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What Is a Watershed?

We often hear the term "watershed" these days. We all live within a watershed. Fish habitat and water quality can be affected by the watershed's condition and by the activities within it. All of us depend upon the water that flows from our watershed. But what exactly is a watershed?

A watershed is the area of land where all surface and groundwater drains into the same body of water, such as a river, wetland, or the ocean. Watersheds can be many millions of acres like the Columbia River Basin, or less than a dozen acres for a single small stream. Since the term "watershed" can be used for drainage areas of any size, the US Geological Survey (USGS) has divided watersheds into distinct units, or "fields," based on size. Sizes range from multi-million acre first-field watersheds to seventh-fields that can be less than 3,000 acres. Third-field, fourth-field, and fifth-field watershed are the most common units used by agencies and other organizations involved in landscape-level watershed planning and restoration.

Third-field watersheds are large river basins. The Umpqua River Basin includes the South, North, and main Umpqua Rivers, as well as Smith River, and has roughly the same boundary as Douglas County. Third-field watersheds are often referred to as "basins," so throughout this website, "basin" will be used to refer to the Umpqua Basin third-field watershed.

Fourth-field watersheds are sub-basins. Just as there are three main rivers in the Umpqua Basin, there are also three fourth-field watersheds, or sub-basins: the Umpqua River fourth-field watershed, the North Umpqua River fourth-field watershed, and the South Umpqua River fourth-field watershed.

Fifth-field watersheds have become the standard size used for research and projects by a variety of agencies and organizations. Therefore, it is convenient for fifth-field watershed to be the unit usually referred to herein by the term "watershed." The Umpqua Basin has 33 fifth-field watersheds. All of the watersheds have their own features, challenges, and potential. The conditions in one watershed may not reflect the conditions in a neighboring watershed.

Although the borders of the watersheds are standardized, the names are not. Different organizations and agencies may call the watersheds by different names, but, in general, all watersheds are named for the creek or the section of river into which all tributaries drain. For example, the Calapooya Creek Watershed includes all land that drains into Calapooya Creek or its tributaries. A very large stream, such as the South Umpqua River, is usually separated into multiple fifth-field watersheds. In this case, watershed names reflect the relative location of the watershed along the mainstem: the Upper Cow Creek Watershed would be near the headwaters of Cow Creek, while the Middle North Umpqua Watershed is somewhere in the middle section of the North Umpqua River.


Chow, Ven Te, 1964. Handbook of applied hydrology. McGraw-Hill.