"1. The source and upper reaches of a stream; also the upper reaches of a reservoir. 2. The water upstream from a structure or point on a stream. 3. The small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries." (U.S. Geological Survey, 2019)


U.S. Geological Survey (USGS). "Water Science Glossary Terms". USGS Water Science School. (2019). Link: https://water.usgs.gov/edu/dictionary.html#W

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