Water Quality Assessment of the South Platte River Basin, Colorado , Nebraska, and Wyoming

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Physical Setting The South Platte River Basin has a drainage area of about 24,300 mil (Dennehy, 1991) and is located in parts of three States (fig. 1): Colorado (79 percent of the basin), Nebraska (15 percent of the basin), and Wyoming (6 percent of the basin). The South Platte River originates in the mountains of central Colorado at the Continental Divide and flows about 450 mi north- east across the Great Plains to its confluence with the North Platte River at North Platte, Nebraska. Altitude in the basin ranges from 14,286 ft at Mt. Lincoln on the Continental Divide to 2,750 ft at the confluence of the South Platte and North Platte Rivers. The basin includes two physiographic provinces, the Front Range Section of the Southern Rocky Mountain Province and the Colorado Piedmont Section of the Great Plains Province. The basin has a continental -type climate modi- fied by topography, in which there are large tempera- ture ranges and irregular seasonal and annual precipitation. Mean temperatures increase from west to east and on the plains from north to south (Gaggiam and others, 1987). Areas along the Continental Divide average 30 in. or more of precipitation annually, which includes snowfall in excess of 300 in. In contrast, the annual precipitation on the plains east of Denver, Colorado, and in the South Park area in the southwest part of the basin, ranges from 7 to 15 in (fig. 2). Most of the precipitation on the plains occurs as rain, which falls between April and September, while most of the precipitation in the mountains occurs as snow, which falls during the winter. The three -State area of the South Platte River Basin has about 2.4 million people, over 95 percent of whom live in Colorado. The basin contains the most concentrated population density in the Rocky Moun- tain region (fig. 3) located along the Front Range urban corridor in Colorado where the mountains meet the plains. Population densities outside the urban corridor are small, centered in small towns located along the principal streams. The principal economy in the moun- tainous headwaters is based on tourism and recreation; the economy in the urbanized south - central region mostly is related to manufacturing, service and trade industries, and government services; and the economy of the basin downstream from Denver is based on agri- culture and livestock production. Land use and land cover in the South Platte River Basin (fig. 4) during the period 1975 -80 (Feagas, and others, 1983) is divided into: 41 percent rangeland, 37 percent agricultural land, 16 percent forest land, 3 percent urban or built -up land, and 3 percent other land. Rangeland is present across all areas of the basin except over the high mountain forests. Agricultural land is somewhat more restricted to the plains and the South Park area near Fairplay, Colorado. Forest land occurs in a north -south band in the mountains. Urban or built -up land is present primarily in the Front Range urban corridor. The `other land' category includes: water (110 mi2), barren lands (160 mi2), tundra (400 mi2), and perennial snow and ice (1 mi2). Barren lands primarily are areas under construction or are areas of strip mining, quarries, or gravel pits. Hydrologic Setting The South Platte River and its major tributaries (Clear Creek, St. Vrain Creek, Big Thompson River, and Cache la Poudre River) originate in the Rocky Mountains and maintain perennial flow generated pri- marily by snowmelt runoff. Prairie streams, on the other hand, are ephemeral and intermittent and contrib- ute little to South Platte River flows except during rain- fall events. Mean annual - runoff patterns (fig. 5) mirror precipitation patterns, with runoff greater than 20 in. in the mountains and runoff less than 2 in. in the plains. Annual mean streamflow and mean monthly stream- flow for a typical mountain stream (Big Thompson River at Estes Park, Colorado, station number 06733000) and a typical plains stream (Bijou Creek at Wiggins, Colorado, station number 06759000) are shown in figure 6. Although the drainage area of the Bijou Creek station is 1,310 mil, its mean annual streamflow was only 9.2 cubic feet per second (ft3 /s), compared to the Big Thompson station with a drainage area of 137 mil and a mean annual streamflow of 127 ft3 /s. Annual flows on Bijou Creek are small for most years, but infrequently can be extremely large; annual variability on the Big Thompson River is less. Monthly flow on Bijou Creek occurs only during the summer rainstorm season, while on the Big Thompson there is an annual baseflow in addition to the spring snowmelt runoff. Many of the streams in the South Platte River Basin originate in the mountains, and then flow through the plains, and have streamflow character- istics that are a mixture of the two types. Among the water - quality sites examined in this report there are 20 sites that have at least 10 years of streamflow data; streamflow statistics for these sites are listed in table 27 in the "Supplemental Data" section at the back of this report. There are three primary aquifers in the South Platte River Basin (fig. 7): the unconsolidated alluvial aquifer along the South Platte River and its tributaries, which includes about 4,000 mil of gravel, sand, silt, and clay (hereinafter referred to in this report as the DESCRIPTION OF THE SOUTH PLATTE RIVER BASIN