Lesson 1: Watershed Basics

Lesson 1: What is a Watershed?

No matter where you live, your home is situated in a watershed: a land area that drains to a central location, such as a lake, river, or ocean. You can think of it as a shallow depression or bowl in the landscape, where the “rim” is a ridge or hill: even if your home is situated on the rim of the bowl, water washing off of your neighborhood is draining to the same place as areas on the opposite side of the bowl—everything is connected. 

The water that you see dripping down your window, pooling in the street, or flowing through a creek leads elsewhere, whether that’s a wastewater treatment facility, a wetland, a larger tributary, or the ocean. Because of this interconnectivity, what may seem like a small action in one area of the watershed can have a big impact on natural systems elsewhere in the watershed, including the plants, animals, and people that depend on them. 

Watersheds come in all shapes and sizes, and they all have important roles in the landscape. The fourth-largest watershed in the world, the Mississippi River watershed, reaches from the Allegheny Mountains in the eastern United States all the way to the Rocky Mountains in the West, encompassing regions from 31 states and two Canadian provinces in its drainage area. The rivers, streams, lakes, creeks, and underground flow that make up this watershed are responsible for moving 60% of all grain exported from the US, providing habitat to 25% of the fish species found in North America, hosting 40% of the nation’s migratory waterfowl during their spring and fall migration, and supplying water to more than 50 cities across the country. That’s a tall order for one watershed!

Check out the infographic for an illustrated overview of these natural resources.

NEEF Infographic: We All Live in a Watershed

Activity: Find Your Watershed

Go to EPA’s How’s My Waterway, and enter your zipcode in the provided field, right under the words "Let's get started!". When you hit "enter" a map will appear on the left with an outline of the watershed in which you live indicated with a dotted line. On the upper right-hand side, above the blue strip, there will be the zip code you entered, accompanied by the name of the watershed just below that. 

The parts of a watershed

As described in the infographic above, the moisture of a watershed is composed of two parts (not counting atmospheric water content)—the part we can see, surface water, and the part we can’t, groundwater. Both of these parts of the watershed play vital roles in the area’s ecology, economy, transportation, industry, and general health.  

Surface water is just that—water that collects on the surface of the earth. This category is composed of rivers, lakes, streams, oceans, wetlands, estuaries…the list goes on! Overall, about 269,995 square miles of the US and its territories are covered by water—that comes to about 7% of the area of the country, but the individual compositions of each state and territory can vary widely.

Canoe trip at Pea Island and Alligator River National Wildlife Refuge

Close to 95% of the fresh surface water found in the United States is contained by the Great Lakes, which together constitute the largest surface freshwater system on earth, supplying drinking water to 40 million people across the US and Canada and providing habitat to 250 species of fish and thousands of other species of plants and animals in the surrounding basin. Beyond the Great Lakes, part of the remaining surface water of the US is found in the country's some 3.6 million miles of rivers and streams, and these waterways supply about 65% of the nation's drinking water, help pump more than $97 billion into the nation's economy through recreation and tourism activities, and whose floodplains help prevent billions of dollars in damages from downstream flooding. 

Throughout history, human communities have sprung up around available surface water resources—some of the world's earliest cities for which we have records were formed around estuaries, which are surface water bodies where freshwater sources like rivers or lakes mix with the saltwater of the sea (think about the Chesapeake Bay, the Pamlico Sound, the Mississippi Delta, or the San Francisco Bay). This trend carries through to today—many major US cities like San Francisco, New York City, and Washington, DC formed near surface water bodies just as the metropolises of the past, because of the valuable services and resources they provide.  

For an up-close-and-personal view of the surface water components of a watershed, take this virtual tour from the Ohio State University Extension’s Ohio Watershed Network: Watershed Tour

Groundwater is the other half of the moisture equation contained in a watershed. Groundwater is water that occupies pore space in the rock and soil layer beneath our feet, filling natural underground storage areas called aquifers. Groundwater slowly moves through these aquifers, going on to feed into surface water sources like lakes, ponds, rivers, and even the ocean. This natural discharge to the surface doesn’t deplete aquifers, as the groundwater is replenished with precipitation that soaks through the surface soil and into the saturated groundwater layer. In this manner, the water cycle helps to create and sustain natural reservoirs of water in the ground—one hundred times more water, in fact, than there is in all of the world’s rivers and lakes!

