example, in the case of the modern Mississippi delta front, retreat is virtually 

 nonexistent and in the case of the Atchafalaya Delta complex, it is significantly 

 reversed. We can then draw a picture of a modern shoreline that is undergoing erosion 

 and transgression, but that is dynamically stable at the Mississippi River delta front and 

 is locally progradational near the Atchafalaya River mouth. 



The 200-km section of shoreline extending west from Marsh Island to the Texas 

 border is distinct in plan view from the rest of the Louisiana coast (Figure I). The 

 complex indentations and barrier/lagoon systems that characterize the shorelines 

 flanking the modern Mississippi River course are not found west of Vermilion Bay. The 

 smooth and relatively straight form of the western half of the coast reflects a 

 depositional history different from that of the rest of Louisiana's coastal plain. Early 

 workers hypothesized that this section evolved during the Holocene as a marginal deltaic 

 sequence of prograding mudflats that were intermittently partially reworked into 

 sand/shell ridges called "cheniers" (Russell and Howe 1935; Price 1955). More recently, 

 Gould and McFarlan (1959) reconstructed the development of the "chenier plain" and 

 adjacent shelf from cores using radiocarbon dating techniques. Their interpretation 

 indicates that, as sea level rose from -5 m to its present level, a transgressive sequence 

 of marine sediments was deposited over the dissected Pleistocene Prairie Formation, 

 first filling estuaries, then later spreading across shallow bay and marsh environments. 



During the final asymptotic stage of post-glacial rise in sea level some 5,000 years 

 ago, the chenier plain began to prograde rapidly, and eventually a wedge of recent 

 sediments 6 to 8 m thick was deposited to a width of 24 km, thus placing the shoreline 

 roughly where we see it today (Figure I). Pulses of sediment from the Mississippi River, 

 transported by coast-parallel currents, were responsible for the various stages of 

 progradation. At times when the Mississippi River introduced sediment in the vicinity of 

 the present chenier plain, the shoreline shifted seaward; during periods when its course 

 took the discharge farther east, sediment influx to the chenier plain was low and wave 

 attack was able to slow or halt the advance (Gould and McFarlan 1959). Cheniers formed 

 during these latter periods and now stand as "islands" in the marsh. 



A new pulse of sediment, the first in some 500 to 1000 years, began adding soft 

 muds to the eastern margin of the chenier plain in the late I940's, coincident with the 

 subaqueous development of a new delta in Atchafalaya Bay (Morgan et al. 1953). 

 Although the delivery of sediments from the Mississippi River down the Atchafalaya 

 River had been in progress since the mid-1500's (Fisk 1952), it was not until the mid- 

 I900's that sedimentation in the bay and areas offshore became noticeable. This large- 

 scale introduction of silts and clays to the coast began when the inland Atchafalaya Basin 

 to the north became essentially sediment filled and sediment began bypassing these 

 basin-lakes for areas to the south. In the early I950's Morgan et al. (1953) documented 

 the occurrence of mud deposition along approximately 50 km of coast from Marsh Island 

 to Rollover Bayou which, in places, formed broad mudflats up to 2 m thick. 



Nearly 30 years have passed since Morgan et al. (1953) first described these coastal 

 mudflats and tied their origin to the Atchafalaya River, to the east. Whereas our 

 understanding of the basic processes for delivering sediments to the eastern margin of 

 the chenier plain (Figure I) has remained the same, our ability to monitor these processes 

 has improved significantly. Ready access to satellite imagery, color infrared 

 photography, and digital current-meter data now allow us to monitor remotely shoreline 

 changes and the processes that govern their behavior. In the following paragraphs we 

 report our initial findings with respect to these questions: (I) What is the present status 



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