The Lewis River is a river in southwestern Washington state, USA. It flows approximately 95 miles (153 km) from its headwaters on the west side of the Cascade Range to its confluence with the Columbia River.
The river is named for Meriwether Lewis of the Lewis and Clark Expedition, although neither Lewis nor Clark explored this particular river. The naming occurred later in the 19th century.
The Lewis River is notable for its significant hydroelectric development. Three major dams, constructed by PacifiCorp, create Swift Reservoir, Yale Lake, and Merwin Lake. These reservoirs provide hydroelectric power, recreation opportunities, and flood control.
The river and its surrounding watershed support a variety of fish species, including salmon, steelhead, and trout. Fisheries management is an ongoing concern due to the impacts of the dams and other human activities. Efforts are being made to improve fish passage and habitat restoration.
The Lewis River area offers numerous recreational activities, including boating, fishing, hiking, camping, and wildlife viewing. Gifford Pinchot National Forest encompasses much of the river's upper reaches.
Concerns regarding water quality, habitat preservation, and the ecological impacts of hydroelectric operations continue to be important issues in the Lewis River watershed.