Mayfield: Cowlitz River, WA

Tacoma Power’s largest electricity generating facility is the Cowlitz River Project. It produces enough renewable hydroelectric energy to serve more than 135,000 Northwest homes. The Cowlitz River Project produces hydroelectric power from the water stored behind Mayfield and Mossyrock dams.

The Mayfield development, completed in 1963, includes a 250-foot-high (above bedrock), 850-foot-long, concrete arch and gravity dam that impounds Mayfield Lake. In addition to the Cowlitz River, inflows from the Tilton River also contribute to Mayfield Lake. An 854-foot-long power tunnel passes through the right abutment of the dam and terminates at a concrete forebay structure. Four penstocks continue from the forebay structure to the four 40,500 kilowatt vertical Francis turbines for an installed capacity of 162-megawatts.