primarily taking place at the seaward ends 



of selected distributaries and that 



marshland loss was beginning to take 

 place. 



By 1971 a large part of the crevasse 

 system was being inundated by marine 

 waters, and marsh loss was becoming 

 significant. The only deposition was at 

 the seaward ends of some of the 

 distributaries and subaqueously in the bay 

 fill front. Note that land loss begins 

 first near the crevasse break. Here 

 sedimentation is extremely slow, depending 

 only on overbank flooding, whereas higher 

 sedimentation rates are still prevailing 

 near the distal parts of the crevasse 

 system. Figure 21 illustrates the 

 crevasse growth and deterioration. 



Figure 22 shows on a single plot the 

 cyclic nature of four of the Mississippi 

 River crevasses; each cycle consisted of 

 growth followed by deterioration. 

 Projection of the present-day trends 

 indicates a life cycle for a crevasse 

 system that lasts 115 - 175 years. 



% 10 



Growth rates during progradation ranged 

 from 0.8 kmVyr to 2,7 kmVyr. 

 Degradation rates averaged from 1.0 to 4.1 

 kmVyr. 



This growth and deterioration cycle 

 of bay fills, although representing a 

 relatively short time period, is similar 

 to the cycle of major delta lobes de- 

 scribed earlier. The delta cycle is on a 

 much longer time scale - a growth period 

 that approaches 800 - 1,000 years and a 

 deterioration period that can be as long 

 as 2,000 years. These bay fills provide 

 an excellent model for evaluation of 

 the future growth of the newly formed 

 Atchafalaya Delta (Wells et al. 1982) and 

 for the deterioration of the former 

 Mississippi River delta lobes. 



The composite curve in Figure 22 

 shows a peak in the early 1940' s, followed 

 by a rapid loss of marshes that continues, 

 with a tenporary reversal during the flood 

 years of the 1970' s, to the present. The 

 rapid degradation of this delta lobe, even 

 though river flow has been maintained, is 

 not well understood. In the Mississippi 

 River Deltaic Plain as a whole the same 

 rapid marsh loss is found. This is more 

 understandable since, with the exception 

 of the Atchafalaya Delta, the other 

 hydro! ogic units are all abandoned, 

 degrading lobes. Across the delta the 

 marsh loss rates have been accelerating 

 rapidly during this century to the present 

 rate of 1.5 percent per year or about 100 

 km^/year (Gagliano et al . 1981; Figure 23, 

 24). 



This rapid degradation rate is cause 

 for considerable alarm. Strong evidence 

 supports the contention by many that 

 superimposed on the natural 

 processes described in this 

 newer changes, both natural 

 that are strongly affecting 

 marshes today. These changes 

 1 ocal to global . 



geomorphic 



section are 



and human, 



the coastal 



range from 



Figure 21. Linear, areal , and volume 

 growth curves for the Cubits Gap subdelta 

 (Wells et al. 1982). 



At the global scale the rate of 

 sea-level rise has accelerated in recent 

 years, as has been discussed (Figure 11). 

 The acceleration has been imputed to the 

 increase in the atmosphere's carbon 

 dioxide resulting from burning fossil 

 fuels and clearing forests. Increased 

 carbon dioxide in turn creates a 



23 



