The Tieton River begins in two forks, the North Fork Tieton River and South Fork Tieton River. The North Fork originates at McCall Glacier on the eastern sides of Old Snowy Mountain, Ives Peak, and the Goat Rocks for which the Goat Rocks Wilderness is named. Collecting headwater streams from other high mountains such as Tieton Peak, the North Fork flows northeast into Clear Lake, then into the west end of Rimrock Lake. The South Fork Tieton River begins at Meade Glacier on the eastern side of Gilbert Peak, also part of the Goat Rocks. The South Fork collects many streams as it flows generally north, entering Rimrock Lake near its east end.