South Platte River Basin

The South Platte River originates in the mountains of the northern Front Range, then flows northeast through Denver before crossing the High Plains into Nebraska near Julesburg. In Nebraska, the South Platte merges with the North Platte River to form the Platte River, where it eventually flows to the Missouri, then the Mississippi River on its way to the Gulf of Mexico. About one-third of the basin’s land area is publicly owned and is primarily forested lands in the western mountainous part of the basin. The eastern, High Plains region of the basin is mainly grassland or cultivated land.

According to the 2015 Colorado Water Plan, more than 85% of Colorado’s population resides in the South Platte Basin, and the Front Range is a center for sectors like tech, aerospace and energy, along with widely publicized industries like craft beer. The South Platte Basin also has the greatest concentration of irrigated agricultural lands in Colorado and the highest agricultural production of any of the state’s river basins. The basin also supports water-dependent ecological and recreational attributes including skiing, boating, fishing, and wildlife viewing and hunting.

The basin’s water supply is highly dependent on both transbasin diversions and return flows. Along with a native flow of about 1.4 million acre-feet per year from precipitation—most of it beginning as snowpack in the northern Colorado Rockies—the river benefits from another 400,000 acre-feet of transbasin diversions from the Colorado River and roughly 100,000 acre-feet from the Arkansas, North Platte, and Laramie river basins. 

Taken together, these sources result in an annual flow of just less than 2 million acre-feet, supplemented by about 30,000 acre-feet of well pumping from nontributary groundwater aquifers. And yet, annual surface water diversions in the basin are roughly 4 million acre-feet, suggesting that much of the basin’s water is used more than once. Many water users depend on return flows from cities and farms upstream. They also rely on storage reservoirs throughout the basin. The largest reservoirs are Horsetooth and Carter Lake in Larimer County. 

Stakeholders in the South Platte Basin face a number of challenges in the coming years and are working together on planning and water projects through the South Platte Basin Roundtable. One of the most significant challenges will be securing a reliable water supply for a rapidly growing population while protecting agriculture, the environment and recreation. According to the 2019 Analysis and Technical Update to the Colorado Water Plan, the basin’s population is projected to grow from about 3.8 million people in 2015 to between 5.4 and 6.5 million people by 2050.

Water use along the heavily used South Platte is governed by the South Platte River Compact of 1923. It divides the waters of the South Platte River between Colorado and Nebraska, giving Colorado the right to fully use the river’s flows between Oct. 15 and April 1. During the irrigation season, Colorado must deliver 120 cubic feet per second to Nebraska at Julesburg. If the flow is less than 120 cubic feet per second, Colorado must curtail certain diversions.

The Platte River Recovery Implementation Program is a stakeholder-driven program that provides Endangered Species Act compliance in the Platte for Colorado, Wyoming and Nebraska, through work with the U.S. Fish and Wildlife Service. Without the regulatory certainty provided by this program, water users in the South Platte Basin would be subject to arduous requirements and possible reductions in water use to ensure continued protection of endangered species.