The Willamette River is a 187-mile 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 me…The Willamette River is a 187-mile 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 18 miles 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 101 miles from the larger stream's mouth on the Pacific Ocean, and the smaller Multnomah Channel enters the Columbia about 14.5 miles further downstream near St. Helens in Columbia County.