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Willamette River
USACE Fremont Bridge Portland.jpg
8 bridges across the Williamette River in Downtown Portland. From front to back: Fremont Bridge, Broadway Bridge, Steel Bridge, Burnside Bridge, Morrison Bridge, Hawthorne Bridge, Marquam Bridge, Ross Island Bridge
Name origin: From a Clackamas Native American village name
Country United States
State Oregon
Cities Eugene, OR, Corvallis, OR, Albany, OR, Salem, OR, Newberg, OR, Wilsonville, OR, Portland, OR
Source Confluence of Middle Fork Willamette River and Coast Fork Willamette River
 - location near Eugene, Lane County, Oregon
 - elevation 438 ft (134 m)
Mouth Columbia River
 - location Portland, Multnomah County, Oregon
 - elevation 10 ft (3 m)
Length 187 mi (301 km)
Basin 11,478 sq mi (29,730 km²)
Discharge for Morrison Bridge, Portland, 12.8 miles (20.6 km) from mouth
 - average 33,010 cu ft/s (900 /s)
 - max 420,000 cu ft/s (11,900 /s)
 - min 4,200 cu ft/s (100 /s)

The Willamette River is a major tributary of the Columbia River, accounting for 12 to 15 percent of the Columbia's flow. The Willamette's main stem is 187 miles (301 km) long, lying entirely in northwestern Oregon in the United States. Flowing northward between the Oregon Coast Range and the Cascade Range, the river and its tributaries form the Willamette Valley, a basin that contains two-thirds of Oregon's population, including the state capital, Salem, and the state's largest city, Portland, which surrounds the Willamette's mouth at the Columbia.

Originally created by plate tectonics about 35 million years ago and subsequently altered by volcanism and erosion, the river's drainage basin was significantly modified by the Missoula Floods at the end of the most recent ice age. Humans began living in the watershed over 10,000 years ago. There were once many tribal villages along the lower river and in the area around its mouth on the Columbia. Indigenous peoples lived throughout the upper reaches of the basin as well.

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