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, about 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 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 flood-control basin 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
Fort Morgan and
Brush, where it turns northeastward. It continues past
Sterling, and runs into Nebraska between
Julesburg, Colorado, and
Big Springs, Nebraska. In Nebraska, it passes south of
Ogallala and joins the
North Platte River near the city of
North Platte. The South Platte River through Denver is on the U.S.
Environmental Protection Agency's (US EPA) 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 mussel. The flows of the South Platte have been greatly modified by human activity. Originally the river was seasonal north of Fort Morgan to its confluence with the North Platte. In the 1800s the river would disappear in July, August, and September due to low flows and a sandy river bottom. By 1910 the increase in agriculture caused the river to reach the border of Colorado and Nebraska due to
return flows and dams holding back water later in the year. Increasing diversions from the upper
Colorado River in the 20th century mean that flows reach the North Platte year round. ==History==