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.
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.
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.
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.
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!
