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.