occurs when water rises above the water table and permeates the surrounding 

 soil. Releases from bank storage occur when stream levels recede and is 

 immediately available. Basin storage occurs in aquifers and the water is 

 released more slowly. 



The type of runoff governs natural streamflow characteristics. Streams 

 dependent on direct or surface runoff tend toward wide fluctuations of 

 streamflow while those dependent on ground-water releases are more stable. 



Gaging stations and partial -record (not continuously monitored) stations 

 operated by the USGS provide basic information on streamflow characteristics 

 in coastal Alabama. Data collected include records of stage (water levels) 

 and stream discharge (water volume passing a point within a given period of 

 time). Mathematical analysis of the data allows different types of 

 streamflow characteristics to be developed, such as annual discharge, average 

 discharge, etc. 



Gaging-station locations in Mobile and Baldwin Counties are presented on 

 the atlas sheets. Table 36 lists the gaging stations by USGS station number 

 and indicates where they are located by stream and atlas quad sheets. The 

 table also indicates the average daily flow in million of gallons per day 

 (mgd) and the median annual 7-day low flow in mgd. 



The median annual 7-day low flow of a stream is the lowest mean discharge 

 for seven consecutive days during a year. By comparing this low flow to the 

 average flow, a stream's ability to assimilate wastes and dilute them in dry 

 weather can be evaluated. 



The monthly mean discharge of a stream can be used to evaluate changes in 

 stream flow on a seasonal basis. Factors affecting seasonal streamflow 

 include precipitation rates, water use, and hydrologic modifications, such as 

 channelization and dams. The monthly mean discharge of streams in cubic feet 

 per second (ftVs) in the Mobile River basin and in the Perdido Bay area are 

 presented in Tables 37 and 38. Figure 15 illustrates the monthly mean 

 discharge of the Alabama, Tombigbee, and Perdido Rivers. 



Average flows can also be expressed as unit runoff in cubic feet per 

 second per square mile (ft^/s/mi^), which is the average number of cubic feet 

 of water flowing per second from each square mile of the drainage area. This 

 figure of unit runoff can be utilized in comparing basins of unequal size 

 since they are reduced to a common base of one square mile. Table 39 

 presents the duration of daily flow and average discharge (ft-Vs) at gaging 

 stations in the study area. Average unit runoff in coastal Alabama varies 

 from less than 2.0 ft3/ s /mi'2 to less than 2.5 ft3/s/mi"2 (O'Neil and Mettee 

 1982). 



Table 40 presents a summary of the watershed area of streams flowing 

 through coastal Alabama and the mean annual discharge of these streams. The 

 total mean annual discharge into Alabama estuaries is over 75,000 ft-Vs from 

 a watershed area greater than 45,000 mi 2. 



142 



