The Function of Watersheds

You’ve likely been stuck indoors during a downpour. After a few hours, the land, streets, and buildings outside seem completely dry. Ever wondered where all that rainwater goes? We know that much of the water seeps into the soil and plants, but where does the remaining water end up? It eventually flows into nearby lakes and rivers, but it follows the path of watersheds to get there.

­

So, where exactly are these watersheds? Here's a clue: You’re sitting in one right now. The Environmental Protection Agency (EPA) describes a watershed as any piece of land that directs water downhill into a watercourse. Simply put, "watershed" is a general term used to explain how water travels across the land to feed rivers, streams, and lakes [source: Environmental Protection Agency]. These watersheds interconnect like pieces of a puzzle, shaping our landmasses.

All landmasses drain into some body of water, whether it's the mighty Mississippi River or a small pond in your backyard. Since water can't travel uphill, the shape of the land determines the boundaries of watersheds. This means that if you live on one side of a hill and your neighbor on the other, you are in different watersheds. The size of a watershed can vary dramatically, depending on the highest points that define it.

A watershed can stretch across thousands of square miles or just a few acres leading to a pond [source: Environmental Protection Agency]. There are millions of watersheds around the world—2,100 small ones in the United States alone [source: NatureServe]. But a watershed isn't just land that gathers rainwater and directs it to rivers. Any substance that ends up in a watershed eventually reaches a body of water, including harmful pollutants like motor oil or paint, or debris from tree removal during construction. These pollutants can contaminate water supplies, erode the land, and harm aquatic ecosystems.

Keeping our watersheds healthy is crucial for maintaining ecological balance. But why does this matter to you? Why should you care about watersheds? In this article, we’ll dive into what watersheds are and why they’re important to you.

What Exactly Is a Watershed?

A watershed is a land area that channels all the water flowing beneath it and draining off it into a body of water. It connects with other watersheds to form an intricate system of rivers and streams, eventually leading into larger bodies of water.

Topography dictates the path of water flow. The ridges around a body of water define the boundaries of a watershed. Picture flipping an umbrella upside down in the rain. Rain that falls on the umbrella’s surface would collect at the center, while any rain outside the umbrella would fall to the ground. The umbrella represents a watershed, gathering all that falls within it.

Waterways within a watershed ultimately channel into a main body of water, such as a river, lake, or stream. The origin of a water source is referred to as headwaters, while the point where headwaters meet other water sources is called the confluence. The final point where water enters the main body is known as the mouth (source: Environmental Protection Agency).

­

Returning to the umbrella analogy, imagine three layers of umbrellas. A group of large umbrellas (the basin) sits on the ground, with smaller umbrellas (watersheds) above them, each with a hole at the bottom. Even smaller umbrellas (catchments) float above these, also with holes. If rain falls on the top level, it drains into the larger umbrellas below, and so on, down to the largest umbrellas.

Of course, this is a simplified model. Water doesn’t just fall onto the land and roll off into streams. Rainwater, along with other substances, is lost through plant absorption, evaporation, and human consumption. These processes vary depending on the area; for example, Georgia’s clay-like soil absorbs less water compared to the loose soil of Kansas.

So, why are watersheds so important? Why must we ensure their health? Keep reading to learn more.

Why Watersheds Are Crucial

Now that you understand what watersheds are, why should their health matter to you? Watersheds have a direct impact on water quality, whether it's for drinking, recreation, or other uses. For instance, algae blooms caused by fertilizer runoff into water bodies harm watershed health, as do pollutants like mercury and lead that seep into the water supply. As states and cities search for uncontaminated drinking water sources, maintaining healthy watersheds becomes increasingly crucial for ensuring clean water [source: Environmental Protection Agency].

Unhealthy watersheds affect wildlife, and the polluted water that results can pose risks to humans. Aquatic life is particularly vulnerable to the impacts of watershed pollution, while new pollutants entering ecosystems disrupt wildlife habitats. This reduces biodiversity by eliminating some species and introducing invasive ones that threaten native species. This shift in the ecosystem can also disrupt the food chain, affecting everything from microorganisms that feed birds and animals to fish that are a source of food for humans.

As noted in the EPA report 'Sustaining Healthy Freshwater Ecosystems,' one freshwater ecosystem can greatly influence another: "Far from being isolated bodies or conduits, freshwater ecosystems are tightly linked to the watersheds or catchments of which each is a part, and they are greatly influenced by human uses or modifications of land as well as water" [source: Environmental Protection Agency].

Erosion is another significant threat. As water flows toward a stream, it picks up dirt along the way. If enough soil accumulates over time, the land along that stream becomes unstable and erodes. For those living along riverbanks, this could mean the loss of their property. For wildlife in these areas, it means the destruction of their habitats.

Rapid global development is likely contributing to many of the issues affecting watersheds today. Development in the Amazon Basin, for example, has put the Amazon river dolphin at risk of extinction [source: Water Resources eAtlas]. Urban development often involves removing vegetation, artificially altering the landscape, and modifying natural drainage systems. These changes all impact a watershed. Additionally, manmade surfaces like asphalt roads or buildings act as what the U.S. Geological Survey calls "fast lanes" for rainfall. Instead of rainwater being absorbed by soil and plants, it flows directly into streams, increasing the risk of flooding due to the excess water [source: U.S. Geological Survey].

Continue reading to discover how we can safeguard the health of our watersheds.

Preserving Watersheds

Now that you're aware of how vital watersheds are, how can you help protect them? Numerous laws exist to safeguard watersheds. The first, the Watershed Protection and Flood Prevention Act of 1954, helped coordinate federal and state efforts for flood prevention. In 1972, the Act was amended to include conservation measures. Then, in 1996, revisions were made concerning loans for groups undertaking watershed preservation and cleanup initiatives [source: U.S. Fish & Wildlife Service]. The World Bank, the United Nations, and other global organizations have been implementing watershed protection programs around the world for years [source: United Nations].

In 1996, the Environmental Protection Agency initiated a program to assist watersheds. The watershed approach is an environmental management strategy aimed at addressing the declining health of watersheds. This approach involves a collaboration between public and private sectors to tackle significant contamination challenges. Groups within specific watersheds are encouraged to work together to balance pollution prevention and environmental improvement with local economic development. These teams, known as watershed teams, monitor their watersheds and engage in restoration and cleanup projects [source: Environmental Protection Agency].

You don’t have to wait for government intervention to make a difference. You can take action on your own to help protect watersheds. By reducing the pollutants that seep into the soil, you can help keep your watershed and water supply cleaner. For instance, recycle your used antifreeze and motor oil rather than disposing of them improperly. Also, trash and pet waste that ends up in storm drains can pollute waterways, so make sure to carry a bag while walking your dog.

Another way to help protect watersheds is by adopting green building practices. The EPA offers several suggestions:

 For more details on how to protect watersheds and our water supply, please refer to the resources linked on the next page.