Water quality in the South Platte River Basin is a product of its environmental setting and hydrologic conditions. Environmental conditions that affect water quality include natural factors such as physiography, climate, geology, and soils and human factors such as water use, population, land use, and water-management practices. However, in the South Platte River Basin, the largest population center in the Rocky Mountain Region is in a predominantly semi-arid environment, so human factors have an especially large effect on water quality.
Mean-annual precipitation (1951-80) in the South Platte River Basin. Precipitation data are from the Colorado Climate Center (1984).
The South Platte River Basin is characterized by a continental-type climate having a wide temperature range (-30 to 100oF) and irregular seasonal and annual precipitation. Precipitation patterns across the basin depend on altitude (mountains to plains). The greatest amount of precipitation (more than 30 inches annually) falls in the mountains as snow. In contrast, annual precipitation on the plains is less than 15 inches. Most of the precipitation on the plains occurs from April to September as thunderstorms.
Land use provides many of the chemical and sediment inputs that affect water quality. Forest and rangeland are sources of nutrients, suspended sediment, dissolved solids, and metals. However, urban and agricultural land uses are larger sources of these same constituents, as well as other chemicals such as pesticides and volatile organic compounds. In the South Platte River Basin, stream-water quality generally is dependent more on adjacent land use than on upstream land use because upstream water often is removed from the river by diversions. In terms of area, rangeland is the largest land use (41 percent), but has a relatively small effect on water quality because of the lack of water and minimal water use. Irrigated agriculture comprises only 8 percent of the basin but accounts for 71 percent of the water use. Urban lands comprise only 3 percent of the basin but account for 12 percent of the water use (or 27 percent if power generation is considered an urban water use).
Major land uses, 1975-80, in the South Platte River Basin (modified from Fegeas and others, 1983).
Population growth areas in the South Platte River Basin, during 1979-90.
Population density in the South Platte River Basin ranges from sparsely populated mountain areas in the headwaters region and rural agricultural areas downstream from Denver to the densely populated Denver metropolitan area and Front Range urban corridor in the central part of the basin.
Water quality is affected by hydrologic conditions such as the quantity of water in streams and aquifers and the source of that water. In the South Platte River Basin, the primary sources of water are snowmelt from the mountains and rainfall on the plains.
Streamflow in a typical mountain stream, the Cache La Poudre River, during the study period showed year-to-year variability. Snowfall, and the resulting streamflow, was about average during 1993. Mountain streamflow during 1993 helped to fill reservoirs, which had been at low levels during the prolonged dry spell of 1985-92. Mountain snowfall and streamflow were below average during 1994. The plains climate also was drier than normal during 1994, and the combined effect of dry mountains and dry plains made 1994 the driest year of the study period. This was followed during 1995 by a wet year in the mountains and the plains, which produced near-record high streamflows in some areas and localized flooding. The large flows during 1995 resulted in increased loads for many chemical constituents compared to the previous 2 years. Results from water-quality samples collected during 1995 would likely be atypical.
The snowmelt peak at mountain sites was about twice as large during 1995 as during the previous 2 years.
The hydrograph for the Cache La Poudre River showed a consistent seasonal pattern of streamflow. Streamflow was high only during the annual snowmelt-runoff peak, which occurred during April through August. In contrast, the hydrograph at a plains site, such as the South Platte River at Cooper Bridge, near Balzac, Colo., showed more monthly variability. This variability was caused by two factors: precipitation and water use. The high-flow peak during May-August 1995 at the Balzac site was caused by unusually large rainfall amounts in the foothills and the plains that coincided with the annual snowmelt peak. The remaining monthly variability at the Balzac site primarily was due to the effects of irrigation water diversions along the river.
Streamflow at plains sites was about 10 times larger during 1995 than during the previous 2 years.
Ground-water levels in the mountains and plains during the study period showed seasonal patterns. Ground-water levels in the mountains fluctuated in response to snowmelt and the subsequent infiltration of water, which resulted in the highest water levels occurring between March and June. The crystalline-bedrock aquifer in the forested mountains primarily is used to supply small quantities of water to individual households, so water use usually does not drawdown water levels except during extreme dry periods. Ground-water levels in the alluvial aquifer in the plains fluctuated in response to the application of irrigation water to fields, which resulted in the highest water levels occurring between July and September. The South Platte alluvial aquifer in the plains primarily is used for irrigation but also is used as a drinking-water supply.
Ground-water-level fluctuations occur in response to different factors depending if wells are located in mountains (snowmelt) or in the plains (irrigation).