Development of the Atchafalaya 

 Floodway levees system was undertaken 

 in 1932, which confined flow to about 

 50 percent of the original floodplain 

 width from the leveed Atchafalaya 

 River to just above Morgan City 

 Reach. 



In 1942 work was undertaken to 

 construct a channel through Teche 

 Ridge for the purpose of diverting 20 

 percent of the Atchafalaya flow to 

 western Atchafalaya Bay. Subsequent, 

 natural enlargement has increased the 

 diversion to about 30 percent of the 

 Atchafalaya flow. 



EFFECT OF ALTERATIONS ON FLOW 

 REGIME AND SEDIMENT LOADS 



Bay, south of Morgan City, Louisiana; 

 and Atchafalaya Bay, west of Morgan 

 City, Louisiana. 



The present distribution of flow 

 to coastal Louisiana is 70 percent at 

 Head of Passes; 21 percent, Atcha- 

 falaya Bay, south of Morgan City; and 

 9 percent, Atchafalaya Bay, west of 

 Morgan City. The high-level outlet, 

 Bonnet Carre Spillway, diverts flood 

 flows in excess of 1,250,000 cfs to 

 Lake Ponchartrain. This outlet has 

 been operated six times, 1937, 1945, 

 1950, 1973, 1975, and 1979. 



In summary, alterations have re- 

 duced flows in the Mississippi, in- 

 creased flows in the Atchafalaya, 

 closed three distributaries, and con- 

 fined flows to three all-stage out- 

 lets and one high-level-flood outlet. 



FLOW REGIME 



SEDIMENT LOADS 



Alterations of the Mississippi 

 River, its drainage basin, and adja- 

 cent floodplain have been extensive, 

 but, the effect of these alterations 

 has been distribution of flow rather 

 than influence on the annual volume 

 of flow delivered to the estuaries. 

 Removal of the Atchafalaya River log 

 raft and subsequent alterations which 

 hastened the natural enlargement pro- 

 cess have resulted in the Atchafalaya 

 River now carrying 30 percent of the 

 total latitude flow of the Missis- 

 sippi under normal conditions (Figure 

 8). This includes all of the Red 

 River and portions of the Missis- 

 sippi . 



Levees have closed distributar- 

 ies and confined flood flows of the 

 Mississippi River drainage system 

 to three all-stage outlets and one 

 high-level outlet. The all-stage 

 outlets are Head of Passes, south of 

 New Orleans, Louisiana; Atchafalaya 



At the latitude of Red River 

 Landing, Louisiana, sediment data 

 have been collected at the following 

 stations: Red River Landing, Louis- 

 iana, (September 1949 to date); 

 Simmesport, Louisiana (September 1951 

 to date); Morgan City, Louisiana 

 (1965 to date); and Wax Lake Outlet, 

 Louisiana (1965 to date). The data 

 consist of suspended sediment meas- 

 urements which measure that portion 

 of sediment load that is suspended in 

 the water from the water surface to 

 about 3 feet above the riverbed. 



The measurements at Red River 

 Landing represent the sediment loads 

 transported by the Mississippi. The 

 average annual load at this station 

 for the period 1950-1959 was 307 mil- 

 lion tons for an average annual vol- 

 ume of flow of 332 million acre- 

 feet. For an equivalent volume of 

 flow the average annual load for the 

 period 1966-1976 was 170 million 



345 



