 The South Fork begins at  slightly south of the Middle Fork and flows generally east to meet the main stem at The South Fork passes under Route 99W south of Monmouth and under Route 51 in Independence.None of the three forks has a named tributary.Before alterations by settlers in the 19th century, the basin was a "boggy, 'braided' marsh with many small creeks spilling into Ash Creek" and the river.