A watershed is the land that drains into a body of water. That is why it is also called a drainage basin. Runoff from rain and melting snow or ice flows over and through the land to collect in ponds, lakes streams and rivers.
A small stream or pond will have its own watershed which may be part of a larger watershed. These smaller streams and rivers are called tributaries. For example, the Chesapeake Bay watershed is actually five different watersheds that join together. Each of these rivers has smaller rivers and streams or tributaries. The same is true for the Mississippi River, the Great Lakes and other large watersheds.
Any land activity within a watershed can affect the water quality of its rivers and streams. Most precipitation eventually either seeps into the groundwater or drains into surface waters before it evaporates or is transpired back into the atmosphere. The runoff into surface water can carry polluting materials. These materials could be from farmland, woodland, urban areas, marinas, waste dump sites and landfills, construction sites or other land-disturbed areas, or where oil on and other chemicals have been spilled or used on streets and roadways. To find a nonpoint source of pollution, we often must examine the entire watershed. Therefore, the understanding of a watershed is very important.