Generally one of the channels formed in a bifurcation is smaller than the other. 

 The smaller slowly loses hydrodynamic efficiency and eventually seals owing to 

 subaqueous levee formation. Thereafter it fills with fine-grained sediment and fuses 

 with adjacent lobes. Thus larger lobes form as a result of coalescence of numerous 

 smaller distributary-mouth bars and adjacent channels (Figure 1 1). 



IMPLICATIONS OF DELTA BUILDING 



Diversion of Mississippi River fresh water and sediment to the central coast of 

 Louisiana will steadily influence the future character of coastal environments in the 

 immediate vicinity of Atchafalaya Bay and its adjacent downdrift coasts, in conjunction 

 with man-made flood control measures, filling of the Atchafalaya Basin, a natural 

 sediment sink, has promoted transport of sediments in significant quantities to the coast 

 since the early I950's. The initial sediments to impact the central Louisiana coast from 

 this progression of events associated with "delta switching" were fine grained. They 

 started a regressive phase that will replace the traditional erosional trends that have 

 characterized central and western Louisiana coasts for hundreds of years. 



In addition to simply supplying sediment to nearshore depositional sites, aggraded 

 bay bottom and resulting delta development have influenced the hydrography of 

 surrounding marshlands. For example, flood levels at Morgan City and in adjacent 

 marshes average over 0.3 m (1.0 ft) higher than in pre-delta years (U.S. Army Corps of 

 Engineers 1974). This change has resulted from the inefficient dispersal of flood waters 

 because of the obstructive effects of deltas at the mouths of both the Lower Atchafalaya 

 River and Wax Lake outlets. Elevated flood levels have the net effect of driving 

 sediment-laden water into marshes lying generally between the Grand Lake-Six Mile 

 Lake complex and the coast (Baumann and Adams in press). It is suggested that this 

 process tends to cause an increased increment of yearly sedimentation which results in 

 aggradation of the marsh surface at a higher rate than in pre-delta years. 



Another set of processes, winter cold-front passage, also accounts for abnormal 

 elevation of water levels in coastal marsh areas surrounding Atchafalaya Bay. Figure 12 

 illustrates a record segment (January 1978) from a tide gauge located at the Amerada 

 Hess platform (Figure 3) on the western side of the Atchafalaya Delta. Water level 

 changes in the bay associated with a cold-front passage and tidal effects are shown on 

 this figure. Winds preceding a cold front generally blow from a southerly quadrant, 

 which promotes setup or water-level elevation in the bay (Figure 12, up to 2100 hr on 16 

 January). It is during this phase in cold-front-related events that local wave action 

 suspends sediments and high water levels force turbid water into the coastal marshes. As 

 the cold front crosses the area from northwest to southeast, winds switch to a northerly 

 quadrant and cause rapid setdown (Figure 12, after 2100 hour on 16 January). Swift 

 movement of water out of the bay, coupled with wind-wave action, is responsible for 

 erosion and redistribution of sediment within the delta (van Heerden and Roberts 1980a). 



The similarity of water level response to cold-front passages at three sites in 

 Atchafalaya Bay is illustrated in Figure 13. The magnitude of the mean fluctuations 

 decreases from Eugene Island to the Lower Atchafalaya River mouth. Maximum average 

 water levels at Deer Island, near the mouth, were nearly 92 cm (3.0 ft) above mean sea 

 level during this study period (January 1979-April 1980). These elevated coastal water 

 levels initiate overbank flooding of surrounding marshes, which promotes aggradation of 

 the marsh surface. 



225 



