It's difficult to define a watershed.
My water resource engineering professor would want me to define it as: "A geographic region, defined by topographically induced flow to a point. It's a model that is more useful as an organizational and cultural feature than for predicting the actual flow and chemistry of a stream because it disregards a number of facts, including that streams are generally fed by groundwater and groundwater flow is not defined by the same ridges and topography as surface water." This is a useful definition for thinking about water resource modeling, but not very helpful if you have to explain the concept to an adult who has never really considered the idea before.
I think a more helpful definition would be: "A geographic region, where all the water and chemicals contained in the water flow to the same point point through a river and it's tributaries, lakes, and other man made water bodies. Throwing trash right into the river can be just as disruptive to the watershed's health as the water running off your driveway when you wash your car 10 miles away at the edge of the watershed." This definition is a bit harsh and focuses less on the community spirit of a watershed than I would like...
I think John Wesley Powell a 19th century geologist and explorer defines it best (the EPA agrees with me too): "that area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community." Mr. Powell went on to hypothesize that state and regional boundaries should be defined based upon watersheds to limit conflict and improve management. Sadly his ideas never came to fruition, but luckily the Chares River is all within Massachusetts
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