Riffe Lake, reservoir on the Cowlitz River in Lewis County, Washington, United States

Riffe Lake is a 23.5-mile reservoir on the Cowlitz River in Lewis County, Washington, formed in 1968 by Mossyrock Dam, the tallest dam in the state. It holds six unnamed islands, three of them tiny sandy islets, and is bordered by Mossyrock Park and Taidnapam Park, both with boat launches.

Riffe Lake was formed in the late 1960s behind Mossyrock Dam, the tallest dam in Washington, built by Tacoma City Light. The rising water submerged the communities of Kosmos, Nesika and Riffe, whose residents were relocated. First called Mossyrock Lake and then Davisson Lake in 1962, it was renamed Riffe Lake in 1976 after Lewis County residents petitioned to honor the flooded town.

Reach the lake from U.S. Route 12 near Mossyrock; the Gary Stamper Memorial Overlook on the northern shore opened in June 2025. Boat launches at Mossyrock Park and Taidnapam Park have been closed periodically since 2017, reopening only when Tacoma Power raises water levels, so check conditions before towing a boat.

Riffe Lake covers six islands, three of them tiny sandy islets, and none has a name. The reservoir also holds rainbow and brown trout, landlocked coho, largemouth and smallmouth bass, bluegill, crappie and perch. The Kosmos Unit of the Cowlitz Wildlife Area, a 520-acre site split by Rainey Creek, draws pheasants, songbirds, ospreys and raptors.