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 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.