basin filled for hundreds of years, but it was not until the early I950's that swamp floors 

 and lake bottoms had accreted to a point that fine-grained sediments were transported to 

 the coast in significant quantities. In addition to basin filling, flood-control levees in 

 Atchafalaya Basin have increased the hydraulic efficiency of the river, which is 

 responsible for delivering proportionately higher loads of both fine and coarse sediment 

 to the coast. Starting in about 1952, accelerated sedimentation in Atchafalaya Bay 

 marked the beginning of subaqueous delta growth (Shiemon 1975). From that time to 

 1973 prodelta clays and silty clays aggraded the bay bottom seaward of both the Lower 

 Atchafalaya River Outlet and the Wax Lake Outlet, an artificial channel dredged in 1942 

 (Figure I). 



As a product of the abnormal 1973 flood, a disproportionate quantity of sediment 

 was transported to Atchafalaya Bay. Prior to this time only a few small shoals were 

 exposed at low tide, and these areas were primarily composed of dredge spoil from the 

 navigational channel which is maintained from the Lower Atchafalaya River Outlet 

 through the Point Au Fer shell reefs. After the massive 1973 flood (Figure 2), however, 

 numerous coarse subaerial lobes appeared on both the eastern and the western sides of 

 the river outlet. This event initiated the sand-rich subaerial phase of delta 

 development. Since that time sands have been prograding over finer prodelta clays and 

 silts. As a product of subaerial delta growth, marshlands expanded rapidly in 

 Atchafalaya Bay. 



69 ' 70 ' 71 ' 72 ' 73 ^ 74 ' 75 ' 76 77 



78 



79 



80 



81 



E 



2> 



Figure 2. Mean monthly discharge for the Atchafalaya River at Simmesport, 

 Louisiana, for 1956-1981. The dotted line represents average annual peak 

 flow, which is approximately 400,000 ft^/s. Note the abnormal discharge 

 years 1973, 1975, and 1979. 



217 



