High Quality Content by WIKIPEDIA articles! The South Fork McKenzie River is a tributary, about 31 miles (50 km) long, of the McKenzie River in the U.S. state of Oregon. It begins at about 4,500 feet (1,400 m) above sea level near Mink Lake in the Three Sisters Wilderness of the Cascade Range. Flowing northwest within Lane County, it meets the McKenzie River about 60 miles (97 km) from the larger river`s confluence with the Willamette River.