Which river forms most of the boundary between the US states of Oregon and Washington?
Lewis River (Washington) flank of Mount Adams, approximately 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 , 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
Oregon City, Oregon city has a total area of , of which, is land and is water. The major waterways of Oregon City include the Willamette River, which flows along the northwest side of the city, and the Clackamas River, which merges with the larger Willamette to the north of the city. The Willamette forms the boundary between Oregon City and West Linn; the Clackamas serves as the boundary between Oregon City and Gladstone. The Willamette Falls Locks in West Linn were the first multi-lift navigational locks in the United States and are now a National Historical Site, no longer in use. The