INCREASING AGE 



^ 



PONCHARTRAIN 

 L BORGNE 



VI 



VERMILION 



FRESH 



INTERMEDIATE 

 ^""'j BRACKISH 

 SAl I 



Mineral 

 order of 



Figure 26. 

 arranged in 



PERCENT 

 MINERAL 

 CONTENT 



content of marsh soils 

 increasing age (data from 



100 



50 







in Mississippi 

 Chabreck 1972). 



delta hydro! ogic units. 



The Ponchartrain-Lake Borgne basin 

 (Unit I) is fed by a number of small, 

 local streams, by the Pearl River, and 

 periodically by diversion of the Missis- 

 sippi River through the Bonnet Carre 

 '■'--  ^'-" The Vermil ion 



The Pontchartrain-Lake Borgne unit is 

 exceptional in that the mean salinity is 

 high, but so is the proportion of fresh 

 marshes. This may be a result of the 

 physiography of the system. The gradient 

 is compressed into the lower half of the 

 basin by the location of the mouth of the 

 Pearl River, the primary freshwater 

 source, and by the small passes into Lake 

 Pontchartrain which restrain free flow of 

 saline water into the lake. 



Within a hydrologic unit of constant 

 size, wetland area and land:water ratio 



a 

 a 



> 

 I- 



<0 



O 

 in 



#100- 



X 

 (0 



K 

 < 



(A 

 III 



i INCREASING »Gg> 



J 



V II 



ATCHAF- MIS8.R. 

 ALAVA DELTA 



III 

 BARA- 

 TARIA 



IV I 



TERRE- PONT- 

 BONE CHAR- 

 TRAIN 



VI 



VER- 

 MILION 



Figure 27. Marsh soil salinity and 

 percent fresh marsh in Mississippi Delta 

 marshes by hydrologic unit, arranged in 

 order of increasing age. Soil salinity is 

 a mean for the whole basin weighted by 

 area of each marsh zone. The fresh marsh 

 is percent of total marsh area (data from 

 Chabreck 1972). 



30 



