thumbnail

Rapid Structured Decision Making for Hypomesus Transpacificus (Delta Smelt) Summer–Fall Freshwater Outflow Management

Open-File Report 2025-1055
Prepared in cooperation with the U.S. Bureau of Reclamation, the Metropolitan Water District of Southern California, and the California Department of Water Resources
By: , , , , , , , , , , , and 
https://doi.org/10.3133/ofr20251055

Links

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.

Plain Language Summary

Decisions on how to store and distribute water in California’s Central Valley are made considering the use of water resources by people, fish and wildlife, and the effects on water quality. Water is stored behind dams throughout the Central Valley for later release into rivers and canals for distribution to meet different water needs. Declining water availability and increasing human demands for water over recent decades have made these decisions increasingly difficult, especially because different uses of water resources often conflict. This report summarizes a facilitated decision-making process, led by the U.S. Geological Survey, involving water, fish, wildlife managers, and those that have an interest in how water is used (interest holders) in the Central Valley. This process provides information for water managers to consider when deciding how to distribute water resources to meet the needs for endangered Hypomesus transpacificus (delta smelt), different runs of Oncorhynchus tshawytscha (Chinook salmon), and Oncorhynchus mykiss (Central Valley steelhead), while maximizing water deliveries for human use and maintaining water quality standards.

Suggested Citation

Healy, B.D., Phillis, C.C., Mahardja, B., Koizumi, C., Pien, C., Parker, N., Conrad, J.L., Ekstrom, J., Leimbach, J., Silberblatt, R., Fischer, T., and Ehlo, C., 2025, Rapid structured decision making for Hypomesus transpacificus (delta smelt) summer–fall freshwater outflow management: U.S. Geological Survey Open-File Report 2025–1055, 36 p., https://doi.org/10.3133/ofr20251055.

ISSN: 2331-1258 (online)

Table of Contents

  • Acknowledgments
  • Abstract
  • Plain Language Summary
  • Introduction
  • Purpose and Scope
  • Legal and Regulatory Context
  • Ecological Context
  • Methods
  • Decision Analysis
  • Discussion
  • References Cited
  • Appendix 1. List of Proposed Objectives Submitted by Participants
  • Appendix 2. Estimating the Consequences of Aquatic Weed Control Action for Delta Smelt
Publication type Report
Publication Subtype USGS Numbered Series
Title Rapid structured decision making for Hypomesus transpacificus (delta smelt) summer–fall freshwater outflow management
Series title Open-File Report
Series number 2025-1055
DOI 10.3133/ofr20251055
Publication Date December 09, 2025
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Southwest Biological Science Center
Description Report: viii, 36 p.; Data Release
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details