Figure 16. Deltaic lobes of Mississippi River deltas (modified from 

 Kolb and Van Lopik 1958). 



increased rapidly. Initially the 



interior marshes deteriorated, and the 

 coastal barrier islands were attached to 

 the ends of the fonner distributaries. 

 Eventually the Lafourche Delta system 

 reached its maximum development and the 

 modern delta lobes (Plaqueinine and Balize) 

 began their progradation. The Lafourche 

 Delta was then subjected to marine 

 reworking and compaction. 



During the past 800 or so years 

 subsidence in the St. Bernard Delta has 

 reached a stage in which little or no 

 freshwater marshes exist, and the 

 reworked barrier islands have been sepa- 

 rated from the mainland. During this same 

 period the Lafourche Delta has lost land, 

 mainly by saltwater intrusion and opening 

 of the marshland behind a coastal barrier 

 still attached to the fonner distributar- 

 ies. 



Meanwhile, in the modern Balize Delta 

 the river has constructed a major delta 

 lobe. The river would abandon this lobe 



in favor of the Atchafalaya River course 

 if manmade river control structures at 

 Simmesport did not limit diversion to 

 about one-third of the Mississippi River's 

 discharge. Even with this limited flow 

 the modern Atchafalaya River will continue 

 to build its delta onto the continental 

 shelf for the next several hundred years. 



Modern Mississippi Delta 



The modern Balize Delta has been 

 constructed during the past 500 years. 

 Because it is relatively young, it offers 

 an opportunity to evaluate the short-term 

 processes responsible for delta building 

 and deterioration. When a break (or 

 crevasse) occurs in the levee of one of 

 the river distributaries, water rushing 

 through the break deposits sediment in the 

 adjacent bay. These bay fill deposits 

 form the major coastal marshes of the 

 subaerial delta. Figure 18 illustrates 

 the bay fill sequences within the modern 

 delta during the past few hundred years. 

 Of the six crevasses shown, four have been 



18 



