Howard Hanson Dam Additional Water Storage Project


The Howard A. Hanson Dam (HAHD) Additional Water Storage Project (AWSP) was authorized by Section 101 (b)(15) of the Water Resources Development Act (WRDA) of 1999 for municipal and industrial water supply and ecosystem restoration, with fish passage being recognized as a key element. This project involves the construction of a downstream fish passage facility that will allow juvenile salmon and steelhead to safely pass downstream through HAHD. The facility consists of vertically stacked ports on the dam’s upstream side, allowing fish to enter at varying reservoir water levels. Smolt then travel through water-filled conduits, passing at safe velocities below the dam.

The downstream fish passage facility will work in conjunction with the upstream fish passage facility and fish habitat projects upstream of HAHD, already completed by Tacoma Public Utilities (TPU). The U.S. Army Corps of Engineers (USACE) and TPU have been working together to prepare the upper watershed for the anticipated reintroduction of salmon which will restore the biological connection to more than 66 miles of high-quality salmon spawning habitat upriver from HAHD. The USACE also actively carries out ecosystem restoration programs.

The State of Washington’s Lead Entity for salmon recovery in the Green/Duwamish and Central Puget Sound Watershed, comprised of local and state governments, community organizations and businesses, believes that this could be the largest single opportunity to increase salmon production in Puget Sound. Completing HAHD fish passage would approximately double the available habitat for ESA-listed salmon and steelhead in the Green/Duwamish watershed.


The project is currently in the design phase, with construction anticipated to begin in 2025. Operations of the facility are scheduled to be completed by the end of 2030.

Project Background

Howard A. Hanson Dam is an earthen dam on the Green River, 35 miles southeast of Seattle. Originally built by the U.S. Army Corps of Engineers in 1962, HAHD is a multipurpose project with missions that include flood risk management, water supply storage, ecosystem restoration, and fish conservation. Flood risk management is focused on the highly developed Green River Valley. Since 1962, HAHD has prevented an estimated $21.5 billion in flood damage. HAHD and its reservoir lie entirely within the TPU municipal watershed and are closed to the public.

Authorized by the Flood Control Act of 1950, and amended by the Water Resources Development Act of 1999, the project currently provides:

  • 104,000 acre feet of flood storage from November to February
  • 5,000 acre feet for ecosystem restoration
  • 20,000 acre feet of municipal water storage
  • 24,000 acre feet for fish conservation

TPU's primary source of water has been the Green River since 1913. Three miles downstream from HAHD, TPU has a diversion dam that allows a portion of the river to be diverted for municipal use. Once the downstream fish passage facility is built TPU will transport adult fish from the diversion dam above HAHD, and will also pass juvenile fish through the facility.

Fish passage was initially not included when Howard A. Hanson Dam or when TPU’s diversion dam was built. At the time Tacoma thought it necessary to limit fish access to the upper watershed to protect public health. This action reduced fish production in the basin, but at the same time, attempts were made to make up the loss with the best tools available at the time – fish production from hatcheries. During the 1980s and 1990s, a greater knowledge of disease transmission potential from fish began to reduce concerns regarding the public health impact of fish above Tacoma’s diversion. In 2006, TPU in partnership with the Regional Water Second Supply Partners, constructed a trap and haul fish passage facility to move adult salmon upstream. USACE, in partnership with TPU, will open the upper Green River watershed above HAHD for salmon spawning. The fish population will be co-managed by the Washington Department of Fish & Wildlife and the Muckleshoot Indian Tribe with oversight by the U.S. Fish & Wildlife Service and the National Marine Fisheries Service.

Fish passage construction is only one piece of the efforts being done in the Green River basin by USACE and other partners across the region. USACE has supported fish habitat in this area for over 15 years by placing woody debris and gravel into the Green River downstream of TPU’s facility. USACE employees collect large logs and other woody debris that float downstream before they can enter and possibly damage the dam’s intake or tunnels. This debris is then transported downstream and reintroduced into the river for salmon habitat. The wood is carried by the river and has improved river habitat as far downstream as Auburn. Adding gravel has increased suitable spawning habitat from the Tacoma diversion dam to the Green River Gorge. 

The facility was initially authorized in 1999, with construction starting in 2005. In 2011, all work on the downstream fish passage facility was paused when current construction costs and the estimated cost of the facility were determined to exceed the authorized limit. Over a decade later on February 15, 2019, a Biological Opinion (BiOp) was issued by NOAA Fisheries, which reignited the downstream fish passage facility project. In addition to the fish passage facility’s construction meeting USACE’s goals to restore habitats and support healthy ecosystems, the BiOp also made it a requirement for USACE to complete a downstream fish passage facility at HAHD by the end of 2030.