Located east of Tacoma, the Green-Duwamish River Basin covers 483 square miles. Originating in the Cascades, the Green River flows northwest and empties into Elliott Bay; the river turns into the Duwamish at the point where the Green and Black Rivers once converged.

Flooding in the Green River Valley was once a costly problem; it affected not only personal property, including crop losses and planting delays for hundreds of farms, but also city property and state and county roads. In 1947, an average of $200,000 total annual losses was estimated. Loss of human lives and of livestock also occurred during flooding periods. Possible methods for flood control included building a dam and reservoir on the Green River, constructing dykes, or excavating a river channel to carry flood waters. The dam, if "economically feasible," would be paid for by federal dollars. The Seattle Chamber of Commerce supported federal flood control, citing not only the high cost of flood damages, but also the potential for further agricultural and industrial development of the Duwamish basin if flooding were brought under control.

Government engineers also supported the idea of a dam; a project using federal funds to construct a dam on the Green River in Eagle Gorge was adopted by Congress in 1950. Originally known as the Eagle Gorge Dam, the dam was renamed Howard Hanson Dam, after the Seattle Chamber of Commerce Rivers and Harbors Committee chairman who was instrumental in the flood control project, when Hanson died in 1957. The dam was completed in 1962, and extensive development of the valley -- both agricultural and industrial -- soon followed. Although flood control remains the primary function of the Howard Hanson Dam, it is also used for irrigation and water supply.