Howard Hanson Dam

Howard Hanson Dam stands on the upper Green–Duwamish River in King County, 63 river miles above the mouth. The Corps Seattle District fact sheet places it in Tacoma’s municipal watershed, 35 road miles east of Tacoma and six miles upstream from Palmer. The dam provides flood-risk reduction, municipal water supply, and summer and fall low-flow augmentation for fish. The public cannot visit; the project is closed.

Construction, including 13 miles of railroad relocation, began in February 1959. The dam became operational on Christmas Day 1961, ahead of a scheduled April 1962 finish. The reservoir collects runoff from 220 square miles of the Green River’s 483-square-mile drainage. Flood damage prevented by the dam from the January 2009 flood is estimated at about $4.6 billion.

The dam is an earth-and-rockfill structure. Outlet works include a 19-foot-diameter horseshoe concrete-lined tunnel. A gated spillway with two 45-by-30-foot tainter gates allows storage to elevation 1,206 feet without using the spillway. The paved spillway chute is 656 feet long. The Corps says the spillway has never been needed. The sources below are U.S. Army Corps of Engineers and U.S. Geological Survey pages.