A watershed is an area of land where all the water that is under it or flows over it collects in a single larger body of water, such as a larger river, a lake or an ocean. Larger watersheds may contain many smaller watersheds. For example, all of the land that drains to Rock Creek is in the Rock Creek watershed while all of the land that drains to the Potomac River, including Rock Creek, is in the Potomac River watershed.

Water in the Potomac River, Anacostia River, and Rock Creek flows into the District from outside jurisdictions. For example, the Potomac River begins in West Virginia, while the Anacostia River begins in Maryland. The quality of water in the District is thus affected by activities throughout the watershed. Stormwater runoff from commercial, industrial, residential and agricultural sites, point source pollutants from wastewater treatment plants and industrial discharges, and combined sewer overflows from as far away as West Virginia and Pennsylvania all contribute to the quality of water in the District.

All of the water in a watershed is connected, so it is vital that we all do our part to protect it. What you do upstream can have serious effects not only on the water in your immediate area, but on the water that flows downstream and the people that depend on it.