Over half of the US population relies on groundwater for residential uses, with the country drawing an estimated 79.3 billion gallons per day in 2010. Some states depend on groundwater especially heavily—Hawai’i, Florida, Idaho, Mississippi, Nebraska, and Iowa all used groundwater for more than 75% of their public supply withdrawals in 2010. (Public supply is a water source that is used to provide water for 25 or more people—think of a water tower.) Groundwater is also a major supplier of water for wetlands and rivers, as it is estimated that these underground water sources contribute about 40% of all streamflow in the United States. 

Activity: Dip a Toe in Surface Water

Is surface water more your speed? Click here to find your state on this list, and read about wetlands located in your state. Report back on your state, and the name of or type of wetland found in your state in the field below.

What is water quality?

So far we’ve learned about the main water sources found in a watershed, as well as why these natural resources are important to both humans and wildlife, but what factors can impact the quality of this valuable resource?

Many different organisms and communities depend on the water of a watershed, but in order for this water to be useful for the vital functions outlined above, it must have good water quality. Water quality is a term used to describe how well-suited water is for a particular task, such as drinking, irrigation, transportation, recreation, etc. This suitability is determined by certain physical, chemical, and biological characteristics of the water and the surrounding area, and includes measures such as temperature, dissolved mineral content, or the number of bacteria present. Water quality standards are enforced by the states and the US Environmental Protection Agency, and different standards apply depending on the intended use of the water—for instance, if water in a stream has a high temperature, it can still be drawn to be used for toilets and showers in a city, but that water may not be suitable for aquatic plant and animal populations that would prefer cooler temperatures in order to stay healthy and thrive in that river. For that reason, the river may not be described as having good water quality for fish and wildlife living in that water body.

Impacts on water quality

The quality of a water body is impacted by the surrounding watershed, including dry areas where you may not be able to immediately see a water body. Pollutants can be carried from land areas and into nearby waterways when it rains, which can be particularly troublesome in developed or more urbanized areas. When precipitation hits impervious surfaces such as pavement, roofs, or vehicles, it can’t soak into the ground, and instead must flow across the hard surface. In the process, this stormwater can pick up any trash, soil, debris, or chemical contaminants that it encounters. Heavy precipitation events can lead to a decrease in water quality by washing these pollutants into local waterways.  

When it rains, what goes down the storm drain?

In extreme cases, waterways can be threatened with more than just the trash found outside. In areas with combined sewers, heavy rainfall or snowmelt can overwhelm the single system used to collect stormwater, domestic sewage, and industrial wastewater, causing the sewer system to discharge the combined wastewater directly into nearby bodies of water.

One of the pollutants that can be ferried into waterways by stormwater may leave you scratching your head—nutrients! In small doses, nutrients, in the form of nitrogen and phosphorous, can be good for aquatic plants, helping them to grow, which in turn helps to feed fish and other wildlife populations up the food chain. However, too many nutrients in the water can lead to some plant and bacteria populations growing out of control, threatening water quality and aquatic animal populations. These nutrients can come from garden fertilizer, decomposing organic matter like yard waste, and even pet waste, all of which can contribute to poor water quality if they wash into a waterway. One such indicator of nutrient overload, or nutrient pollution, is the presence of harmful algal blooms.

When an algae population receives an influx of nutrients like nitrogen and phosphorus in warm, slow-moving water, it can multiply out of control. The layer of algae that forms on the surface of the water is called an algal bloom, and it can partially block sunlight from reaching plants and tiny organisms below, stunting their growth.  When the sun-starved plants and organisms die, they undergo decomposition, a natural process that consumes dissolved oxygen from the surrounding waters. When the large population of algae in the bloom at the water’s surface dies, decomposition occurs at such a wide scale that the water can become depleted of oxygen, falling far below normal levels. This lack of oxygen can threaten and even kill off populations of fish and other animals living in the water, creating a dead zone. Mobile marine animal populations move and seek out areas with higher levels of oxygen, which leads to a shift in habitats and can reduce fish abundance for fishery catches. These fish kills and associated habitat shifts are disastrous for the local ecosystem and can be detrimental for fishing industries that rely on the affected waters.

