Howard A. Hanson Dam (HAHD) is a multipurpose project for flood risk management, municipal and industrial (M&I) water supply, ecosystem restoration, and low-flow augmentation. The project provides flood risk management benefits to the highly developed Green River Valley.
The HAHD project is located in King County, Washington, on the Green River 35 miles southeast of Seattle, 25 miles east of Tacoma, and 7 miles upstream from Kanasket. The dam itself is at river mile 64.5. The project lies entirely within the City of Tacoma municipal watershed and is closed to the public.
The authorization for the dam, initially named the Eagle Gorge Dam, came from Public Law 81-516, Rivers and Harbors Act of 1950. Construction was complete in 1962. The authorized project purpose was for 106,000 ac ft of flood storage from November to February and for 24,200 ac ft of water storage for summer low flow augmentation for conservation and preservation of fish life.
In drought years, the summer low-flow augmentation to pool elevation 1141’ was not adequate to keep the river wetted downstream of the Tacoma Water diversion dam. In response to requests from resource agencies and the Muckleshoot Indian Tribe, the Corps and Tacoma (non-Federal Sponsor) completed a project under Section 1135 of the 1986 Flood Control Act (FCA) to provide an additional 5,000 ac ft of summer low flow water supply and adaptive pool operation for the conservation of salmon and steelhead.
The Additional Water Storage Project (AWSP) was authorized in Section 101(b) (15) of the Water Resources Development Act of 1999 (Public Law 106-53). Tacoma Public Utilities, Water Division, is the project sponsor for the AWSP. The AWSP was authorized with two purposes: Municipal and Industrial (M&I) water supply and ecosystem restoration (added to the study in 1994). The project includes changes to the existing project to increase non-flood season summer water storage in two Phases. The Phase 1 provides 20,000 ac-ft for M&I water supply for the City of Tacoma. Fish passage features for Endangered Species Act (ESA) compliance are also included. Additionally, Phase 2 of the project (to be constructed at a later date) includes storage of 2,400 acre-ft for M&I and 9,600 acre-ft for low-flow augmentation, as well as restoring and mitigating additional fish and wildlife habitat. The non M&I water supply components are intended to provide for re-establishment of self-sustaining runs of salmon and steelhead. The M&I water is provided to support the economic development of the Puget Sound area.