Stormwater discharge, also known as stormwater runoff, has gradually become a much bigger problem in the United States than it has been in the past.

This is primarily due to the fact that as more and more land is developed, particularly in urban areas, the hydrology (or way that water flows) changes. With this change comes a rise in the number of areas where water from rain or melting snow cannot infiltrate into the ground. This leads to increases in the volume of stormwater runoff which carries with it pollutants such as fertilizers, pesticides, sediments containing pollutants from other sources, oils, and septic tank overflows, among others. In addition, highways, roads, and parking lots contribute stormwater discharges that contain combustion products from gasoline and heavy metals.

Because stormwater runoff is not treated to remove contaminants, these contaminants go directly into lakes, streams, rivers, and other waterbodies, often affecting their water quality as well as the plants and organisms that live in them. A study that was commissioned by the United States National Research Council in 2008 identified urban stormwater runoff as a leading contributor to water quality problems in the United States.