HYDROGRAPHY OF THE MARINE WATERS 41 



Una's inland waters is shown on Figure i8. According to this, the Cape Fear 

 River Basin has the largest drainage and is about twice the size of the three 

 main basins that drain into the sounds. Another interpretation issued by the 

 North Carolina State Planning Board extends the drainage basins to include 

 all the coastal tributaries as though part of the major basins; for example, 

 the Meherrin-Chowan River Basin and all the waters tributary to Albe- 

 marle Sound, including Currituck Sound, are considered as in the same basin 

 with the Roanoke River. This interpretation, the figures for which are in 

 parentheses on Figure i8, is especially significant, for it shows that the 

 northern drainage into Albemarle Sound equals the combined areas of the 

 Neuse and the Tar which flow into Pamlico Sound. The Corps of Engineers 

 (1935) states that the average daily run-off for the area tributary to Albe- 

 marle, Currituck, Croatan, and Roanoke sounds is 26,950 acre-feet as 

 compared with 17,504 acre-feet for the drainage tributary to Pamlico and 

 Core sounds. As mentioned above in the section on Circulation in the Sounds, 

 the source of these data is unexplained and rather questionable; nevertheless, 

 there is a noteworthy correlation between these discharge figures and the 

 over-all areas just mentioned. 



Figure 19 shows the rate of discharge from these river basins as it varies 

 with the seasons over a period that was selected as having the most complete 

 records and including both drought and wet conditions. The most downstream 

 permanent gaging stations were used for the flow data under consideration, 

 but even these stations were some distance from the river mouths, beyond 

 any backing-up effects characteristic of estuaries; consequently, the actual 

 discharge from the rivers may be quite different when rainfall, evaporation, 

 and transpiration effects are considered. For this reason only the figures for 

 run-off in inches, ^^ which are on a per unit of area basis, have been used. 

 This gives all four rivers approximately equal weight in the computed aver- 

 ages and demonstrates seasonal and annual changes in relation to rainfall. 

 A correlation with precipitation is noticeable in the fall, winter, and spring. 

 The fall often has little precipitation and little discharge. The winter and 

 spring have more rainfall, greater discharge, and, according to Figure 20, 

 a high frequency of floods and freshets. Summer is usually a rainy period, but 

 apparently because of transpiration and evaporation, the discharge does not 

 always rise accordingly and floods are not as frequent as the precipitation 

 data at first suggest. On an annual basis there is more agreement between 

 discharge and precipitation, though the latter still fluctuates more than the 

 former. 



Other factors being equal, these fluctuations in river discharge must cause 

 inverse fluctuations in salinity in the sounds. Such a relationship has been 



16. Run-off in inches is the depth to which a drainage area would be covered if all the water 

 draining from it in a given period were uniformly distributed on its surface. 



