A watershed is a connected series of streams, rivers, and lakes that collects water from a specific area of land. Watersheds are important habitats for animals and plants, and offer a source of drinking and recreational water for many communities.

The quality of the water in a watershed, also sometimes referred to as the health of the watershed, is important to preserve or remediate (to repair after damage). Water quality generally refers to the appearance, smell, and, above all, ability to serve as drinking water (a quality known as potability).

At the beginning of the twentieth century, most watersheds were unaffected by man-made pollution; pollution occurred only from natural sources such as animal waste. Before increased levels of man-made contamination, most watersheds were able to dilute pollutants or filter them out through surrounding wetlands (grassy areas that hold water throughout most of the year).

Today, pristine and uncontaminated watersheds are rarely observed. Pressures from modern urban development, mainly runoff of pollution and decreased air quality, make monitoring water quality a necessity.

Water quality

A number of factors determine water quality. Scientists measure levels of chemicals and observe living creatures whose presence (or in some cases, absence) in water indicates that the water is contaminated. A well-known example of a contamination is from the bacterium Escherichia coli.

Escherichia coli (commonly referred to as E. coli) lives in the digestive tract of humans and other warm-blooded animals. The bacteria do not usually survive or thrive (exist in high numbers) in other environments, such as fresh water.

High numbers of the bacteria in water is usually evidence of contamination by human or animal intestinal waste. Because the detection of E. coli requires simple and relatively inexpensive tests, such testing is a fundamental part of programs that monitor fresh water quality.

In addition to the detection of E. coli, there are other ways to determine water quality exist. For example, determining the number of species in a certain area of a stream can provide information on upstream water quality, especially if the species do not live and reproduce in contaminated water.

Tests to measure water quality are performed at different sites along the watershed at different times. Repeated test sites and test times are necessary to develop a watershed history. Also, results can vary according to location. For example, a low level of E. coli at one point in a stream does not necessarily guarantee that another area is uncontaminated.

Water quality

Inorganic, or non-living, indicators of water quality (for example, amounts of certain chemicals) tend to be easier to measure throughout the course of the watershed. Scientists often used a combination of tests to determine water quality.

One important inorganic measurement determines the level of nitrates in the water. Nitrate (NO3) is a chemical form of the element nitrogen. Another chemical form of nitrogen, nitrites (NO2), form important compounds that permit the growth of algae and some plants.

Nitrate is hard to measure directly, but can be determined from nitrite levels. Too much nitrate leads to the explosive growth of algae, which overuses the available oxygen in the water, crippling the survival of other water species.

The dissolved oxygen level is another chemical indicator of water quality. The level of oxygen in the water can be lowered by chemical conditions or the presence of biological material such as sewage. If maintained, the low amount of oxygen makes the watershed incapable of supporting life.

Other measurement tools measure the presence of other important chemicals in the watershed such as salt (the salinity of the water) and phosphorus (a component of agricultural fertilizers that can enter watersheds via runoff from lawns, golf courses, and agricultural land). Phosphorus is another nutrient that can stimulate the explosive growth of algae.

Some tests are performed because of the location of the watershed. For example, if the watershed is near a mine, then monitoring to detect the acid that can flow from mining activities may be warranted. Alternatively, if the watershed is near a nuclear power plant or uranium facility, testing for the presence of radioactive compounds is often warranted.

Watersheds that incorporate urban areas are often monitored for the presence of petroleum. Watersheds in rural areas are monitored for the presence of farm fertilizer and pesticides.

Historically, local, regional, and federal governments conducted most watershed quality monitoring. Increasingly, however, citizens groups and committees are seeking funding to conduct tests.

Governments generally support such civic efforts, and training is available in many areas for those desiring to learn proper sampling techniques. The quality of freshwater in watershed areas is often improved with civic awareness and involvement.

Restoring watersheds

Restoration of a watershed returns the ecosystem to as close as possible to its state prior to a specific incident or period of deterioration. If a watershed has deteriorated abruptly due to a sewage spill, restoration may consist of only a few procedures. When deterioration of a watershed occurs gradually, however, restoration can require lengthy, complicated, and costly operations.

The restorative process includes the remediation of the water quality, repairing the source of the water damage, and repopulating the watershed with animal, fish, or bird species. In some cases, it is sufficient to make the restored habitat attractive to native species and to allow natural repopulation.

Watershed restoration is important, not only for the benefit of the species living in the area, but also for those who will use the water that emerges from the watershed. A contaminated watershed affects all the watersheds downstream from the contamination site. Communities that are miles away from a contaminated watershed can be adversely affected.

