Watersheds 101

Often also referred to as a “drainage basin,” a watershed is a region of land where rainfall and snowmelt drains into a stream, lake, or river system.

Watersheds provide both a level of organization for landscapes and a scale for understanding their natural hydrological patterns. By extension, they also form an extremely useful unit for organizing human activities and managing lands as well as waters.

Watershed boundaries are generally defined by ridges of higher ground, but their scale need not be dramatic – small increments of rising and falling land in low-lying areas like the Everglades or the Okefenokee Swamp are just as effective in determining where their water flows as lofty crags in the Rockies or Cascades. Whatever the boundary, rain falling on one side flows toward the low point of one watershed, while rain falling on the other side of the boundary flows toward the low point of a different watershed.

The watershed concept operates at numerous scales. For instance, Freshwater Ecoregions of the World, a global collaboration led by the World Wildlife Fund and the Nature Conservancy, has identified 48 freshwater ecoregions for the continental U.S. These “super-sized” watersheds each consist of a large river watershed or multiple watersheds and support significantly different collections of aquatic life. Large individual watersheds are, in turn, usually composed of several smaller “sub-sheds.” Each of the many streams and rivers that feed the Mississippi River, to take one example, has its own watershed.

At a much finer scale, the Hydrological Unit system divides the nation into 2,264 small watersheds, each of which is assigned a HUC, or Hydrologic Unit Code. The US Geological Survey developed this standardized watershed classification system in the mid 1970s, and these units are often used effectively in conservation planning and action.

Watersheds and Conservation

Watersheds also provide logical organizing units for human activities. Federal agencies like the Environmental Protection Agency and the National Resource Conservation Service (NRCS) use them to define their local and regional authorities, membership, responsibilities, and funding. State fish and game agencies use watersheds to set priorities for fish management and aquatic species conservation, and state conservation districts are also organized by watersheds.

Many conservation organizations large and small likewise organize their efforts by watersheds. The Nature Conservancy created its Freshwater Conservation Initiative to help increase their effectiveness at identifying places harbor critically important freshwater biodiversity, abating threats at key sites, and sharing information and expertise. Many other local and regional efforts are underway to protect our nation’s watersheds, and the EPA can maintainsa list of local watershed organizations and partners along with other resources and additional information about watershed conservation.

Why do Watersheds Matter?

Clean water is essential to life – humans, plants, and animals all depend on adequate supplies of it to survive.

Viewing landscapes as watersheds helps us diagnose the health of local and regional freshwater ecological systems like wetlands, river and stream corridors, and aquifers –systems that provide essential ecosystem services like drinking water, flood control, transportation, and the food harvests of our fisheries. People also enjoy the beauty and recreational benefits they supply for activities like swimming, boating, and sport fishing.

The rivers, streams, and lakes of our watersheds also important crucibles of evolution that feed and shelter numerous rare and endangered freshwater plants and animals. Even watersheds and streams without imperiled species deserve the best possible care and stewardship that we can give them, because human health and quality of life are tied to their well-being. Aquatic insects, mollusks, and crustaceans are very effective indicators of environmental quality. Because they all require specific conditions to thrive – the correct amount of water, water clarity, temperature, oxygen levels, and substrates – these species can help identify emerging or persistent threats to human and animal health.

U.S. Watersheds and Earth’s Biodiversity

The United States has an extraordinary diversity of freshwater fishes – about 10% of the world’s known freshwater fish species. We have approximately 800 species, far more than any other temperate zone country. Roughly 66% of U.S. fish species do not occur anywhere else in the world.

The U.S. also harbors almost 40% of the world’s salamanders and is home to the earth’s greatest diversity of freshwater mussels and snails. Watersheds of the U.S. contain almost one-third of all known freshwater mussel species and 17% of described snails. We also support over 60% of the world’s crayfishes, 96% of which are endemic to the U.S., occurring nowhere else in the world.

In terms of their ratio of at-risk species, freshwater-dependent animals – mussels, crayfishes, stoneflies, amphibians, and fish – represent the country’s most vulnerable collection of species. 69% of freshwater mussels, the most imperiled group, are in trouble, and 51% of crayfish and 37% of freshwater fish are also at risk.

Human Threats and Impacts on Watersheds

What are local and neighborhood activities that can negatively affect water quality? For example, distinguish between sources: residential impacts (stormwater runoff from lawns) vs. industrial sources and agriculture. This will help readers identify their own potential problems.

