Cougar Reservoir (also known as Cougar Lake) is a reservoir on the South Fork McKenzie River in the U.S. state of Oregon. It is in Lane County, 46 miles (74 km) east of Eugene and about 3 miles (5 km) south of the community of Rainbow in the Willamette National Forest. In 1963 the United States Army Corps of Engineers built Cougar Dam on the river primarily to generate hydroelectricity and control flooding, and the reservoir formed behind the dam.