The McKenzie River is a 90-mile (145 km) tributary of the Willamette River in western Oregon in the United States. It drains part of the Cascade Range east of Eugene and flows westward into the southernmost end of the Willamette Valley. It is named for Donald McKenzie, a Scottish Canadian fur trader who explored parts of the Pacific Northwest for the Pacific Fur Company in the early 19th century. As of the 21st century, six large dams have been built on the McKenzie and its tributaries.

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.