Howard A. Hanson Dam is a U.S. Army Corps of Engineers flood control dam located near the headwaters of the Green River in King County. It's primary purpose is flood control in the winter and fish enhancement in the summer.
Before it was constructed, floodwaters of the Green River periodically spread almost unimpeded across the Green River Valley. Over the past 70 years the Valley has flooded more than 30 times, heavily damaging lands and buildings. People still remember the December 1959 flood when houses were flooded up to the second floors, and much irreplaceable topsoil was washed into Puget Sound.