The flow of water that results from precipitation and which occurs immediately following rainfall or as a result of snowmelt. When a rainfall event occurs, several things can happen to the precipitation. Some of the precipitation infiltrates into the soil surface, some is taken up by plants, and some is evaporated into the atmosphere. Stormwater is the rest of the precipitation that runs off land surfaces and impervious areas. Stormwater discharges are generated by precipitation and runoff from land, pavements, building rooftops and other surfaces. These hardened surfaces are called ‘impervious surfaces’ and they do not allow rainfall to infiltrate into the soil surface like natural vegetation, so more of the rainfall becomes stormwater runoff. Storm water runoff accumulates pollutants such as oil and grease, chemicals, nutrients, metals, and bacteria as it travels across land. Heavy precipitation or snowmelt can also cause sewer overflows that may contaminate water sources with untreated human and industrial waste, toxic materials, and other debris.