The Willamette River is the largest tributary of the Columbia in Oregon and the largest river wholly within the state (the Snake, which forms a portion of Oregon’s border with Idaho, is a bigger river but is not wholly within Oregon). Not only is the Willamette a major river, it flows through the most populated part of the state. The Willamette River Basin comprises 11,748 square miles and includes 70 percent of the population of Oregon.
The 4,000-square-mile Willamette Valley, destination of thousands of Oregon Trail emigrants in the 1840s and 1850s, has rich farmland and annually produces about half of all the sales of farm products in the state. The river basin is bordered by the Cascade Mountains on the east, the Coast Range on the west, and the Calapooia Range on the south. The northern end is its discharge into the Columbia 86 miles inland from the ocean. There, the Columbia flows through Oregon’s largest city, Portland, which has an urban-area population of more than 1 million.
The Willamette basin is about 180 miles long and 100 miles wide; the mainstem of the river is 185 miles long. The Willamette is the 13th largest river by volume in the United States. Largely because of its location between two mountain ranges and its proximity to the ocean and frequent weather systems, the Willamette has more runoff per square mile than any other large river in the lower 48 states.
According to a 2004 report on the Willamette basin, there 18 species of native amphibians, 15 species of reptiles, 154 bird species, 69 mammal species, and 31 native fish in the basin. There are 25 major dams in the Willamette basin, including 11 hydropower dams operated by public and private utilities, one multipurpose dam on the Tualatin River, and 13 multipurpose reservoirs (flood control, hydropower, recreation) operated by the U.S. Army Corps of Engineers.