Restoration can be divided into two broad categories, reestablishment and rehabilitation. Reestablishment is the alteration of the various characteristics of a watershed in order to restore the site’s former function.

Reestablishment can involve construction to rebuild physical parts of the watershed. Often, reestablishment results in the enlargement of the total area of the watershed, as acreage that has fallen into disuse is reutilized to restore the former dimensions of the watershed.

Rehabilitation seeks to repair the watershed, not necessarily to restore its original function. Correcting the damage from a sewage spill, for example, is considered watershed rehabilitation. The rehabilitated portion of the watershed does not necessarily function at its previous best (optimum) level. Nonetheless, the short-term damage is repaired.

The restoration of watershed ecosystems is increasing, as the realization grows of the importance of the watersheds to the health of animals and plants that are part of the ecosystem and to the communities that depend on the watershed for their drinking water. For example, the U.S. government has committed to improve or restore 100,000 acres of wetlands each year and 25,000 miles of stream shoreline.

Risks in restoration

Scientists are increasingly aware of the dangers of not using native plant and animal species during restoration efforts. In many places around the world, the introduction of a nonindigenous (nonnative) species has caused trouble. The foreign species, which may not have any competition or natural enemies, can grow explosively and out-compete the native species.

The fast-growing kudzu plant, for example, was planted in the southeastern United States during the 1930s as a means to prevent soil erosion (wearing away). Native to China and Japan, and without its natural controls, the kudzu proved hardier than expected and by 2002, kudzu occupied an estimated two million acres of forest and watershed land in the United States.

Kudzu can grow up to one foot per day, and densely covers trees and other vegetation, eventually causing their death from lack of sunlight. Kudzu eradication efforts remain an ongoing concern in southern states.

Impact of fire and logging on watersheds

The presence of forests in a watershed enhances the capability of a watershed to acquire and retain water. Conversely, the loss of trees, whether naturally due to fire, or deliberately such as the removal of trees for lumber or to permit construction of a road, can have adverse impacts on a watershed.

The presence of trees and other vegetation contributes to the water-bearing capacity of the watershed. The roots of the trees and other plants stabilize the soil. Roots also help retain moisture. Additionally, watershed vegetation shades and cools the ground, minimizing the loss of moisture.

Moisture in the form of rainfall is gathered on the leaves of trees and trickles slowly to the soil. The slow addition of moisture to the soil allows the soil to retain more water because the soil does not become saturated (holding the maximum amount of water), as would happen if it were deluged with water.

The riparian zone

An example of the importance of forests to a watershed is the riparian zone. The riparian zone is a narrow strip of vegetation that is found bounding the edge of a natural water body such as a stream or river. The riparian zone enhances watershed stability and quality because plant roots minimize erosion of the shoreline.

A water body that is clear of mud is better able to support fish and other life. The riparian vegetation is also the source of wood and larger debris that helps create pools in the water body. Such pools enhance the ability of the water to support life.

The loss of the riparian vegetation can be detrimental to a water body. Erosion of the shoreline is increased because there is no supporting network of roots to stabilize the bank. Large sections of the shoreline may give way in a landslide.

Runoff of material into the water is also increased. The cloudier water will contain less oxygen; therefore, sunlight cannot penetrate as far into the water. The result is a habitat that is less suitable for life. Torrential rains can also lead to flooding because the flood-waters are not held back by vegetation.


An event such as a forest fire represents a natural means of deforestation (the large-scale removal of trees from a woodland). Typically, a forest fire does not removal all the vegetation.

Rather, the effect is to thin out the forest, which can lead to promotion of future growth by the opening up of space and the nutrient supply created by the decomposition of the tree remains. A natural removal of forests by fire can actually have beneficial results for a watershed.

In contrast, the unnatural deforestation and loss of vegetation associated with clear cut lumbering (removing all trees from an area for their wood) is never beneficial for a watershed. The mass loss of vegetation often lowers.

Loss of surrounding forest can also alter the movement of water through the watershed. A healthy watershed will have a fairly constant rate of water movement. Without the water retention provided by forests, rainfall can result in flooding.

A fluctuating water supply can also be a serious concern when a watershed supplies drinking water for communities. As a result, many communities are passing legislation to protect their watersheds. Development, logging and road construction are restricted or kept to a minimum in the watershed zone.

If houses are built, often barriers to minimize erosion and runoff (excess water that does not soak into the soil) must be maintained. Some communities require riparian zones to remain undisturbed before and after construction.