These meteorologically driven water 

 level changes are common events. Tropical 

 storms are much more unusual . When they 

 occur water levels can be dramatically 

 elevated. The water level height/fre- 

 quency curve for Shell Beach, southeast of 

 New Orleans (Figure 12), shows that wind 

 tides as high as 3.5 m have been recorded, 

 and l.S-m tides occur about once every 

 eight years. On a coast with a slope of 

 about n.2 mm/km (Byrne et al . 1976) a 

 1.5-m tide can cause flooding hundreds of 

 kilometers inland. The ecological effects 

 of such flooding can be dramatic. 



GEOLOGICAL PROCESSES 



The Mississippi River, the largest 

 river systan in North America, drains an 

 area of 3,344,560 km^ (Coleman 1976). The 

 average discharge of the river at the 

 delta apex is approximately 15,360 cimecs 

 with a maximum and minimim of 57,900 and 

 2,830 cijmecs, respectively. Sediment 

 discharge is generally about 2.4x10^^ kg 

 annually. The sediments brought down by 

 the river to the delta consist primarily 

 of clay, silt, and sand. The sediments 

 are 70 percent cl ay. 



The river has had a pronounced 

 influence on the development of the 

 northern Gulf of Mexico throughout a long 

 period of geologic time. In the Tertiary 

 Period (70 - 1 million years before the 

 present) the large volumes of sediment 



< 



UJ 

 CO 



z 

 < 



> 

 O 

 CD 



< 



100 50 



OCCURRENCES/ 100 YEARS 



Figure 12. Tide levels at Shell Beach, in 

 the Pontchartrain-Lake Borgne basin, 

 associated with nine major storms (Wicker 

 et al. 1982). 



brought down by the Mississippi River 

 created a major sedimentary basin, and 

 many of the subsurface deposits, 

 especially those that formed in localized 

 centers of deposition, have been prolific 

 hydrocarbon-produci ng reservoi rs . 



In more recent geologic times, 

 changing sea levels associated with the 

 advance and retreat of inland glaciers 

 during the Pleistocene Ice Ages have 

 strongly influenced the sedimentary 

 patterns off the coast. In order to 

 understand the development of the 

 present-day coastal wetlands it is 

 necessary to view the progradation of the 

 delta and its adjacent coastal plains in 

 relationship to several time scales. 

 These scales range from the long periods 

 of geologic time associated with changing 

 sea levels to the changes in the last 100 

 years in the patterns of minor subdeltas 

 that foniied the most recent deltaic lobe, 

 the Balize Delta. In addition, the heavy 

 sediment load deposited by the river 

 during the last several million years has 

 caused excessive subsidence. This factor 

 has to a large degree controlled the 

 construction rate and the rate of coastal 

 wetland loss throughout much of the recent 

 geologic history. 



Pleistocene Sea Levels 



During the Pleistocene Epoch, some 

 1.8 - 2.5 million years long, sea level 

 fluctuated several times. Most 



authorities agree on at least four major 

 low sea-level stands and four or five high 

 level stands. In addition to these major 

 changes in sea level, numerous more rapid 

 fluctuations took place. The minor 

 changes in level undoubtedly affected the 

 development of the delta marshes, but in 

 the younger Pleistocene deposits it is 

 extremely difficult to document the pre- 

 cise changes. At the lower sea-level 

 stands, the ocean surface was 150 - 200 m 

 below its present level. During the 

 higher stands water surfaces were slightly 

 above or near present sea level. These 

 fluctuations resulted in periodic valley 

 cutting during the low stands and valley 

 filling or terrace formation during the 

 high sea-level stands. This concept is 

 diagrammed in Figure 13. Fisk's 1944 

 paper should be consulted for details of 



14 



