38 



coasts, salt marshes appear to be maintaining themselves 

 through marsh building or accretion despite a worldwide 

 rise in sea level. In Louisiana, however, this is not true as 

 large expanses of coastal marshes are being permanently 

 flooded by rising sea level (Figure 34). Vertical marsh 

 accretion has not kept pace with coastal submergence 

 over the past 30 years. The marsh is accreting at a rate of 

 0.33 inches yearly, while submergence is occurring at 0.5 

 inches per year (DeLaune, et al. 1983). The rate of subsi- 

 dence here is more than five times as high as the average 

 rate of global sea level rise over the past century ( Boesch. 

 et al. 1983). Currently, an estimated 40 square miles or 

 25,000 acres of coastal marshes are lost each year (Fruge 

 1982). Besides direct losses, salt water intrusion is killing 

 freshwater vegetation in tidal freshwater marshes and 

 converting these types to more brackish wetlands or open 

 water. It also has accelerated the advance of the preda- 

 ceous oyster drill into productive oyster beds. 



The causes of Louisiana coastal marsh loss are numer- 

 ous and complicated (Craig, et al. 1980). A combination 

 of factors both natural and man-induced are responsible. 

 Coastal subsidence, rise in sea level and the cyclical pro- 

 cesses of Mississippi River Delta growth and deteriora- 

 tion represent the major natural forces. The Mississippi 

 River is trying to shift its course into the Atchafalaya 

 River, but the U.S. Army Corps of Engineers is only 

 allowing 30% of the Mississippi and Red River flows to 



be moved down the Atchafalaya. This is still enough to 

 get some marsh building in Atchafalaya Bay. An estimat- 

 ed 1 20.000 acres of marsh will be created here in the next 

 30 to 50 years, but this will not offset heavy marsh losses 

 in other areas of Louisiana (Louisiana State University 

 1983). Man"s impacts include channelization and levee 

 construction along the Mississippi River, canal dredging 

 for navigation and energy operations, and subsidence 

 from extraction of groundwater, minerals, oil and gas. 

 Channelization and canal construction have increased 

 marsh erosion and salt water intrusion along the coast. 

 Man-made levees have disrupted the natural marsh build- 

 ing process by preventing overflow of sediment rich 

 waters. 



Efforts must be made to reduce man's adverse impacts 

 on Louisiana's coastal marshes. Specific wetland preser- 

 vation and restoration actions should be taken immediate- 

 ly. These actions include diverting Mississippi and 

 Atchafalaya River flows into areas experiencing salt wa- 

 ter intrusion and accelerated wetland loss, creation of 

 new marsh through careful placement of dredged materi- 

 al, improved water management in existing marsh areas, 

 and reducing petroleum industry canal dredging through 

 increased use of directional drilling. Future research stud- 

 ies should improve our understanding of the importance 

 of causal factors and address mechanisms to improve the 

 future for this rapidly diminishing resource. 



"MISSISSIPPI RIVER ACTIVE DELTA (195^ 



Courtesy of USFWS National Coastal Ecosystem Team 



i MISSISSIPPI RIVER ACTIVE DELTA 0978) | 



us RSH A WlDLfE SEHVCE 



WSnONAL COftSTW- ECOSrSTtMS TEAM 



SLCex. UXJGUNA 



US nSH « VflLDLFE SERVICE 



NATIONAL COASTRL ECOSYSrae TEAM 



SUXU,U3UISUVNA 



Fig. 34. Louisiana's coastal marshes are being permanently flooded by Gulf of Mexico waters at an accelerating rate. Example shows marsh 

 changes between (a) 1956 and (b) 1978. 



