related to petrochemical production. 

 Increased salinity in eastern and cen- 

 tral Terrebonne basin has also induced 

 the landward extension of the nursery 

 ground for brown and white shrimp. 



In addition to navigation canals, 

 the Terrebonne basin has been the site 

 of intense petroleum production activ- 

 ity. Terrebonne Parish has recently led 

 the coastal parishes in terms of numbers 

 of permit requests for marsh alteration 

 (378 in 1981), mostly for exploration 

 and rig access canals (Louisiana De- 

 partment of Natural Resources 1982). 



Oyster production has always been 

 significant in the Terrebonne Hydrologic 

 Unit. Large areas of formerly produc- 

 tive oyster ground are now closed, how- 

 ever, because of high coliform levels. 



ATCHAFALAYA HYDROLOGIC UNIT (VI) 



The Atchafalaya hydrologic unit is 

 the only coastal basin in Louisiana that 

 is presently undergoing net accretion — 

 increase in land area--rather than net 

 erosion. The Atchafalaya system is 

 characterized by the dominance of 

 riverine over marine processes. Abun- 

 dant sediments are creating a new delta, 

 with new habitats, and the unit does not 

 exhibit the wetland loss that is found 

 in the other basins. 



Since the mid-1500' s, the Atchafa- 

 laya River has been a distributary of 

 the Mississippi system. Its present 

 course to the gulf is 307 km (191 mi) 

 shorter than the modern Mississippi 

 River. The Atchafalaya route has a much 

 steeper gradient than the present Mis- 

 sissippi route. In the 1950' s, it 

 became rlear that the natural course of 

 the lower Mississippi River would even- 

 tually follow this more direct route to 

 the gulf (Fisk 1952). 



The Atchafalaya basin is bounded by 

 natural levees of the Teche Delta system 

 (3,500 years B.P.) and Pleistocene al- 

 luvial terraces on the west and by the 

 modern Mississippi and the Lafourche 

 levee systems (1,500 years B.P.) on the 

 east. The construction of artificial le- 

 vees within the basin has substantially 



reduced the width of the natural Atcha- 

 falaya alluvial plain. At Morgan City, 

 the Atchafalaya River cuts through the 

 Teche levee system, where approximately 

 70% of the flow is transported southward 

 for about 35 km (22 mi) until it dis- 

 charges into Atchafalaya Bay. The re- 

 maining 307o of the flow is discharged 

 into Atchafalaya Bay via Wax Lake 

 Outlet--a man-made channel (Roberts et 

 al. 1980). 



The entire Atchafalaya drainage 

 basin and the hydrologic unit boundary 

 are shown in Figure 55. The eastern 

 boundary of the hydrologic unit follows 

 the east bank and protection levee of 

 the river to the Avoca Island cutoff. It 

 then follows the shoreline to Point-au- 

 Fer, where it drops south to the 3-mi 

 limit. The western boundary extends 

 along the east bank protection levee of 

 Bayou Sale community to the town of 

 Burns. From Burns, the boundary follows 

 the shoreline of East Cote Blanche Bay 

 to Point Chevreuil. From Point Chev- 

 reuil, the boundary extends south to 

 South Point, following the shoreline of 

 Marsh Island to Mound Point. At Mound 

 Point, the boundary goes south to the 

 3-mi limit (Wicker et al. 1980b). 



These boundaries exclude areas to 

 the east and west that are influenced by 

 the flooding of the Atchafalaya River. 

 Although the Terrebonne basin as a whole 

 has a high rate of wetland loss, some 

 areas adjacent to the lower Atchafalaya 

 River have accreted during the past 

 decade as a result of riverine sediment 

 inputs (Baumann and Adams 1982) . Only 

 about 40% of the total suspended sedi- 

 ment load transported by the lower 

 Atchafalaya River and Wax Lake Outlet 

 was retained in Atchafalaya Bay during 

 the period from 1967 to 1977. The 

 remaining sediments, all fine grained, 

 were transported to peripheral marshes 

 in the Vermilion-Cote Blanche Bay com- 

 plex, and offshore, where the predomi- 

 nantly westward-trending currents carry 

 material to southwestern Louisiana. 



Geological History 



The distribution of flow between 

 the Mississippi and Atchafalaya Rivers 

 has not been constant. Not until 1839, 



133 



