record flood of 1973. In 1974 little 

 additional deposition, except for 

 dredge material, was noted because of 

 the absence of an extreme flood crest 

 (B) . In 1975 another major flood peak 

 doubled the bar exposure to about 

 12 mi (30 km ) (D) (Rouse et al. 

 1978). 



Another high water year was ex- 

 perienced in 1979 (Figure 3). Data 

 for a new topographic survey are cur- 

 rently being compiled. Preliminary 

 indications from recent aerial photo- 

 graphy and island transects show 

 marked aggradation or vertical build- 

 up of existing islands, welding of 

 several islands, and a reduction in 

 the number of active distributaries. 

 Only a small increase in the area 

 of the delta was noted, indicating 

 the possibility of a new phase of 

 deltaic development — a subaqueous ma- 

 rine delta, forming just seaward of 

 the Point Au Fer shell reef (Van 

 Heerden 1980). 



VEGETATION RESPONSE 



The following table lists the 

 acreages of vegetation colonizing the 

 emergent islands: 



1978 1,113 



1,783 



(1 acre = 0.0040 km ) 



Vegetation progradation showed a 

 marked increase during a year fol- 

 lowing major flood, as was evident 

 the year following the 1975 flood 

 (Sasser personal communication). A 

 slight decrease was noted during sub- 

 sequent low water years due to a 

 lack of sediment nourishment and 

 erosional processes. Figure 8 il- 

 lustrates vegetation progradation, 

 covering approximately 4.5 mi (11.7 

 km ) of the delta as of the end of 

 the 1978 growing season. Preliminary 

 results from a 1980 vegetation inven- 

 tory indicate another large increase 

 in acreage resulting from the 1979 

 flood. All developing habitats in 

 the delta have been documented as 

 being freshwater habitats (Montz 

 1978). 



REGIONAL IMPACTS OF DELTAIC PROCESSES 



During the investigation of sub- 

 aerial delta growth in Atchafalaya 

 Bay it became apparent that the im- 

 pact of the sediment-laden discharge 

 from the Atchafalaya River extended 

 far outside the confines of Atcha- 

 falaya Bay. In fact, estimates indi- 

 cate that less than 50 percent of the 

 fine-grained sediment transported to 

 the bay is actually deposited there. 

 Satellite imagery, available since 

 1972, tends to support this conclu- 

 sion (Figure 9). Turbid flows were 

 found to impact as little as 300 mi 

 during Low discharge and as much as 

 1,200 mi during flood periods. 



To study these regional impacts, 

 Landsat images collected between 

 1972-1977 were studied. Turbidity 

 patterns which appear on the imagery 

 were excellent indicators of the 

 major components of estuarine circu- 

 lation when correlated with a mathe- 

 matical model tuned with wind and 



358 



