While the previously described process is being repeated in the newly 

 created subdelta, important events are continuing in the older area. First, 

 with the decrease in frequent flooding and deposition of inorganic sediments, 

 vegetation density increases over the broad interdistributary areas. The 

 formation of peat layers proceeds with little contamination from the inorganic 

 sediments. Second, the dense bar fingers and adjacent levees continue to weigh 

 down on the underlying water-saturated clays deposited in open water (before 

 delta progradation began). The compaction of the underlying clays allows the 

 bar fingers to accumulate a thickness not possible in their depositonal environ- 

 ment alone. Finally, as this subsidence proceeds with little addition of 

 inorganic deposits, open water gradually covers more and more land. Depressions 

 along levee flanks form, enhanced by the subsidence of the levees and bar 

 fingers. Ponds in the middle of the interdistributary marsh expand and 

 coalesce. These processes of abandonment take place more gradually than those 

 of delta construction. 



As open-water areas expand, increased wave erosion occurs because of 

 increased fetch. Longshore currents smooth the coastline of the delta. 

 Destruction of the delta proceeds more rapidly until an abandoned distributary 

 is reopened, reinitiating the process of delta growth. 



The angle at which new distributaries leave the main channel of the river 

 (when levee crevasses form) determines the success of that potential subdelta 

 growth. Acute angles of distributary formation allow large volumes of water- 

 relative to sediment transport - to pass through the channel. In contrast, 

 as the angle of departure from the main channel increases to and beyond 90°, 

 the proportion of water volume to sediment load decreases. Under such condi- 

 tions, the new channel tends to shoal rapidly. Furthermore, distributary 

 networks leaving the main channel at acute angles tend to exhibit simple 

 branching patterns with few major distributaries. Because of the greater 

 relative sediment load, bifurcation of distributaries departing at a greater 

 angle results in complex branching and anastomosing of channels (Russell, 1936). 

 The vast branching network of natural channels of the Delta NWR is, therefore, 

 attributable to simple geologic processes. 



The Cubit's Gap subdelta, bound on the south by Pass a Loutre, on the west 

 by the Mississippi River, and on the north by Baptiste Collette subdelta, has 

 developed via constructional and abandonment phases since 1838. The subdelta 

 rapidly prograded between 1869 and 1905. Since 1932, abandonment of the sub- 

 delta has given way to the destructional phase, as evidenced by the increased 

 acreage of open-water bodies (Morgan, 1972). The decline of the subdelta may 

 in part be attributed to (1) decreased sediment inflow, resulting from the arti- 

 ficial leveeing of the Mississippi River and (2) bayor and man-made cuts within 

 the Cubit's Gap subdelta. 



Climatology . The climate of this region is typicaly of northern gulf 

 coastal areas - warm and humid with hot summers. The prevailing winds provide 

 a moderating effect on daily temperature extremes in summer and winter. Precipi- 

 tation maxima occur in summer and winter/early springs; minima occur during 

 autumn. Mean annual precipitation is nearly 152 cm, or 60 inches (Gagliano et 



70 



