Resource: What Is a Watershed?

Print Background Essay

As a concept, a watershed is simple—it is any area that funnels rainwater to a common destination. If the common destination is a ditch that runs through a neighborhood, then the watershed is all of the neighborhood’s surface area that drains into that ditch. If the destination is a lake, then the watershed is all of the surface area that drains into that lake.

But watersheds are complex, too. Smaller watersheds can interconnect to form larger ones, and large watersheds are part of giant watersheds, and so on, all nested together in the largest watershed. So, rainwater in the neighborhood drains into a ditch, but the ditch drains into a creek, and the creek drains into a river or lake, which ultimately drains into a bay or ocean. The neighborhood can be a watershed by itself, but may also be part of other watersheds.

Wherever we may be, it is important to consider our local, nearby watershed because what enters the water at any point in the watershed can have an impact as it travels downstream. Everything we do on or to the land has potential to impact everyone downstream from us.

For example, sediments loosened by farming, construction, off-road recreational activities, and logging, wash into waterways every time it rains. Motor vehicles spray oil, antifreeze, grease, and brake dust onto roads, and then rainwater runoff carries it to waterways. Fertilizer, pesticide, insecticide from yards and farms, and bacteria from uncollected pet and livestock waste enter waterways, too. Even litter gets there. Soaps, detergents, hair products, cosmetics, medicines, and anything else that washes off or out of our bodies make its way through sewage systems to waterways. Even though natural processes mitigate some of the effects, these pollutants degrade the quality of water in a watershed, negatively affecting the plants and animals downstream—including humans.

Individuals can decrease pollutant entry to waterways by employing sound agricultural and landscaping practices, collecting pet waste and litter, keeping motor vehicles in good repair, minimizing use of cleaning products and other pharmaceuticals, and keeping local runoff from reaching sewer and drainage systems and having them leach through soil where water is stored.

Watersheds are affected on a grand scale by flood control in populated areas, water diversion for agricultural purposes or other human use, dams for hydropower generation, the dredging of navigational channels, and other projects. Ideas for best management practices on any scale are evolving as understanding about watersheds and their interconnectivity increase.

Local communities can form partnerships at any watershed level to educate and assist stakeholders in protecting water resources. Many large watersheds such as the Chesapeake Bay and Galveston Bay watersheds already have organizations that coordinate efforts between stakeholders.

To learn more about the water cycle, check out Water Cycle Animation, The Hydrologic Cycle, and Water Vapor Circulation on Earth.

To learn about environmental impact on water supply, check out What’s in Your Stream?.

Print Questions for Discussion

• What is a watershed?
• Give two examples of watersheds in different areas. You can select the area where you live and one other.
• Look at a watershed in a specific area. One example is the Great Lakes. Where does the water leaving the lakes go? Where does the water feeding into the lakes come from? Is there a well-known watershed near your home?
• Approximately how much of the United States’ land is part of the Mississippi River watershed?
• Can you trace rain that falls on your school’s roof through all of its potential watersheds?