basins, much of the sediment load 

 carried by the increasing flow was 

 deposited in these areas before it 

 reached Atchafalaya Bay (Figure 2). 

 Progressive sedimentation began to 

 drastically reduce open water areas 

 in the basin. Grand and Six Mile 

 Lakes in the lower basin filled rap- 

 idly during the period from the 

 1930' s through the 1960's. By 1975 

 only small remnants of open water 

 remained . 



It was not until the early 

 1950' s that sedimentation at the 

 coast began to initiate noticeable 

 effects. This occurred only after 

 the channel through the basin had 

 developed well enough to convey silts 

 and small amounts of sand to the bay. 

 During the 1950' s and 1960's silts 

 and clays transported to the coast 

 began to be deposited near the mouths 

 of the outlets in the bay. By the 

 early 1970' s a thick platform of 

 silty clay deposits covered not only 

 Atchafalaya Bay, but adjacent off- 

 shore areas as well. As much as six 

 feet of bay fill was deposited 

 between 1952-1972 as the delta front 

 advanced to the Point Au Fer shell 

 reef (Shlemon 1975). Prior to 1972 

 very little sand-sized sediment was 

 being deposited in Atchafalaya Bay. 



The years 1973-75 were unprece- 

 dented flood years on the Atchafalaya 

 River (Figure 3). River discharge 

 doubled normal conditions during the 

 peak flow periods. More importantly, 

 both the volume and size distribution 

 of sediments reaching Atchafalaya Bay 

 changed dramatically (Table 1). An 

 extraordinary increase in the amount 

 of sand, scoured from the basin and 

 transported to the bay, was noted 

 during this period (Roberts et al. 

 1980). 



ATCHAFALAYA DELTA DEVELOPMENT 



CHANGES IN BATHYMETRY 



Bathymetric changes in Atcha- 

 falaya Bay over the decade 1967-77 

 have been impressive. The 1967 map 

 (Figure 4) shows silty distal bar de- 

 posits represented by the 4-foot con- 

 tour were beginning development in 

 the bay. By 1972 these deposits cov- 

 ered most of the bay. 



Following the 1973-75 high water 

 years, an extensive network of sandy 

 distributary mouth bar deposits had 

 emerged in both Wax Lake and lower 

 Atchafalaya River lobes (Figure 5). 

 Approximately 15 mi of these de- 

 posits had become subaerially exposed 

 (Rouse et al. 1978). A seaward ex- 

 tending, branching network of dis- 

 tributary channels had also devel- 

 oped . 



The 1977 bathymetric chart 

 emphasizes the tremendous volume of 

 coarse-grained sandy material de- 

 posited during the 1967-77 decade. 

 Figure 6 illustrates the areas of net 

 accretion/erosion and their magni- 

 tudes in Atchafalaya Bay over the 

 period (Roberts et al. 1980). Areas 

 with accumulations of 7-8 feet are 

 generally regions of dredge spoil 

 accumulation. Small areas of bay 

 scour are noted, due primarily to the 

 increasingly restricted routes by 

 which water can exit the bay. 



As the initial subaerial phase 

 of delta growth was monitored, using 

 repetitive satellite imagery, it was 

 noted that substantial increases in 

 bar exposure became apparent only 

 after major flood crests (Figure 7). 

 Little growth was noted between these 

 flood peaks. For example, 6 mi 

 (15.5 km ) of bar exposure are shown 

 on this image (A) following the 



352 



