The South Platte River is one of the two principal tributaries of the Platte River and itself a major river of the American West, located in the U.S. states of Colorado and Nebraska. It drains much of the eastern flank of the Rocky Mountains in Colorado, as well as much of the populated region known as the Colorado Front Range and Eastern Plains. It forms the Platte at its confluence with the North Platte River in western Nebraska. The river serves as the principal source of water for eastern Colorado. Its valley along the foothills in Colorado has provided for agriculture in an area of the Colorado Piedmont and Great Plains that is otherwise arid. Its drainage basin also includes a portion of southeastern Wyoming in the vicinity of the city of Cheyenne.
The river is formed in Park County, Colorado southwest of Denver in the South Park grassland basin by the confluence of the South Fork and Middle Fork, approximately 15 miles (24 km) southeast of Fairplay. Both forks rise along the eastern flank of the Mosquito Range, on the western side of South Park, which is drained by the tributaries at the headwaters of the river. From South Park, it passes through the Platte Canyon (sometimes called Waterton Canyon) approximately 50 miles in length, receiving the North Fork before emerging from the foothills southwest of the Denver suburb of Littleton, where it is impounded to form Chatfield Reservoir, a major source of drinking water for the Denver Metropolitan Area.
The river flows north through central Denver, which was founded along its banks at its confluence with Cherry Creek. The valley through Denver is highly industrialized, serving generally as the route for both the railroad lines, as well as Interstate 25. On the north side of Denver it is joined somewhat inconspicuously by Clear Creek, which descends from the mountains to the west in a canyon that was the cradle of the Pike's Peak Gold Rush. North of Denver it flows through the agricultural heartland of the Piedmont (a shale region that was formed through erosion by the ancestor of the river following the creation of the Rockies). It flows directly past the communities of Brighton and Fort Lupton, and is joined in succession by Saint Vrain Creek, the Little Thompson River, the Big Thompson River, and the Cache la Poudre River, which it receives just east of Greeley.
East of Greeley it turns eastward, flowing across the Colorado Eastern Plains, past the towns of Fort Morgan and Brush, where it turns northeastward, flowing past the town of Sterling and into Nebraska near the town of Julesburg. In Nebraska, it passes south of the town of Ogallala and joins the North Platte near the town of North Platte, Nebraska.
The South Platte River through Denver is on the U.S. EPA's list of impaired waterbodies for pathogen impairment, with E. coli as the representative pathogen species.