Corps of Engineers begins annual Howard Hanson Dam reservoir summer refill

Published Feb. 21, 2017

SEATTLE – U.S. Army Corps of Engineers water managers will begin refilling the Howard A. Hanson Dam reservoir near the Green River headwaters February 21, 2017.

It takes roughly three months to reach the maximum pool elevation target of 1,167 feet above sea level, the refill goal since 2007.

The stored water augments downstream flows, helping endangered Chinook and steelhead, as well as other fish.  Regulating downstream flows assures sufficient water levels for successful fish migration, spawning and other life-cycle events. The City of Tacoma and its water supply partners also use some for their water supplies.

The spring water supply outlook is currently average based on average snowpack conditions in the Green River Basin and wet basin soils following typical precipitation patterns the past few months.

The dam’s primary function is to provide flood risk management for the Green River in King County. During winter the reservoir is kept nearly empty to maximize available space in the reservoir for flood water storage. During a flood event water is stored in the reservoir to manage downstream flood risk while making releases to target river flows within downstream channel capacity, provided reservoir holding capacity is available. Water in the reservoir is released as soon as possible to make storage space for the next potential flood event.

When flooding probability diminishes, the dam begins its second major function - water conservation. The reservoir is gradually filled and the water collected augments summer season low flows.

Water managers carefully monitor reservoir levels and dam safety is a top priority for the Corps of Engineers. The Corps has an aggressive dam safety program and safety is a daily focus at Howard Hanson Dam. Corps dam safety professionals regularly assess the dam and verify proper operations and maintenance are taking place.


Current reservoir information is available on the Corps’ Seattle District Reservoir Control Center website at
Bill Dowell

Release no. 17-006