Nonpoint source pollution is water quality degradation from sources which are spatially diffuse. Nonpoint source pollution is produced from landscapes which are relatively undisturbed. Nonpoint source pollution can also be accelerated or caused entirely by human activities. The nonpoint sources of pollution (natural- and human-related) which cause slight-to-high impairment of Wyoming rivers and lakes are listed below.

The nonpoint source pollution of Wyoming rivers and lakes from natural background sources and from human activities is illustrated (Plate 31A and Plate 31B, respectively) by type of pollutant (color designations) and by length of rivers where existing water uses are impaired by failure to meet criteria established by the Wyoming Department of Environmental Quality. Available data on water quality impairment by nonpoint source pollution show stable or declining trends in Wyoming.

Sediment (yellow) is contributed from natural background sources and from irrigated crop production, riparian zone grazing, and range lands. Nonpoint sources of sediment constitute the most extensive water quality impairment problems in Wyoming. Highly erodable soils are found throughout the lowland basin areas of the state (see Chapter 4). Erosion on hillslopes is a particular problem in the Bighorn drainage.

Nutrients (purple) are a significant nonpoint source pollutant in the Green and Bighorn drainages affecting Flaming Gorge and Boysen reservoirs, respectively. Much of the phosphorus entering Flaming Gorge appears to be related to the erosion of phosphate rich soils and is contributing to a eutrophication problem in the upper reservoir.

Salinity (white) is added to rivers and lakes from natural sources, irrigated crop production, and oil field discharges. The hot springs at Thermopolis contribute 20 percent of the Bighorn River salinity.

Flow alterations (light blue) and habitat alterations (green) have affected streams throughout the state, especially below reservoirs and where streams parallel roads and highways.

Cumulative impacts on water quality from point source and nonpoint source pollution affect the larger rivers in Wyoming, especially the Bighorn, Powder, North Platte, and Green (Plates 30, 31A-31B). However, of the 19,437 miles of rivers and streams capable of supporting fish populations in Wyoming, only seven miles of Haggerty Creek (in the Little Snake subdrainage of the Green River) are totally unfishable as a result of pollution. All reservoirs are fishable, and all rivers and lakes in the state are swimmable as mandated by the Federal Water Quality Act of 1987.

Plates 31A -31B were digitized from Hogan (1988).