The South Fork Republican River is a river that arises in Lincoln County, Colorado, United States, and flows east-northeastward for about 171 miles (275 km) through Kit Carson and Yuma Counties, Colorado, and Cheyenne County, Kansas, to a confluence with the Republican River in Dundy County, Nebraska. Bonny Reservoir is located on the South Fork Republican River in Yuma County, Colorado. Use of water from the South Fork Republican River is governed by the , a water agreement among the U.S. states of Colorado, Kansas, and Nebraska signed on 12-31-1942