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The definition of a drainage basin is: The land area where precipitation runs off into streams, rivers, lakes, and reservoirs. 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 drainage basins, like the area that drains into the Altamaha River contain thousands of smaller drainage basins. The term "watershed" is often used in the same context as drainage basin.

When precipitation falls on a drainage basin, it either flows overland towards the lowest part(s) of the basin or it seeps into the ground and moves "down-gradient" because of gravity. Rivers, lakes, and swamps are often found in the valleys of drainage basins because they are the low points in the basins. Of course, drainage basins come in all sizes, from less than an acre (think about a little depression that collects water after a rain) to thousands of square miles (the area into which water that falls drains into the Mississippi River).

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