Lake Tapps is a reservoir in Pierce County, Washington. It was created in 1911 by Puget Sound Energy and operated for hydroelectric power until it ceased power production in 2004. In December 2009 PSE sold the lake to the Cascade Water Alliance, a municipal corporation whose members are five cities and two water districts. Cascade provides water to almost 350,000 residents and more than 20,000 businesses. It plans to eventually use Lake Tapps as a municipal water supply source.