What is a Dead Zone?

You get it—pollution on the ground can be washed into local waterways, threatening water quality. However, did you know that there are variables that impact water quality beneath your feet? Surface water isn’t the only place where a watershed can be vulnerable. Groundwater can be threatened as well, particularly in the case of overdrawing, or removing water from underground aquifers faster than it can be replenished.

Groundwater is a vital resource for communities across the world, making up 30.1% of the planet’s fresh water and providing reliable sources of water for home, agriculture, and industry use.  While groundwater is invaluable for many communities, it is not inexhaustible. When it is drawn out of the earth faster than it can be recharged by precipitation seeping through the surface or by underground flow through the aquifer, wells can become prohibitively expensive and even run dry. 

In extreme cases, the volume of water pumped out of the ground may be great enough that the ground begins to compact, or subside. This occurs because in some places, the water in aquifers is helping to hold the ground up. When the water is removed, the rock and dirt falls down on itself, settling to fill in the places that used to be occupied by water, permanently reducing the aquifer’s storage capacity. This isn’t a sinkhole—land subsidence is typically a gradual change spread out over a wide area, making it less obvious. However, there are some areas where the change has been drastic—for instance, the ground in some areas of San Joaquin Valley, California, has fallen up to 30 feet (nine meters), and California, Texas, and Florida have experienced hundreds of millions of dollars of damage related to land subsidence. Across the United States, the total area affected by land subsidence is the size of New Hampshire and Vermont combined!

Again—an action on one side of the watershed, like littering, not picking up yard waste, or using too much water, can have a big impact on water resources across the watershed!

Natural filtration

While there can be threats to water quality hiding in the watershed, there are also many features of a watershed that serve to improve water quality—many of them natural features that operate all on their own! For instance, one major pollutant in waterways can be dirt—while you may think of dirt as part of the ecosystem, too much of it eroded from the nearby land and stirred up in the water column can be unhealthy for fish, and can even cloud the water so much that aquatic plants can’t get the sunlight they need to grow. Aside from the threats posed to water quality by the dirt, or sediment, itself, these tiny particles can also carry dangerous passengers, in the form of chemical pollutants that have bonded to the miniscule dirt particles and are being carried along for the ride.

This problem of turbid (cloudy) waterways is a significant issue in some areas of the country, with an estimated 1.5 billion tons of soil entering the nation’s waters each year! Luckily, there is a solution already in place, as long as we don’t uproot them: trees. Streamside trees and other plants naturally filter out soil and sediment as rainwater flows over roots, and through leaves and branches. Pollutants bonded to the soil are also removed from the water—80% of phosphorous and nitrogen (found in fertilizers) is removed by the natural forest filter before reaching a stream. This can prevent harmful algae growths in downstream waters. Sediment isn’t the only pollutant that trees can help filter from the water—one mature tree in a riparian area can filter as much as 200 pounds of nitrates runoff per year.

Trees aren’t the only part of the watershed working to improve water quality—check out the infographic below for a look at how wetlands work for us!

Wetlands Work for Us

While watersheds can face threats from pollution and overuse of water resources, there are also many ways in which these landscapes work to take care of themselves, and maintain their water quality—if their natural features are allowed to flourish and thrive!

Activity: Track Pollution - A Hazardous Whodunit

Now that you’re more familiar with the type of factors that can threaten water quality in a watershed, try your hand at pinpointing the sources of these contaminants! Many homes, businesses, and individuals aren’t even aware of the impact that their actions can have on the surrounding landscape—in this virtual whodunit, you can track the pollution to the source and help spread the word about the importance of water quality and healthy watersheds! Tracking pollution: A hazardous whodunit.

Looking for something a little closer to home? EPA’s MyEnvironment offers water quality reports for water features around your zip code. Go to the page, enter your zip code, and see what you find. Report back in the field below on your findings.

Please provide us with your email address to receive your Watersleuth badge.  (Please note: Your school's email security settings may block messages from outside senders. If you don't receive your badge within a few hours, please check with your school administrator).