Mississippi 



Amife 



Tickfaw 



Pearl 



eo 



i \^[j^EP[MILIOli_.* 



, "^ ^T^Lwi, n I PONTCHARTRAIN. 





jATCHAFALAyA\ 



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G U i /^ 





MISSISSIPPI 

 ,'^^v|R DELTA 



O F 



M £ X / C 



Figure 10. Freshwater inflows to the Mississippi Delta. (Oata from IJSGS 1978). 

 Discharges are in cuinecs. All discharges are for water year 1978 except Mississippi 

 River, which is a long-term mean representing the combined average annual discharge 

 above the confluence of the lower Mississippi (10400 cumecs) and the Atchafalaya 

 (5000 cumecs) Rivers. 



centimeter per year (Figure 11a). This is 

 double the rate anywhere else along the 

 eastern United States coast (Table 3). 

 Superimposed on this long-term trend is a 

 seasonal variation in mean water level 

 that itself has an excursion of 20 - 25 

 cm. This bimodal variation (Figure lib) 

 occurs consistently throughout the 

 different salinity zones of the delta, 

 with peaks in the spring and late sunmer. 

 In the Barataria basin the spring maximum 

 increases in an inland direction, that is 

 from salt toward fresh marshes, possibly 

 because of the considerable volume of 

 surplus precipitation during this time of 

 the year (Baumann 1980). 



The seasonal changes in water level 

 are attributed to several interacting 

 factors. Water level varies inversely 

 with barometric pressure which averages 



1,021 millibars (mb) during December and 

 January and 1,015 mb during early summer 

 and fall. Several investigations have 

 shown that water level decreases nearly 1 

 cm for each mb increase in barometric 

 pressure (e.g. Lisitzin and Pattullo 

 1951). Thus the expected mean seasonal 

 range in water level as a response to 

 barometric pressure is approximately 6 cm 

 or 25 percent of the total observed range. 

 In addition, the seasonal warming 

 (expansion) and cooling (contraction) of 

 nearshore waters contribute to a seasonal 

 high in the late summer and a low in 

 January and February. 



These astronomical events can be 

 modeled and compared to the actual water 

 levels. When this is done (Byrne et al. 

 1976) there is always a significant 



12 



