The Lewis River is a tributary of the Columbia River, about 95 miles (153 km) long, in southwestern Washington in the United States. It drains part of the Cascade Range north of the Columbia River. The drainage basin of the Lewis River covers about 1,046 square miles (2,709 sq km). The river's mean annual discharge is about 6,125 cubic feet per second (173.4 m3/s). Unlike nearby Lewis County and Fort Lewis the Lewis River was not named for Meriwether Lewis, but rather for A. Lee Lewis, an early settler who homesteaded near the mouth of the river.
The Lewis River rises in the Cascades in northeastern Skamania County, high on the west flank of Mount Adams, approximately 75 miles (121 km) northeast of Portland, Oregon. It flows generally southwest through Gifford Pinchot National Forest, across central Skamania County, passing south of Mount St. Helens. It forms the boundary, along the Dark Divide on its north side, between Cowlitz County, to the north, and Clark County, to the south. Towns along the river include Cougar, Ariel, and Woodland. In its lower 10 miles (16 km), it turns sharply south, then west, and enters the Columbia from the east, opposite St. Helens, Oregon, approximately 15 miles (24 km) north of Vancouver, Washington. The mouth of the Lewis River is at Columbia river mile (RM) 87 or river kilometer (RK) 140.
Near the confluence with the Columbia River, the Lewis River is joined by the East Fork Lewis River. The main Lewis River, sometimes called the North Fork Lewis River, forms the boundary between Clark and Cowlitz counties, while the East Fork divides Clark County in half. The East Fork flows westward from headwaters on the western flanks of Lookout Mountain in Skamania County. Parks along the East Fork include Moulton Falls, Lucia Falls, Lewisville, and Paradise Point. A variant name of the East Fork is the South Fork Lewis River.