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. 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. Irrigated agriculture comprises only 8 percent of the basin but accounts for 71 percent of the water use. Water use in the South Platte River Basin for 1990.