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What is a Watershed?

Updated: Jan 26, 2023

When you hear or see the word watershed, what do you imagine? Perhaps the area around a body of water, such as a river or harbor. Or perhaps a larger land area, like a town or village. Well, you would be right on both accounts, but neither fully describes what a watershed is. We all live in a watershed—the geographical area that drains to a common waterway, such as a river, stream, lake, estuary, wetland, or, ultimately, the ocean. Watersheds come in all shapes and sizes. They cross town, county, state, and national boundaries. No matter where you are, you're in a watershed.

Watersheds can be as small as a person’s footprint or as large as to encompass all the land that drains into Buzzards Bay. Ridges and hills that separate two watersheds are called the drainage divide. A watershed consists of surface water—lakes, streams, reservoirs, and wetlands—and all the underlying ground water. Larger watersheds contain many smaller watersheds.

We all know that water flows downhill and carries with it whatever it touches on the way—such as pollutants. Nutrients such as nitrogen and phosphorous, bacteria, oil, gas, pharmaceuticals, and other pollutants that migrate into surface waters or ground waters from land sources in these towns travel through the watershed, working their way towards the Westport River and ultimately Buzzards Bay. What and how much eventually reaches the bay depends on pollutant source, soil type, bedrock formation, land use, proximity to surface waters or water table, and precipitation amount and duration.

Not all water drains out of the watershed, however. Some amount is stored as ground water or surface water, taken up by vegetation, lost as evaporation, or infiltrates the soil. This is the water budget for a watershed, and determines the capabilities of a watershed to maintain a natural balance of water vital to ecosystem health, as well as the health of our community. Land use especially influences the stability of a watershed. For example, New Bedford can be considered a sub-watershed, yet is one that, because of its large population and amount of impervious surface, produces a lot of stormwater as direct runoff into drains, which pipe directly into the bay. This also makes the city susceptible to flooding and drain overflow.

In contrast, Westport has quite a lot of open space and a relatively small population, allowing more precipitation to go back into the ground or into ponds, lakes, and streams, which act to filter the water before it reaches Buzzards Bay. Precipitation is also allowed to recharge local groundwater aquifers, which are the major sources of drinking water from wells in Westport. However, when pulses of ever increasing contaminated stormwater and groundwater are allowed to enter the Westport River or infiltrate wells, the capability of the watershed to accommodate and filter these pollutants is compromised. As a result, land use changes and practices, as well as pollutant sources, must be seriously considered throughout the watershed. No town is an island when it comes to water quality remediation. The strategies required to restore water quality or prevent degradation will be most effectively achieved through planning and implementation on the watershed level and with the cooperation and partnership of watershed towns and residents.


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