Howard Hanson Dam spans the Green River approximately three miles upstream from Tacoma Water’s Headworks diversion dam, where water from the Green River is diverted by Tacoma Water. The Howard Hanson Dam was built in 1961. The U.S. Army Corps of Engineers (USACOE) operates the dam, controls water levels in the reservoir, and regulates flow in the Green River for flood control in the winter and flow-augmentation during low-flow periods (typically late summer through fall). The primary authorized use of the project is to provide flood control for the Green River Valley during the winter. Secondary authorized uses include augmentation of low summer flows downstream of the dam, irrigation, and water supply. The dam impounds the Green River, forming the Eagle Gorge Reservoir.

Tacoma Water, the Regional Water Supply System (RWSS), and the USACOE are working in cooperation on the Howard Hanson Additional Water Storage Project. This project – a component of the Second Supply Project - increased the amount of water stored behind Howard Hanson Dam for use as municipal supply by increasing the surface elevation from 1147 feet (above sea level) to 1167 feet. This will increase the stored volume of water by approximately 75 percent. Concurrent with this increased storage volume, a new downstream Fish Passage Facility is being constructed to enhance survival and passage of out-migrating juvenile fish. This facility will predominantly take water off the top water elevations from the reservoir, rather than the deep elevations as is done with the current outlet structure. The downstream migration facility is not expected to be completed until 2013, however, storage for municipal use began in 2006 and has already been used to supplement Tacoma’s water supply.

Prior to completion of the downstream fish migration facility, Tacoma and the RWSS will have the ability to store up to 10,000 acre-feet (AF) of water behind Howard Hanson Dam during the spring. Tacoma Water’s share of the stored water is 15/36, or up to 4,167 AF, which equates to 14 MGD (million gallons per day) if used at a uniform rate over a 90-day period. Upon completion of the downstream fish passage facility, the amount of water available to the RWSS will double (20,000 AF total).

The Additional Storage Project will provide flexibility in how sources are used and will provide additional water when it is most needed to meet demands.