The U.S. Environmental Protection Agency (EPA) defines watershed as the area of land where all of the water that is under it or drains off of it goes into the same place. The World Bank includes the receiving water body in its definition: A watershed is the specific land area that drains water into a river system or other body of water. Depending on the scale of the receiving water body, watersheds can include thousands of square miles of drainage area (e.g., Mississippi River watershed) or only a few square miles (e.g. Ralston Creek watershed, Johnson County, Iowa). Regardless of the size of any given watershed, receiving water bodies may be highly vulnerable to contamination from a variety of natural as well as anthropogenic sources, and water quality in receiving waters can vary seasonally as well as spatially.  Wildlife populations and domestic animals can negatively impact water quality depending on the number of animals and their proximity to water sources. Human activities and land uses within a watershed may impact water quality in streams and lakes and can affect wildlife habitats as well as humans who use those water sources for recreation (fishing, swimming) and as drinking water sources (for municipal water supplies). In the Midwest, intense agricultural activities including row cropping and livestock production may have impacts on water quality within watersheds over broad geographic areas, depending on seasonal precipitation patterns and conservation practices. Larger metropolitan areas as well as smaller urban areas can contribute significant contaminant loads to surface waters within watersheds.

Ambient surface water quality in the United States became a national concern during the late 1960s and early 1970s, resulting in grassroots level efforts to identify problems and legislative action to establish watershed protection programs. More than 30 years after the passing of the Federal Water Pollution Control Act (1972), efforts to limit point sources of contaminants to surface water sources have been considered very successful, although some problems remain. Point sources of pollution are defined as “any discernible, confined and discrete conveyance, including but not linked to any pipe, ditch, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft, from which pollutants are or may be discharged. This term does not include agricultural storm water discharges and return flows from irrigated agriculture.” Nonpoint source contaminants include any sources of water pollution that do not fit within the point source definition.  

Nonpoint contaminants have been more difficult to identify, quantify and prevent than point sources and federal funding is not adequate to tackle the vast number of impacted watersheds across the nation.