when log jams were removed from the 

 Atchafalaya River did the flows begin to 

 increase steadily. In the early 1950' s 

 it was postulated that the main flow of 

 the Mississippi River would eventually 

 be diverted to the Atchafalaya course 

 (Fisk 1952). To prevent this from hap- 

 pening, the USACE constructed a control 

 device, the Old River Structure, at 

 Simmesport, Louisiana (Fisk 1952). 

 Completed in 1963, this structure was 

 designed to limit the Atchafalaya' s flow 

 to 30% of the combined flow regimes of 

 the Mississippi and Red Rivers. During 

 periods of peak flow, more than 30% of 

 the total Mississippi drainage has 

 flowed through the structure, and peak 

 flood stages produce most sedimentation 

 (Roberts et al . 1980). 



Recent Changes in the Atchafalaya 

 Hydrologic Unit 



Increasing sediment loads have been 

 deposited into the lower Atchafalaya 

 basin above Morgan City during the past 

 50 years or more. The Atchafalaya River 

 flows through a system of freshwater 

 lakes, swamps, and bayous. Most of the 

 larger lakes in the lower basin (Grand 

 Lake and Six Mile Lake) were filled 

 between 1917 and 1960. With loss of the 

 lake storage and channelization of the 

 main streams, sediment was carried into 

 Atchafalaya Bay in increasing volumes. 

 From 1858 to the early 1950' s, little 

 change was noted in the bathymetry of 

 Atchafalaya Bay (Thompson 1951). During 

 the 1950's and 1960's, increasing vol- 

 umes of silt and clay contributed to the 

 beginning of a subaqueous delta at the 

 mouth of the Atchafalaya River and Wax 

 Lake Outlet (Cratsley -1975; Shlemon 

 1975). 



By 1972, a layer of sediment about 

 2 m thick had been deposited in Atcha- 

 falaya Bay. In the floods of 1973-75, 

 the large sediment load (mostly sand) 

 increased from the average 42.6 x 10 6 

 metric tons/yr to 88.9 x 10 6 metric 

 tons/yr (Roberts et al. 1980). Between 

 1973 and 1975, a well-developed sub- 

 aerial delta emerged (Rouse et al. 

 1978). By 1976, more than 1400 ha 

 (3,500 acres) of subaerial delta were 



present (Roberts et al . 1980). The 

 growth of the delta from 1967 to 1977 is 

 illustrated in Figure 56; the change in 

 all habitat categories from 1955 to 1978 

 are indicated in Table 22. 



Recent changes in land use reflect 

 the rapidly increasing influence of 

 water and sediment inputs and the eco- 

 nomic influence of petroleum recovery. 



The basin consists primarily of 

 habitats controlled by riverine pro- 

 cesses. Most of the hydrologic unit is 

 made up of estuarine open water, fresh 

 marsh, and cypress-tupelo swamp (Table 

 22) . Major changes in habitat types 

 from 1955 to 1978 reflect the increasing 

 spring flooding with the concommitant 

 sediment input (Baumann and Adams 1932). 

 During that time period there was a 

 complete loss of salt marsh from the 

 area; a 140% increase in fresh marsh; an 

 increase in fresh aquatic beds, rivers, 

 streams, and fresh scrub-shrub habitat; 

 and a loss in area of estuarine open 

 water. Most of this change is related 

 to the increase in flooding and sedi- 

 mentation. 



During the same period, area 

 covered by spoil banks and canals has 

 increased 91% and 105%, respectively, 

 reflecting the importance of the basin 

 as a site for oil and gas activity. 

 Urban industrial and agricultural hab- 

 itats have increased 49% and 41% in 

 area, respectively, the former related 

 to oil production, the latter to the 

 lucrative market for soybeans. 



The primary economic activity in 

 this basin, based on revenues, is oil 

 and gas drilling. The Atchafalaya 

 River and associated channels and 

 canals provide a transportation network 

 to offshore oil and gas rigs. The popu- 

 lation centers in the lower basin — 

 Morgan City, Berwick, Amelia, and 

 Patterson — are predominantly areas of 

 boat-tug-barge construction, and oil rig 

 construction, assembly, supply, and 

 support. Before the oil boom, economic 

 activity in the lower basin was based 

 primarily on renewable resources 

 (shrimping, sugar cane, and lumber), but 



135 



