Project Synopsis

Project Name: Howard Hanson Dam Additional Water Storage
Project Customer: City of Tacoma
Funding Type: Construction General
Type of Project: Water Supply, Ecosystem Restoration
Location: Howard A. Hanson Dam is located northeast of Enumclaw, Washington.

Project Description. The existing Howard Hanson Dam project is a flood control and summer conservation project on the Green River. Under Section 216 of the 1970 Flood Control Act the Howard Hanson Dam project was evaluated for the purposes of water supply storage, improved fish passage through the dam, and improvements to the surrounding habitat. The Howard Hanson Dam Additional Water Storage Project is a dual purpose water supply and ecosystem restoration project.  The purpose of this project is to meet the Municipal and Industrial (M&I) water supply needs of the Puget Sound area, and provide downstream and upstream ecosystem restoration for fish and wildlife in the Green River watershed.  This project is being implemented in two phases.

The Phase I pool raise includes construction of all mitigation features having to do with raising the pool to elevation (el.) 1167 feet, and all ecosystem restoration features. This includes a fish passage facility, right dam abutment seepage mitigation, and fish and wildlife habitat mitigation.  The general objective of the mitigation and restoration projects is to restore in part the natural processes that have been disrupted by the dam and to offset impacts to habitat that will be caused by the additional water storage and larger reservoir.  The dam and reservoir currently trap 100% of gravel and wood that reaches the dam.  Consequently, downstream reaches have become gravel and wood 'starved', resulting in an armoring of the streambed and decrease in habitat complexity that adversely affects fish spawning and rearing.  For this reason there are three habitat projects located immediately downstream of the dam that include annual gravel nourishment, placement of loose wood that collects behind the dam each year and construction of two engineered log jams.  The Phase II pool raise includes construction of all remaining project mitigation features required for a pool raise to el. 1177 feet.  With the additional 2,400 ac-ft of M&I water plus 9,600 ac-ft of LFA water to be stored under Phase II, a combined total of 32,000 ac-ft of additional water will be stored at Howard Hanson Dam.

Construction for the habitat mitigation and restoration began in 2003 and will be completed in 2006 or 2007.  Construction of the fish passage facility is in progress and is scheduled to be completed in 2009.

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