Information about floods and flooding can be found in the Earth Science-Climate Guide: http://libguides.sbuniv.edu/c.php?g=113770&p=91222
"Watershed--the land area that drains water to a particular stream, river, or lake. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large watersheds, like the Mississippi River basin contain thousands of smaller watersheds." -- https://water.usgs.gov/edu/dictionary.html#W