U.S. Army Corps of Engineers water managers will begin refilling the Howard Hanson Dam reservoir near the Green River headwaters mid-March.
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 Corps said. The city of Tacoma and its water supply partners also use some for their water supplies.
The spring water supply outlook is currently below average based on below average snowpack conditions in the Green River Basin, the Corps said. However, the current forecast suggests more than adequate spring runoff to fully refill the reservoir.
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, 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. Hanson Dam flood risk reduction operations have helped avoid an estimated $21 billion in flood damages in the heavily-populated Green River Valley.
When flooding probability diminishes, the dam begins its second major function – water conservation storage. The filled reservoir augments summer season low flows. Augmentation is primarily for fish conservation, ensuring enough water is flowing for aquatic resources, including spawning and rearing salmon.
The Corps also supports fish habitat by placing woody debris and gravel into the Green River downstream of Tacoma Water’s diversion dam. Large logs and other woody debris collected floating upstream of the dam is transported downstream and reintroduced into the river for salmon habitat. The wood has improved river habitat as far downstream as Auburn. Adding gravel increases suitable spawning habitat down to the Green River Gorge, an area heavily used by spawning Chinook salmon.
Dam safety is a top priority for the Corps of Engineers and water managers carefully monitor reservoir levels. The Corps has an aggressive dam safety program and safety is a daily focus at HAHD. Corps dam safety professionals regularly assess the dam and verify proper operations and maintenance are taking place.
Reservoir information is available on the Corps’ Seattle District Reservoir Control Center website at bit.ly/NWS-RCC.