these have been overshadowed by the 

 petroleum industry in recent years. 

 Shell dredging in Atchafalaya Bay is 

 still a major economic activity, al- 

 though limestone from out of State is 

 becoming economically competitive with 

 shells (R. Dugas, Louisiana Department 

 of Wildlife and Fisheries; pers. coram.). 



The population of St. Mary's Parish 

 rose from 48,833 in 1960 to 70,831 in 

 1980, a 45% increase. This increase is 

 largely related to employment offered by 

 the oil companies and their associated 

 industries . 



Atchafalaya Bay and Associated Wetlands 



The Atchafalaya basin is strongly 

 influenced by the seasonal flooding of 

 the Atchafalaya River. High floods oc- 

 cur between January and June. The 

 average peak flows have been about 

 11,300 m 3 /sec (400,000 cfs) at Simmes- 

 port; the average annual flows have been 

 5,100 m 3 /sec (181,000 cfs) between 1956 

 and 1975 (Figure 57). During 1973 to 

 1975, the river received average yearly 

 flows of 8,800 m 3 /sec (313,000 cfs) at 

 Simmesport, with 19,800 m 3 /sec (700,000 

 cfs) in April 1973, and 17,000 m 3 /sec 

 (600,000 cfs) in April 1975. The flows 

 were similar for the same years at 

 Morgan City (Figure 58). Peak flows 

 during the flood of 1973 are suspected 

 to have been substantially greater than 

 reported at Simmesport because of the 

 opening that year of the Morganza 

 Spillway, a control structure that 

 diverts Mississippi River flow into the 

 lower Atchafalaya basin during high 

 discharge periods. 



Within the basin, precipitation 

 usually has little effect on flood stage 

 elevation or timing. The particularly 

 high flows of 1973-75 were accompanied 

 by similarly high levels of suspended 

 sediment. 



early 1970's, the majority of the sedi- 

 ment reaching Atchafalaya Bay was silt 

 and clay, because sands were being de- 

 posited until that time within the 

 basin. The overall sediment increase to 

 Atchafalaya Bay since the 1970' s has 

 been primarily due to an increase in the 

 amount of fine sand, an important event 

 in deltaic development, as it marked the 

 shift of the locus of sedimentation 

 downstream from the lower Atchafalaya 

 basin above Morgan City to the Atchafa- 

 laya Bay area. 



Coastal submergence is occurring in 

 the Atchafalaya hydrologic unit, just as 

 in the rest of the MDPR, caused by 

 worldwide sea level rise combined with 

 subsidence of the land and increasing 

 river stages. The rate of sediment ac- 

 cumulation is more than keeping up with 

 coastal submergence over the area, how- 

 ever, and land elevations are in- 

 creasing. 



Tidal and wind energy are also 

 influential on the development of the 

 emerging deltaic system in Atchafalaya 

 Bay and the surrounding wetlands . The 

 seasonal tidal levels at Eugene Island 

 from 1935 to 1970 are shown in Figure 

 59. The astronomical tides in Atchafa- 

 laya Bay are 60% semidiurnal and 40% 

 mixed semidiurnal and diurnal. The mean 

 tidal range in Atchafalaya and Vermilion 

 bays is about 0.4 m (1.3 ft), and the 

 extreme range is 0.8 m (2.6 ft). The 

 mean tidal range at Eugene Island is 

 about 0.6 m (2 ft), with a 0.7 m (2.3 

 ft) extreme. Meteorological tides (wind 

 tides), which are independent of the 

 lunar tides, are also important in the 

 shallow Atchafalaya Estuary. With a 

 strong southeasterly wind, a wind tide 

 may reach 0.5 m above normal; a strong 

 northerly wind may depress water levels 

 up to 1.0 m (Barrett 1970). Hurricane 

 tides can have an even greater impact on 

 water levels. 



Suspended sediment loads in the 

 lower Atchafalaya River averaged 41.6 x 

 10 6 metric tons/yr during 1967-71 

 (Roberts et al. 1980). During the flood 

 years 1973-75, the load nearly doubled, 

 averaging 86.8 x 10 6 metric tons/yr 

 (Roberts et al. 1980). Prior to the 



Seasonal effects on the water level 

 at Eugene Island are as follows; high 

 water levels due to streamflow occur 

 during the spring (March, April, and 

 May) and the late summer, while the high 

 water level associated with the early 

 fall is due to the rise of the Gulf of 



138 



