Watershed

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Watershed

A watershed refers to land that is drained by an interconnected system of rivulets, streams, rivers, lakes, and groundwater to a particular point. Both rain and snow contribute to the watershed. The topography of a region sets the boundaries for a watershed, which are defined by the highest regions around a stream. Any drop of water falling on land within the water shed will drain into the watershed. Any drop falling on land outside highest points of the watershed will drain downward into another watershed. Watersheds can range in size from just a few square miles to many hundreds of square miles. The related terms catchment and drainage basin refer to the total area of land that drains into a water body.

Watersheds are important for many reasons. They are a direct link to the water quality of a region. They impact the amount and intensity of flooding. Fisheries yields are related to watershed health. Watersheds are key natural areas that serve as habitats for many plants and animals as well as for human recreational use.

The major predictor of the health of a watershed is the percentage of impervious cover found within its boundaries. Impervious cover is any surface that does not allow water to soak through it, such as asphalt surfaces, concrete surfaces, houses and buildings. It is estimated that watersheds with impervious cover over more than 10% of their area will experience declines in stream quality. When impervious cover exceeds 60%, the stream quality will be severely damaged.

Impervious cover affects watersheds in a variety of ways. By blocking the movement of precipitation into groundwater, stream flow decreases since many streams are fed in part from groundwater. Impervious cover also increases the amount of stormwater entering streams, which can lead to increased flooding, can change the shape of stream channels and can lead to increased erosion. In addition, the amount of pollutants washed into streams, in the form of bacteria, pesticides and herbicides, oil and gas and sediment, increases. This is usually accompanied by a decrease in the biodiversity of a region as both aquatic and terrestrial habitat is impacted.

Development of land and waterways has threatened a variety of ecosystems through out the United States, specifically because of impacts to the water-shed. For example, salmon runs in the streams of the Pacific Northwest have decreased drastically due to watershed development diminishing the health of streams where the salmon spawn. Coral reefs in the Florida Keys have been damaged due to coastal development resulting in pollution and sedimentation in stormwater runoff. Rare salamanders found in Texas springs have become endangered due to poor land use practices affecting aquatic habitat in the watershed.

In general, a group of basic management tools that work at the watershed level can mitigate some of the threats to stream quality. These tools include watershed planning, land conservation, maintaining ecological buffers such as wetlands, improved site design, erosion management and mitigation, stormwater treatment, control of water discharges not related to storms, and long-term watershed monitoring. Because of both the ecological and human dependence on watersheds, the improvement of watershed management practices is becoming recognized as a priority by local, regional, and national governments.

See also Hydrologic cycle; Water pollution.