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. Its drainage basin includes much of the eastern flank of the Rocky Mountains in Colorado; much of the populated region known as the Colorado Front Range and Eastern Plains; and a portion of southeastern Wyoming in the vicinity of the city of Cheyenne. It joins the North Platte River in western Nebraska to form the Platte, which then flows across Nebraska to the Missouri. The river serves as the principal source of water for eastern Colorado. In its valley along the foothills in Colorado, it has permitted agriculture in an area of the Colorado Piedmont and Great Plains that is otherwise arid.
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 50 miles (80 km) of the Platte Canyon and its lower section, Waterton Canyon. Here, it is joined by the North Fork before emerging from the foothills southwest of the Denver suburb of Littleton. At Littleton, the river 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. Other water issues involve the appearance of the New Zealand Mud Snail , and zebra mussels.
The South Platte was originally called the Rio Chato (see the report of the Humano and Bonillo Expedition). In 1702, it was named the Rio Jesus Maria by Captain Jose Lopez Naranjo, the Tewa Irish scout and Captain of War of the K9's Indian Auxiliaries who was ordered by the Viceroy of New Spain to search the Tierra Incognita for a French incursion into New Mexico.
In an arid region of the United States, the South Platte is marked with several dams. The first notable water impoundment on the South Platte is Antero Reservoir. "Antero" is derived from the Spanish word "first," as it was the first dam on the South Platte River near the river's origin. The next dam is Spinney Mountain Reservoir. At capacity Spinney Mountain covers 2,500 surface acres. A bottom release dam, Spinney releases to the east of the inlet and feeds a two mile section of the South Platte known as the Dream Stream. The Dream Stream feeds Eleven Mile Reservoir only two miles below Spinney Mountain Reservoir. Eleven Mile Reservoir will carry 97,000 acres. The Eleven Mile Reservoir Dam gives way to Eleven Mile Canyon, which runs through Forest Service land. From Eleven Mile Canyon, the South Platte runs northeast to Cheesman Reservoir, named for Denver water pioneer Walter S. Cheesman. At its completion in 1905, the dam was the world’s tallest, at 221 feet above the streambed. The reservoir and related facilities were purchased in November 1918 by the Denver Water Board. Cheesman was the first reservoir of Denver's mountain storage facilities and has been designated a National Historic Civil Engineering Landmark. Cheesman Reservoir feeds Cheesman Canyon. Six miles below Cheesman Reservoir is the town of Deckers; there, the river bends north for approximately 17 miles to the confluence with the North Fork of the South Platte. In the late 1980s, plans called for a reservoir to be built that would have flooded the entire section from Deckers to the confluence with the North Fork. In 1990 the Environmental Protection Agency vetoed the permit, calling the project an "environmental catastrophe." From the confluence, the river flows towards Denver and enters Strontia Springs Reservoir. Below Strontia Springs the South Platte runs through Waterton Canyon before entering Chatfield Reservoir. Chatfield marks the seventh and final dam on the South Platte until it merges with the North Platte.