This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2000, Number 2]
WHAT IS A WATERSHED?
A few years ago the only people who used the word watershed were specialists such as hydrologists, geographers and planners. Today the word pops up in almost any news story involving environmental issues. Frequent use of the word however does not mean that people really understand its significance. The meaning of watershed depends on whether you are in North America or somewhere else in the English-speaking world! By its traditional English definition the word describes the high point or ridgeline of topography that divides two valleys. In North America the word watershed is used to describe the area of the whole landscape system that collects and directs flow to a particular river system. Outside North America, the words catchment or drainage basin are used. Watershed areas include the total land area from which water input may be derived. Large watersheds, like the area that drains into the Mississippi River, contain thousands of smaller watersheds.
For many years hydrologists and engineers have used watersheds for calculation of inputs, outputs and changes of storage in the hydrologic system.
ß Inputs - rain, snow etc. ß Outputs - river flow leaving the defined watershed, evaporation and transpiration ß Changes in storage - adding or losing water stored in lakes, reservoirs, soils and ground water.
These hydrologic calculations are important because they provide the scientific basis for estimating flood risks, quantifying the availability of water resources and developing strategies for water management. When only ground water resources are being considered, the area contributing water to an aquifer system is called its phreatic catchment. Because of subsurface geologic structure, surface area watersheds and ground water phreatic catchment areas may not exactly coincide.
So why do we hear so much about watersheds these days? The answer is because of environmental concerns about water quality. As water moves across and through a watershed, its quality can deteriorate because of contact with agricultural chemicals, urban development, mining operations etc. Deteriorating water quality has both ecological and economic effects. Prevention is much less expensive than cure (cure may not even be feasible) and there is now wide acceptance of the need to view the watershed as an integrated hydrologic system of surface water and ground water for purposes of land- use planning and resource protection. There are in excess of 4,000 watershed associations throughout the US that have been formed by citizens as a means of proactively educating about pollution “cause and effect.” Every water molecule in a watershed deserves our concern and attention. One person’s downstream is another person’s upstream. The objective of having all our rivers drinkable, fishable and swimmable is still a long way off. Rising population, increasing urbanization and greater resource demand are powerful reasons to keep vigilant watch on the health of our watersheds.
Additional watershed information is available on many Internet web sites. For example: http://www.epa.gov/owow/watershed is the home page on watershed information managed by the Office of Watersheds and Wetlands, United States Environmental Protection Agency. This site offers access to technical reports, educational outreach for communities and school children and a search engine that can help you find out more about your local watershed.
http://www.watershed.org/wmc/ is the home page for the Watershed Management Council that promotes the protection of watersheds.
http://www1.umn.edu/bellmuse/mnideals/watershed/watershed2.html - is an interactive on-line game about watershed management for teachers and their students in grades 4 through 8 developed by Bell Museum of Natural History at the University of Minnesota.
[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2000, Number 2]