THE DELTAIC PLAIN 



The deltaic plain is the site of a series of seven deltaic lobes extending seaward at 

 different times during the last 6,000 to 7,000 years. Except for the modern "bird's foot" 

 delta, each lobe advanced into the shallow waters of the continental shelf and was 

 distinguished by numerous distributaries. These channels continued to bifurcate, thus 

 aiding the distribution of the river sediments and progradation of the coast. Through 

 time, the recurring channel changes created the intricate "horse's tail" pattern of levee 

 fingers extending into the wetlands. Fluvial-marine materials deposited in the prodelta, 

 interdistributary and intradelta environments built up an estimated 75% of the deltaic 

 plain (Kolb and Van Lopik 1958; Frazier 1967). Most of this land is an abandoned subdelta 

 composed of alluvial ridges, beaches, marsh and water surface, where accretion has been 

 replaced by subsidence and erosion. 



in the paludal environments, the organic bulltongue (Sagittaria ) and other grass- 

 derived materials develop in place. They are not altered by alluvial deposits. In these 

 tracts organic material continually decays and accumulates as peats, in effect, building 

 the marsh "down" rather than "up." Decomposition maintains an organic layer that 

 thickens with subsidence to a depth of 3 to 6 m (10 to 20 ft) (Russell 1942; Kolb and Van 

 Lopik 1958). 



On a regional basis, some southeastern Louisiana surfaces may sink as much as 5 m 

 (17 ft) per century (Kolb and Van Lopik 1958). In many areas aggradation simply cannot 

 keep pace with subsidence. Small ponds often develop that expand rapidly as wind-driven 

 waves attack the poorly consolidated sediments that make up the shore (Gagliano and van 

 Beek 1970). 



Further, the construction of flood levees and the dredging of drainage, navigation, 

 petroleum, and logging canals upset the sedimentation balance, influenced salt water 

 intrusion, and disrupted the natural flow regimes. Consequently, the Mississippi's natural 

 processes were altered and erosion began to overshadow deposition. Sediments are now 

 channeled off the continental shelf. This waste of sediments deprives the coast of the 

 "material" that sustained the balance and prevents the building of new marshes. There is 

 nothing available that can offset the rapid rate of wetland loss (Fruge 1981). Salt water 

 moves inland and kills the root mat that "holds" the marsh together^ In the I90p's this 

 reversal in the natural cycle has accelerated from a loss of 17.3 km /yr (6.7 mi /yr) in 

 1913 to nearly 104 kmVyr (40 miVyr) in 1980 (Gagliano 1981). 



THE CHENIER PLAIN 



On southwestern Louisiana's near -sea -I eve I grasslands the surface is broken by a 

 series of long, narrow sand ridges, locally called cheniers (Howe et al. 1935). Referred to 

 as the chenier plain, the area was formed by wave action pushing sand up onto shore 

 (Russell and Howe 1953; Price 1955). Each chenier marks the position of a once active 

 shoreline (Schou 1967). When the Mississippi occupied one of its western courses, clays, 

 muds, and sands were carried westward by littoral currents advancing the chenier plain 

 as a mud coast. Interruptions in the progradation process allowed coarser particles to 

 accumulate as a ridge. An increase in sedimentation caused the shoreline to advance 

 leaving the conspicuous, oak-covered chenier as the region's most impressive and 

 continuous topographic feature (Howe et al. 1935). 



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