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, 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 (and do not yet have standardized boundaries).

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