The Mobile/Baldwin County study area lies within the South Atlantic-Gulf 

 Region and includes four subregions (Escatawpa River, Tombigbee River/Mobile 

 River/Mobile Bay, Alabama River, and Perdido Bay drainages) which are sub- 

 divided into a total of eight cataloging units, listed below (U.S. Geological 

 Survey 1974, 1981). These hydrologic units are delineated on the Atlas 

 sheets, along with the cataloging unit code. 



03170008 

 03170009 



Escatawpa River Basin 

 Western Coastal Unit 



03160203 

 03160204 

 03160205 



Tombigbee River Basin 

 Mobile River Basin 

 Mobile Bay Unit 



03150204 



Alabama River Basin 



03140106 

 03140107 



Perdido River Basin 

 Eastern Coastal Unit 



STREAM AND RIVER DISCHARGE 



Factors that control streamflow are of two basic types: meteorological 

 and terrestrial. Meteorologic factors include precipitation, temperature, 

 and wind. Terrestrial factors include topographic and geologic features and 

 the geographic distributions of cultural features and vegetative cover. 

 Since coastal Alabama is not a self-contained hydrologic unit, streams which 

 flow through the area may be influenced to a greater or lesser degree by 

 factors outside the immediate geographic area. 



Runoff of precipitation is the source of all streamflow, and consists of 

 two basic types; surface runoff and subsurface runoff. Surface runoff occurs 

 immediately after it rains when the amount of rainfall has exceeded the 

 combined rates of evaporation and ground infiltration. This direct runoff is 

 transported to stream basins by running over the land surface or by inter- 

 flow, the movement of water through the soil without entering the groundwater 

 table. Factors important in determining direct runoff rates include 

 drainage-basin area, land-surface slope and permeability of the ground 

 surface. 



Runoff also reaches streams by releases from ground water storage. Basin 

 and bank storage are two components of this type of release. Bank storage 



141 



