A watershed is an area that sends all of its water to the same place. As you know, water runs downhill. Surface water collects into larger and larger streams, then into larger and larger rivers. Eventually, it winds up in the sea. You can see an example of watersheds in the Santa Barbara, Ca area here: watershed

As the water moves, it picks up things like nutrients, pollution, soil, and living things. What happens upstream has a large influence on what happens downstream.

Sometimes watersheds are pretty obvious. If you live in a valley, you can see where all of the water on the surface goes. You won’t see all of the water though, because some water goes underground. It might have a much longer journey to the sea.

You can look at watersheds at different scales. For example, you might study the watershed that winds up in a small stream. That area will probably be small. You could also study the watershed that is drained by the Mississippi River. That will be HUGE. It will contain many, many smaller watersheds.

Why might an ecologist study areas by watershed instead of by artificial boundaries like counties?

If you are interested in watersheds, you might be interested in studying ecology, geology, or environmental science.

A watershed is a line "dividing drainage basins." Often these are formed by ranges of hills or mountains. This means that rain or water on one side of the watershed will not cross over to the other side. If it rains on a slope of a mountain, the rain will always run down the mountain to the same side. In that way, scientists can divide up the country into different water supply areas. For example, if you cross the mountains north of Santa Barbara, you will cross the watershed since rain on the other side of the mountains won't make it over here.

A watershed is an area where all of the water flowing downhill reaches a particular place, or flows into a particular river. So, for example, rain that falls on the west side of the Sierra Nevada, in the central valley of California, flows into the Pacific through the Sacramento River, which empties into the San Francisco Bay. Hence, the central valley is the San Francisco Bay watershed.