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Watersheds in Washington

What is a Watershed?

A watershed, or drainage basin, is an area of land from which rainfall and/or snow melt drains into a common stream or other body of water. Small watersheds nest together to form larger watersheds. For example, the Cedar River in King County has many tributaries. Each tributary has its own watershed. The entire land base that drains into the Cedar River, i.e., all of the tributary watersheds, comprises the Cedar River watershed. The water from the Cedar River ultimately flows into Puget Sound, thereby being part of an even larger watershed, the Puget Sound watershed.


Why use watersheds in land-use and conservation planning?

Watersheds provide a natural framework for land-use planning, including conservation planning. The distribution and abundance of many species, such as fish, freshwater mussels, aquatic invertebrates, and aquatic plants are directly dependent upon a healthy, functioning hydrologic system. From the human perspective, when we use, divert, or pollute water within a watershed, we are affecting all downstream inhabitants. As our population grows and water resources become more limiting, the potential for conflict becomes more acute. Using watersheds as the framework for water-dependent resources makes sense from both the environmental and human perspectives.

Watersheds in Washington

The Washington State Department of Ecology, in cooperation with other state natural resources agencies, has divided the state into 62 Water Resource Inventory Areas or  WRIAs to delineate the state’s major watersheds.  The WRIA watershed boundaries  were formalized by law in 1971 to be used as the basis for state management purposes.

According to the current Hydrological Unit boundary classification system, Washington has portions of 72  watersheds.  HUs and WRIAs are similar, although HUs extend beyond Washington into neighboring states and into British Columbia.

More information about Washington watersheds is available on the Department of Ecology web site:

Washington State Department of Ecology - Watershed Planning

Healthy Watersheds, Healthy People

Washington Water Resource Inventory Areas (WRIAs)

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