Two-Minute Takeaway: What is a Watershed?

Graphic by Erica Simek Sloniker / TNC

By David Ryan, Field Forester

A watershed is an area of land where all water that enters it, either through precipitation or springs, drains to a common outlet or outflow point. Or, to switch the phrasing, that piece of land is shedding water to a single point. 

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A watershed can be practically any size. It can be smaller than your bathroom or large enough to capture all the water that flows into major bays. For example, the Willapa Bay watershed in southwest Washington is more than 700,000 acres. 

A watershed is not simply a matter of creeks, rivers and other open bodies of water; watersheds encompass all the water that flows over the land as well as the water flowing below ground. Water enters and exits a watershed at different rates and in different ways. Factors such as amount and type of precipitation, infiltration of water into the soil, evaporation, transpiration through plants, water storage and water use by people all impact the "water budget" — an account of all the water inputs, outputs and changes in a watershed.

Liane Davis enters observations into an electronic data recorder while measuring the width and depth of a stream that feeds into Ellsworth Creek near Willapa Bay. Photo by Harley Soltes.

Large watersheds can contain many smaller watersheds, analogous to nesting dolls —it is all dependent on the outflow point. All the land that drains water to a single outflow point is the watershed for that point. Consideration of entire watersheds is important because streamflow and water quality are affected by the actions — whether human or mother-nature induced — happening in the land area above. Landslides, roads, windthrow and development are among the long list of things that can alter the quality and character of a watershed.

So, consider that wherever you are standing at any given time, you are standing in a watershed. And think about how what is happening around you may be affecting the water in that landscape.

Explore the Ellsworth Creek watershed