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Watersheds are areas of land that drain to a specified point on the landscape, or watershed outlet. The boundary of a watershed is defined by high elevation points that direct flow "downhill" to the outlet. An example would be the Mississippi River watershed, which would contain the streams, rivers, and all land that a raindrop could travel to reach the Gulf of Mexico.

Watersheds in the eastern part of Georgia drain to the Atlantic Ocean, commonly referred to as Atlantic Slope drainages (shown in red). Watersheds in the western part and most of southern Georgia drain to the Gulf of Mexico, passing through the Mobile Basin in Alabama or other independent basins that flow through Florida before reaching the Gulf. These drainages are commonly referred to as Gulf Slope drainages (shown in tan). Some watersheds in the northern part of the state flow to the Mississippi River via the Tennessee River System and continue on to the Gulf of Mexico; commonly referred to as Mississippi Embayment drainages (shown in brown). Historically, each of these drainage systems harbored different groups of fish species or different fish faunas. These drainages along with complex geology and ecology provide the state with a diverse array of fish and other aquatic fauna.

These major drainages can be split further into smaller watersheds. The United States Geological Survey (USGS) has classified all of the watersheds in the United States, using a classification system to define where the watershed occurs. The Hydrologic Unit Code, or HUC code, identifies the region (2 digits), subregion (4 digits), accounting unit (6 digits), cataloging unit (8 digits), watershed unit (10 digits), and the subwatershed unit (12 digits). The USGS has defined 52 8-digit HUCs (cataloging units) for the state of Georgia. Examples of an 8-digit HUC in Georgia includes the Etowah River (03150104), which belongs to the accounting unit 031501 called the Coosa-Tallapoosa, in subregion 0315 (The Alabama River Basin).

For an Adobe Acrobat copy of the 8-digit HUC map for the Georgia use the following link to download the maps (with HUC numbers or HUC names).