Human activities can pose a host of threats to watersheds, though it’s often difficult to pinpoint a precise location for water-quality and -quantity problems. As a result, the distinction is often made between point-source and non-point-source threats. Point sources, once identified, often become the focus of investigations and news reports, as with industrial waste and chemical discharges, whether they are accidental or intentional.

Non-point sources are more diffuse – by definition, they do not come from single outlets. But even when their broader classes are identified, they can be exceedingly difficult to control, because common industrial, agricultural, and residential activities are often incompatible with strategies capable of protecting the species and habitats of our watersheds. The greatest cumulative threats created by this variety of activities are degraded water quality and alterations to natural water patterns.

Degraded water quality stems largely from water’s sensitivity to pollutants, toxins, and sediments. As rainfall and snowmelt flow over and through a watershed, they readily collect these by-products of our activities from the surrounding landscape and concentrate them into waterways.

The commercial, residential and agricultural sectors all add to increased sediment from places like construction sites, crop and forest lands, and eroding streambanks. The removal of streamside and upland vegetation can increase erosion and, along with it, surface-water sediments that greatly change habitat conditions for aquatic species.

Pollutants are equally drawn from many categories of human activities: oil, grease, and chemicals from urban and industrial runoff; fertilizers, herbicides, and pesticides from lawns and fields; bacteria, nutrients, and organic waste from municipal wastewater, animal and livestock operations, and household pets; even prescription medicines disposed of through our wastewater systems.

We create significant alterations to natural hydrological patterns through interventions like:

  • dams, which flood upland habitats, reduce the amount of water flowing through streams, and change seasonality of peak flows;
  • channelization of streams and diversion of water for irrigation purposes and other uses; and
  • pumping of groundwater for irrigation and municipal use.

Conservation Actions for Watersheds

Protecting watersheds is a complicated business. Because water flows downhill, species are affected by activities taking place anywhere upstream or uphill in the watershed, even many miles away. Conservation actions must therefore address threats to water quantity and quality over large areas that are upstream from imperiled species and habitats.

Humans’ sheer numbers ensure that our individual and collective activities on the land, large and small, have profound impacts on the health of our watersheds. This recognition has led to changing practices in many areas – some focused on greater efficiency while others function on larger, more strategic and systemic levels.

Replanting and restoring riparian buffers along streams and rivers is one of the most effective strategies for improving the health of urban and rural watersheds. They control soil erosion, reduce and filter sediments and pollutants, and reduce flooding in addition to providing fish and wildlife habitat. This strategy also enjoys widespread support in agricultural settings, with federal and state agencies providing millions of dollars in financial incentives annually through programs like the Conservation Reserve Program (CRP), Environmental Quality Incentives Program (EQIP), Wildlife Habitat Incentives Program (WHIP), Wetlands Reserve Program (WRP) operated by NRCS and the Stewardship Incentives Program (SIP) administered by the U.S Forest Service.

Urban and suburban settings can make use of a similar strategy through the use of porous street buffers and stormwater swales. These techniques can reduce or delay stormwater runoff and thereby reduce the capture of pollutants and increase groundwater recharge, providing an effective alternative to conventional concrete curbs, gutters, and culverts that channel high volumes of degraded water into our watersheds.

The extensive application of chemical fertilizers is probably the most well-known source of nutrient-related imbalances we’ve introduced into natural systems in recent decades, but by no means the only one. Areas like the Chesapeake Bay, where high nutrient loads from the entire watershed have drastically diminished the quality of water and once-rich aquatic habitats, communities are facing mandatory improvements to their municipal wastewater facilities in coming years.

Such concerns have led to innovations like nutrient banking, a market-based approach toward reducing excess loads of nitrogen and phosphorous in our waters. As with the proposed carbon cap-and-trade model, these water-quality markets allow point sources like waste water treatment plants and industrial facilities to purchase credits from non-point sources when they have gone above their “cap” in nutrient pollutants. Importantly, many of those non-point sources are farmers, who can use the revenue they draw from water-quality markets to fund further improvements in their land and water management practices. The hope and expectation is that these water-quality markets can encourage improved best management practices in both urban and rural settings, leading to greater reductions in all sectors’ negative impact on the health of their watersheds.

Many of these practices are integrated by conservation and land-use planners who pursue a more integrated and strategic approach toward community development, often referred to as green infrastructure.

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