Forecast Informed Reservoir Operations: Howard A. Hanson Dam

FIRO is a reservoir-operations strategy that better informs decisions to retain or release water by integrating additional flexibility in operation policies and rules with enhanced monitoring and improved weather and water forecasts (American Meteorological Society; 2020). Since 2014, CW3E and federal, state, and local partners have developed, tested, and implemented FIRO at reservoirs across the Western United States. FIRO is now being assessed nationwide.

Water stored behind Howard A. Hanson Dam

PROJECT STATUS

FIRO Evaluation

Primary Viability Assessment In Progress

OVERVIEW

Howard A. Hanson Dam is located on the Green River, 21 miles east of Auburn, in King County, Washington. The dam is owned and operated by the U.S. Army Corps of Engineers Seattle District. Built in 1961, it serves three authorized purposes: winter flood risk reduction, municipal water supply for the City of Tacoma, and summer and fall flow augmentation for fish, including federally listed endangered Chinook salmon and steelhead trout.

Tacoma Water and its partners rely on storage at the dam to meet peak summer water demand for approximately 700,000 people across the South Puget Sound region, and to keep enough water flowing downstream to support critical fish habitat. Since 1962, the dam has prevented an estimated $21.5 billion in flood damage to downstream communities along the Green River, a heavily developed corridor encompassing cities, residential neighborhoods, and major industrial and commercial centers in the Green River Valley. The Green River basin is heavily influenced by atmospheric rivers, which account for more than 93% of all flood damage costs in Washington. The motivation for implementing FIRO at Howard A. Hanson Dam is to mitigate the increasing risk of failure to refill due to changing hydrology. Full reservoir refill is critical for chinook salmon and steelhead flows in the Green River and for a reliable water supply to the City of Tacoma.

The FIRO Steering Committee was convened in September 2023, and the Work Plan was completed in August 2024. FIRO at Howard Hanson Dam is now being evaluated through the Preliminary Viability Assessment, targeted for publication in December 2026.

FIRO PROGRESS

Timeline of FIRO progress at Howard A. Hanson Dam

Forecast Informed Reservoir Operations implementation progress at Howard Hanson Dam. Green circles with checkmarks indicate completed milestones, blue and gold circles without checkmarks indicate upcoming milestones.

KEY FEATURES

1^st

FIRO project in the Pacific Northwest

Cross-sector partners from federal, state, tribal, and research institutions

operational objectives, including flood risk reduction, municipal water supply, and fish flow augmentation

Endangered Salmon and Steelhead

The Green River downstream of Howard Hanson Dam supports federally listed endangered Chinook salmon and steelhead trout. Reservoir releases during summer and fall directly influence instream flows critical to these species’ habitat and life cycles, making fish considerations a central element of the FIRO viability assessment at Howard Hanson.

The Muckleshoot Indian Tribe, whose treaty fishing rights are tied to the Green River system, is an active member of the project’s collaborative team. CW3E and project partners are evaluating how forecast-informed operations can preserve and potentially enhance flows that support fish habitat while maintaining flood risk reduction and water supply reliability.