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Mansfield University Home Page > Department of Geography and Geology Home Page > What Can I Study? > Watershed Management > What is a Watershed?

What is a watershed?

A watershed is defined as the area of land where all of the water that is under it or drains off of it goes into the same place, according to the EPA website. I think of a watershed as a large bathtub. When a drop of water hits anywhere in that bathtub it eventually finds its way to the drain. The bathtub defines the watershed boundary. On land, that boundary is determined topographically by ridges, or high elevation points. Water flows downhill, so mountains and ridge tops define watershed boundaries. A watershed can be as large as the Mississippi River Watershed (that is one big bathtub) to as small as the little creek running through your back yard being a watershed. We can talk about watersheds and sub-watersheds, with a sub-watershed just being a smaller watershed in the bigger watershed.

This is a map showing the geographic extent of the Susquehanna River Watershed.  Three sub-watersheds are shown:  the North Branch Susquehanna watershed, the West Branch Susquehanna watershed and the Lower Susquehanna watershed.

So, if we talk about the Susquehanna River Watershed, the North Branch of the Susquehanna would be a sub-watershed, and the Tioga River would be a sub-sub- watershed, and Johnson Creek would be sub-sub-sub-watershed….do you get where I am going with this? We start off on the large scale and work towards the small scale and depending on what you are doing and what you are defining, the watershed may change.

This map shows the geographic extent of the North Branch Susquehanna River Watershed.

The Tioga River is a sub-sub watershed and Johnson Creek, a tributary of the Tioga, is a sub-sub-sub watershed.

If you want to talk about the pollution entering the Chesapeake Bay, you would talk about the Susquehanna River Watershed. If you want to talk about mining impacts of Tioga County, Pennsylvania, you would talk about the Tioga River Watershed. The bottom line is watersheds come in all shapes and sizes and exist everywhere. They cross county, state, and national boundaries. No matter where you are, you're in a watershed!

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