Three patterns have been recognized during the 12-year period from 1969-81: (I) 

 simultaneous erosion and accretion at the shoreline, (2) increasing length of shoreline 

 fronted by mudflats, and (3) shift in the locus of sedimentation to the west. No attempt 

 has been made to plot previous shorelines, and our contention is simply that the presence 

 of mudflats indicates an instantaneously prograding shoreline. The segments of coast 

 between mudflats are typically those that are eroding most rapidly. The processes of 

 erosion and accretion are cyclical in both time and space, as becomes evident from close 

 examination of Figure 3. 



ATCHAFALAYA/CHENIER PLAIN CONNECTION 



Turbid water that enters the Gulf of Mexico from the Atchafalaya River and flows 

 along shore as a muddy plume is herein described as the Atchafalaya mud stream. This 

 sediment-laden water is visible from aircraft and shows up well in LANDSAT imagery as 

 partially saturated returns in band 5. Mud stream dimensions vary and are controlled by 

 river discharge, tide stage, wind speed and direction, and residual currents. The plume 

 persists, however, throughout the year and trails off to the west in approximately 75% of 

 the images (unpublished data compiled by R. H. W. Cunningham, USACOE, New Orleans). 



The well-defined seaward extent of the sediment plume on 9 February 1979 during 

 rising river stage is evident in Figure 4. This image is typical of many in that turbid 

 water is found not only in Atchafalaya Bay and offshore, but also in bays to the west. 

 The inset to Figure 4 shows suspended sediment concentrations taken on the day of the 

 satellite overpass along a transect that runs down the navigation channel and ends at the 

 seaward edge of the sediment plume. Suspensate concentrations, determined by 

 millipore filtration, are reported for surface waters only, and thus represent a 

 conservative estimate of sediment throughout the water column. 



Within Atchafalaya Bay concentrations range from 250 to 400 mg/l (0 to 20 km. 

 Figure 4, inset), but increase to more than 800 mg/l seaward of the shell reef barrier (25 

 to 35 km). The sudden increase in concentration is perhaps a result of wave resuspension 

 of soft sediments that are deposited rapidly as prodelto clays seaward of the bay mouth. 

 Beyond this extremely turbid zone, concentrations decrease across the shelf to the plume 

 edge (50 to 63 km). Outside the sediment plume, concentrations are I mg/l or less. 



Composition of sediment in the mud stream is the same as that in the lower 

 Atchafalaya River, primarily silt- and clay-sized particles with median diameters of 2 to 

 6 microns. Clay mineralogy is montmorillonite, illite, and kaolinite in the ratio 3:1:1. 

 Data reported by Roberts et al. (1980) indicate that 63% of the sediment that enters 

 Atchafalaya Bay is silt and clay sized. Using a mass-to-volume conversion of 425 kg/m , 

 Wells and Roberts (1981) determined that this silt and clay load is 146 X 10 m-^ per year. 



Evidence that sediments which enter the Gulf of Mexico from Atchafalaya Bay are 

 transported to the west, as indicated by satellite imagery, is also provided by current 

 meter moorings. Beginning in the spring of 1980, current meter data were taken at 

 numerous stations in and seaward of Atchafalaya Bay. Typical records of speed and 

 direction at three of these stations are shown in Figure 5. Data are from mid-depth 

 current meter moorings made with Endeco 174 ducted-impeller, magnetic recording 



44 



