Clark County, to the south. Towns along the river include Cougar, Ariel, and Woodland. In its lower , it turns sharply south, then west, and enters the Columbia from the east, opposite St. Helens, Oregon, approximately 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. River modifications The North Fork is impounded for hydroelectricity and flood control in its middle course by Swift Dam, forming Swift Reservoir; Yale Dam, forming Yale Lake; and Merwin Dam, forming Lake Merwin. Horseshoe Lake in Woodland is a former oxbow of the North Fork. Beginning in 1940, the construction of Washington State Highway 99 (later to become the Interstate 5 corridor in this area), resulted in the construction of a dike that straightened the river to the east of the highway before it flows under what is now Interstate 5 near the Woodland southern boundary. See also List of rivers of Washington Tributaries of the Columbia River References External links USGS: Lewis River Basin North Fork Lewis River Photo Essay Documentary produced by Oregon Field Guide Rivers of Washington (state) Tributaries of the Columbia River Rivers of Skamania County, Washington Rivers of Clark County, Washington Rivers of Cowlitz County, Washington Mount Adams (Washington) Gifford Pinchot National Forest Midway High Lakes Area