including in drier years, and could be considered to avoid curtailment of water exports.