A river of central and northeast Colorado and west-central Nebraska flowing about 724 km (450 mi) eastward to the North Platte River to form the Platte River. River, Colorado and western Nebraska, U.S. The river rises in central Colorado and flows southeast and then northeast across the Nebraska boundary to join the North Platte River and form the Platte River. The South Platte is 442 mi (711 km) long. Its reservoirs and dams, particularly around Denver, are used for flood control, irrigation, and power generation