South Platte (plăt) [key], river, c.450 mi (720 km) long, rising in the Rocky Mts. in many branches, which then join in central Colorado. It flows in a narrow canyon E and NE to Denver, then NE across the Great Plains to join the North Platte in central Nebraska, where it forms the Platte River. On the Great Plains the river is broad and shallow and loses much water by evaporation. Grazing and irrigated agriculture are important there. The river basin has many private irrigation dams. The upper course of the South Platte is part of the Bureau of Reclamation's Colorado–Big Thompson project.