The Willamette River is a tributary of the Columbia River in the U.S. state of Oregon. The upper tributaries of the Willamette originate in mountains south and southeast of the twin cities of Eugene and Springfield. Formed by the confluence of the Middle Fork Willamette River and Coast Fork Willamette River near Springfield, the main stem meanders generally north from source to mouth. The river's two most significant course deviations occur at Newberg, where the stream turns sharply east, and about downriver from Newberg, where it turns north again. Near its mouth, the river splits into two channels that flow around Sauvie Island. The main channel enters the Columbia about from the larger stream's mouth on the Pacific Ocean, and the smaller Multnomah Channel enters the Columbia about further downstream near St. Helens in Columbia County. Incorporated Cities along the Willamette's course include Springfield and Eugene in Lane County; Harrisburg in Linn County; Corvallis in Benton County; Albany in Linn and Benton counties; Independence in Polk County; Salem in Marion and Polk Counties; Keizer in Marion County; Newberg in Yamhill County; Oregon City, West Linn, Milwaukie, and Lake Oswego in Clackamas County (Small portions of the later two extend into Multnomah County), and Portland in Multnomah County. (Parts of Portland are also in Washington County, but the Willamette neven makes contact with Washington County.) Significant tributaries include the McKenzie, Long Tom, Marys, Calapooia, Santiam, Luckiamute, Yamhill, Molalla, Tualatin, and Clackamas rivers. Arising at above sea level, the river loses in elevation between source and mouth or about 2.3 feet per mile (43 cm per km). The gradient is slightly steeper from the source to Albany than from Albany to Oregon City. At Willamette Falls, the river plunges about . For the rest of its course, the river is extremely low-gradient and is influenced by Pacific Ocean tidal effects from the Columbia. The main stem of the Willamette varies in width from about . Contributing forks The Middle Fork and the North Fork—a tributary of the Middle Fork—rise on the western side of the Cascades between Three Sisters Wilderness on the north and Diamond Peak Wilderness on the south. Originating in Douglas County, the Middle Fork receives the North Fork northwest of Oakridge in Lane County and flows northwest through the mountains to the southern end of the Willamette Valley. The Coast Fork, draining part of the Calapooya Mountains south of