The Willamette River is a major tributary of the Columbia River in the Pacific Northwest of North America. 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, which contains two-thirds of Oregon's population. The state's largest city, Portland, surrounds the Willamette's mouth at the Columbia. Due to prolific rainfall in the basin and sediments from the glacial Missoula Floods, the Willamette Valley is one of the most fertile agricultural regions in North America, and was thus the destination of many 19th-century pioneers traveling west along the Oregon Trail. Since 1900, more than 15 large dams and many smaller ones have been built in the Willamette's drainage basin. They are used primarily to produce hydroelectricity, to maintain reservoirs for recreation, and to prevent flooding. The river and its tributaries support 60 fish species, including many species of salmon and trout; this is despite the dams, other alterations, and pollution (especially on the river's lower reaches)
