1800 



1820 



1840 



1860 



1880 1900 

 Date 



1920 



1940 



1960 



1980 



Figure 22. Composite subaerial growth curve, Mississippi River 

 subdeltas. Total subaerial land determined from averages at 10-yr 

 internals (Wells et al, 1982). 



"greenhouse" effect that is wanning the 

 earth's surface and melting the polar ice 

 caps. The net affect of both true sea- 

 level rise and coastal subsidence has been 

 a change in the coastal submergence rate 

 from about 0.27 cm/yr during 1948 to 1959, 

 to nearly 1.3 on/yr between 1959 and 1971. 

 Although these data are for a gauge at 



1.5 



1 25- 



1 0- 



0-5 



Years spanned by estimate 



Midpoint Reference 



1 Adams et at 1976 



20ozier 1983 



3Gaqliano f. Van Beck 1970 



1 940 

 YEAR 



Figure 23. The accelerating wetland loss 

 rate in the Mississippi Delta (based on 

 data from Dozier 1983). 



Bayou Rigaud in the Barataria basin, the 

 trend is similar along the whole Louisiana 

 coast (Gosselink et al . 1979). 



In order to remain at intertidal 

 elevations marshes must accrete vertically 

 as rapidly as they are sinking. The rapid 

 rate of marsh degradation indicates that 

 they are not doing so, an observation 

 supported by recent research (Delaune et 

 al. 1983). One reason is that the 

 Mississippi River no longer supplies as 

 much sediment to the coast as it has 

 historically. Keown et al . (1980) 

 reported that sediment supplies are only 

 about 60 percent of what they used to be, 

 despite the presumed increase in erosion 

 that accompanies forest clearing on the 

 upper watershed. The reduction is 

 presumably due to the construction of dams 

 on the upper reaches of the river and its 

 tributaries. The dams also remove the 

 coarser sediments selectively, so that the 

 sediments reaching the coast are depleted 

 of the sand that is the main foundation 

 material for delta growth. This means 

 that the river can no longer support as 



24 



