What is a Watershed? Am I In One?

Earth.com staff writer

Everyone lives in a watershed and everyone’s drinking water comes from a watershed, so it is important to know a little something about this exciting concept.

What is a Watershed?

A watershed is an area of land where all water that falls on that land drains to a single point. A watershed is also a drainage basin, where all water flows downhill towards a river, lake, or other body of water. The land that drains to a watershed is called the catchment area. High ridges divide watersheds since water that falls on one side of the ridge drains to a different place than water that falls on the other side of the ridge.

For example, all the land that drains water towards the Mississippi River is in the Mississippi River watershed. This watershed is the largest in North America. Can you believe that 31 states have land that drains towards the Mississippi!

While the Mississippi is the largest watershed, it is made up of many other sub-watersheds. The Arkansas River watershed is a smaller part of the larger Mississippi watershed. The Arkansas drains parts of Colorado, Kansas, New Mexico, Nebraska, Texas, Oklahoma, and Arkansas. Other major rivers, like the Ohio, Missouri, and Platte, all have their own watersheds within the larger watershed of the Mississippi. Diving even smaller, each creek and stream that flows into the Arkansas or Ohio rivers is its own smaller watershed.

Various illustrations that visually demonstrate drainage basins. Amadeu Blasco/Shutterstock.com

What’s Your Watershed?

Can you figure out the watershed you live in? Start by locating the largest watershed and work all the way down to the smallest one. For example, if you lived in downtown Denver, Colorado, you would be in the massive Mississippi watershed, the big Platte River watershed, the moderate South Platte River watershed, and the small Cherry Creek watershed.

Bonus: can you figure out what watershed your tap water comes from?

A map of the major watersheds in the continental U.S. Rainer Lesniewski/Shutterstock.com

Western States Could’ve Been Defined by Watersheds

John Wesley Powell, the first person to lead a boat trip down the entire Grand Canyon, was a scholar of the American West in the late 1800s. He wanted western states’ boundaries to be the boundaries between watersheds. He thought that this arrangement would result in better water management in a generally arid landscape. Unfortunately for the watersheds, Powell’s idea was not adopted. The northern border between Idaho and Montana follows a watershed boundary, but that’s it! While the borders between California and Arizona are the Colorado River, that is not a boundary between two watersheds. Rather, that border just splits one watershed up.

western watershed map — John Wesley Powell’s map of western watersheds. Each color blob shows what Powell would’ve liked to be its own state. Photo via Wikimedia.

Dividing arid watersheds between populous states has created immense difficulties. The Colorado River now faces huge water problems between the seven states that compose its watershed. In short, each state has overclaimed how much water it can draw from the river. When added up, the total amount of water allowed to be drawn between the seven states is about twice the annual discharge of the entire river. Utah, Colorado, Arizona, California, New Mexico, Nevada, and Wyoming now must make tough choices about how to divide up a resource that is much smaller than any of them planned for. Had politicians followed Powell’s guidance, perhaps the management of water resources like the Colorado would make more sense.

The Little Colorado River, true to its name, has very little water for how much land it drains. LHBLLC/Shutterstock.com

What Affects How Much Water a Watershed Produces?

Many factors influence the volume of water that exits a watershed. Some very small watersheds produce copious amounts of water. The Elwah river on the Olympic Peninsula in Washington state discharges an average of 1500 cubic feet of water per second. A cubic foot is about the same size as a basketball. The Elwah’s watershed is just 320 square miles, or about the area of New York City. In contrast, the Little Colorado River watershed, which drains northeastern Arizona, has a flow of just 432 cubic feet per second even though it drains over 25,000 square miles of land! That means the Elwah watershed produces almost 300 times more water per acre than the Little Colorado. Why is the Elwah so much more productive than the Little Colorado?

Precipitation and Watershed Area are Major Factors

One obvious factor that impacts this difference is a drastic difference in precipitation. The Little Colorado drains arid, desert lands, while the Elwah drains lush rainforests. Land area is another basic factor in watershed productivity. A watershed with a larger area and the same annual precipitation will discharge more water than one with a smaller area.

However, precipitation and the area of the watershed are not the only factors that impact the productivity of a watershed.

A diagram that illustrates how water infiltrates into a groundwater aquifer. stihii/Shutterstock.com

Infiltration, Evaporation, and Transpiration

Infiltration is the ability of water to sink into soil and aquifers rather than running off into rivers. Soil, vegetation, and rock composition all affect the infiltration capacity of the land. More vegetation, for example, will slow water down as it moves across the landscape. This slower-moving water is more able to seep into the ground. If the geology allows, this water can eventually seep into underground aquifers instead of running out to the sea. The easier it is for water to infiltrate the soil, the less water makes it to a river.

Evaporation is when water changes from a liquid to a gas. If it is hot outside, water is more likely to evaporate into the air. When rain evaporates on the land, it doesn’t run into rivers.

Transpiration happens when plants use water as they photosynthesize. When plants use water, they subtract the water they use from the discharge of the river of the watershed they are in (with some cool exceptions, like how the Amazon creates its own rain).

Water Use by Humans

Human uses of water change how much water flows through a watershed. In water-rich areas, the water use by humans is minimal compared to the water nature provides. However, in drier regions, rivers are usually used heavily. The Colorado river is utilized so much that it fails to reach the ocean. Humans take every drop out of the Colorado until there is no river left. Most of the human water use is for animal agriculture, crop agriculture, and industrial applications. A small amount of human water use is done by individuals. In California, a very populous state, urban areas use only 20% of human water use in the state.

Another major human alteration to watersheds is dams. Nearly every river has a dam on it, and most rivers have multiple dams on them. There are almost 100,000 dams in the U.S. alone! Dams regulate the flow of rivers, typically to be more predictable than the flow would be otherwise. The reservoirs created by dams change the character of a watershed immensely.

Humans have fundamentally altered some watersheds, such as the Owens River in California. Historically, the Owens river drained into an inland lake on the eastern side of the Sierra Nevada. In the early 1900s, Los Angeles built an aqueduct to transport the Owens river to satisfy Los Angeles’ growing water needs.

Urban Areas Change Wateresheds

Urban areas drastically change watersheds, also. Humans replace soil with square miles of hard surfaces, like concrete and asphalt. When it rains on these surfaces, the water that would’ve soaked into the ground instead runs across parking lots, streets, sidewalks, and basketball courts towards storm drains, sewers, or river channels. These storm drains, along with stormwater runoff, carry urban pollution to creeks, rivers, and the ocean. Cities are trying to reduce the problems associated with impervious surfaces, such as concrete, by creating rain swales, making more parks, and educating the public about storm drains.

Watershed and Pollution

Since watersheds concentrate water to a single point, watersheds also concentrate any pollution that water carries to that single point. Often, fertilizers, pesticides, and sediment from agricultural fields runoff into rivers. This runoff causes all sorts of environmental issues. Government agencies monitor water quality and use that quality as an indicator of watershed health.