PROCESSES OF WETLAND EROSION 

 IN THE MISSISSIPPI RIVER DELTAIC PLAIN 



Sherwood M. Gagliano and Karen M. Wicker 



Coastal Environments, Inc. 



Baton Rouge, LA 70802 



ABSTRACT 



A process-response study of changes associated with deterioration and erosion of wetlands in 

 the Bayou l'Ours subdelta area of the Barataria Basin, LA was made. Using geological, 

 archaeological, and historical data along with aerial photographs and field observations, natural 

 processes and their acceleration because of human activities were examined. Implications of these 

 findings with regard to wetlands management were identified. 



The prehistoric progradation of Mississippi River distributaries formed a complex of natural 

 levee ridges. After abandonment of the subdelta, fresh and intermediate marshes, characterized 

 by unbroken carpets of grass with little channelized flow, formed in low-energy, naturally 

 impounded, and semi-impounded hydrologic units defined by the ridges. Floating marsh surfaces 

 and highly organic soils formed and were held in place by the skeletal framework of elevated 

 alluvial ridges and lake rims. 



Hydrologic and water chemistry conditions were abruptly changed during recent decades when 

 the ridges were cut by canals dredged for oil and gas activities. The canals provided avenues 

 through which marine tidal processes invaded marshes composed of fresh to intermediate salinity 

 tolerant grasses. 



Under natural conditions, tidal invasion of fresh marshes is slow and mineral sediment accumu- 

 lation along tidal channels and lake rims protects interior organic soils. Canals cause rapid invasion 

 of tidal processes and massive marsh die-back followed by rapid tidal scouring of organic soils. 



The Bayou l'Ours area provides a model for the effects of marine tidal invasion on high organic 

 substrate marshes that has important implications for management. Areas most susceptible to 

 erosion and deterioration can be identified. Early symptoms of tidal invasion can be detected and 

 remedial measures can be taken which may result in the prevention or at least retardation of marsh 

 loss. 



INTRODUCTION 



Changes associated with deterioration and erosion of wetlands were studied in the Bayou l'Ours 

 subdelta area of the Mississippi River deltaic plain, LA (Figure 1). A process-response approach 

 was taken. The geological history of the area was reconstructed and historic data were analyzed 

 to gain a better understanding of the processes that have unfolded during modern decades. The 

 work was initiated in reference to several marsh management projects, but has led to a better 

 understanding of the transgressive or destructive phase of this old subdelta and how these 

 transgressive processes have been accelerated by human activity. 



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