100 The Willamette River is a tributary of the Columbia River, approximately 240 mi long, in northwestern Oregon in the United States. Flowing northward between the Coastal Range and Cascade Range, the river and its tributaries form a basin called the Willamette Valley containing the largest inhabitants centers of Oregon, including Portland, which sits along both sides of the river near its mouth on the Columbia. Its lush valley is fed by prolific precipitation on the western side of the Cascades, forming one of the most productive agricultural regions of North America that was the destination for many if not most of the emigrants along the Oregon Trail. The river was an important transportation route throughout much of the early history of the state, furnishing a means of conveying the vast timber and agricultural resources of the state to the outside world.
The Willamette rises in three separate forks in the mountains south and southeast of Eugene, at the southern end of the Willamette Valley. The Middle Fork and North Fork raise on the western side of the Cascades between Three Sisters south to Diamond Peak, with the Middle Fork in receipt of the North Fork northwest of Oakridge and flowing northwest from side to side the mountains to the southern end of the Willamette Valley. The Coast Fork rises in the lower mountains south of Cottage Grove, flowing north to join the Middle Fork 2 mi southeast of Eugene.
From Eugene, the joint river flows NNW across the plain of the southern Willamette Valley to Corvallis, and then follows a zigzag course past Albany and around the isolated hills in the central valley, passing west of downtown Salem. From Salem it flows north in a roundabout course across the northwest plain of the valley, reaching the hills at Newberg, where it turns sharply ENE along the hills, passing through an opening in the hills at Oregon City, the position of the Falls of the Willamette and the head of navigation. From Oregon City it flows northwest, past Lake Oswego and Milwaukie on the south edge of Portland, then passing between east and west Portland, where it is spanned by a series of urban bridges. Downstream of downtown Portland it flows northwest through the industrial port area of Portland Harbor, then splitting into two channels around Sauvie Island, both of which hook around to enter the Columbia from the west, with the main channel entering on the north edge of Portland and the smaller Multnomah Channel entering just about 15 mi NNW at St.Helens.