a long period when the level of the 

 world's oceans was almost 100 m (300 ft) 

 lower than It is today. While sea level 

 was low, a gigantic trough (the Missis- 

 sippi Trench) was eroded offshore of the 

 present MDPR, across the Prairie Terrace 

 Formation (the present Continental 

 Shelf). As melting continental glaciers 

 caused sea level to rise, riverborne 

 sediments from the Mississippi drainage 

 began filling the trench, and the modern 

 deltaic plain began to develop. The 

 rate of increase in sea level gradually 

 diminished until sometime between 3,000 

 to 5,000 B.P., when it stabilized at 

 nearly its present level (see Figure 4) . 



During the period when sea level 

 was rising, and since its relative sta- 

 bilization, river sediment has been 

 deposited along the nearshore portion of 

 the Continental Shelf. The magnitude of 

 this riverine transport can be appre- 

 ciated by multiplying the annual sedi- 

 ment load of the river, about 142 

 million metric tons/yr (Roberts et al. 

 1980) by the 7,000 years during which 

 the MDPR has been building. 



A key element in the geologic 

 development of the MDPR has been the 

 process of river diversion (Kolb and Van 

 Lopik 1958; Frazier 1967). About once 

 every 700 to 1000 years, the main flow 

 has been partially diverted, probably 

 during a major flood, to a shorter, 

 steeper path to the Gulf of Mexico. In 

 each case, the new path gradually cap- 

 tured the majority of the flow and began 

 to build its own delta lobe. It is gen- 

 erally agreed that there have been about 

 seven major changes in stream dominance 

 during the past 5,000 years (Frazier 

 1967), resulting in the production of a 

 series of delta lobes (Figure 5) . In 

 the area surrounding the dominant 

 stream, sediments accumulated over time, 

 first forming shallow bays, then inter- 

 tidal flats with higher natural levees 

 adjacent to the channel margins. 



The intertidal flats were colonized 

 by a variety of freshwater wetland 

 plants such as cattails, arrowhead, and 

 bulltongue. These plants augmented the 

 deposition of more sediment, forming 

 true subaerial land, which was colonized 



by woody plants, like willows. The 

 highest levees and stranded beach ridges 

 from reworked sediments were colonized 

 by upland vegetation. 



The steps by which a delta system 

 forms in the MDPR are currently being 

 quantified in detail by monitoring the 

 formation of the emerging Atchafalaya 

 Delta in Atchafalaya Bay (e.g., Roberts 

 et al. 1980). In an active delta, sed- 

 iments come directly from the river, 

 primarily during spring floods. Land is 

 built in three ways, as can be seen in 

 the newly-forming Atchafalaya system. 

 The delta aggrades as sediments are 

 deposited in shallow coastal waters 

 (Roberts et al. 1980). Deposition may 

 occur on older deteriorating peripheral 

 marshes as turbid waters flow over them. 

 Such infilling is taking place in the 

 marshes adjacent to Atchafalaya Bay 

 (Baumann and Adams 1982). Downdrift to 

 the west of an active delta can induce 

 coastline accretion as fine sediments 

 are deposited. Recent accretion along 

 the Chenier Plain coast is an example of 

 this process (Wells and Kemp 1981). 



Although the rate of worldwide sea 

 level increase slowed about 3,000 to 

 5,000 years ago (Emery and Uchupi 1972), 

 sea level has not remained static. 

 Average annual sea level along the U.S. 

 coast has been rising over the past four 

 decades at a rate of about 1.3 cm/decade 

 (Hicks 1981). In comparison, apparent 

 sea level rise from tide gauge measure- 

 ments along the MDPR coast is about an 

 order of magnitude greater than this 

 figure (over 1 cm/yr or 0.4 in/yr) . 

 Most of this anomaly is attributable to 

 subsidence in coastal wetlands (Swanson 

 and Thurlow 1973). 



Coastal submergence critically in- 

 fluences the future of the MDPR because 

 of its effects on coastal wetlands. 

 Subsidence in the MDPR can be attributed 

 to three interacting factors: (1) crus- 

 tal downwarping and associated tectonic 

 processes, (2) compaction of sediments, 

 and (3) sediment dewatering (Adams et 

 al. 1976). Marsh vegetation is sensi- 

 tive to changes in water level, and most 

 emergent wetland vegetation in Louisiana 

 has an inundation tolerance range of 



10 



