tons, a reduction of about 50 per- 

 cent. At Simmesport, Louisiana, on 

 the Atchafalaya River the average 

 annual sediment load for the period 

 (1951-1959) was 134 million tons for 

 an average annual volume of flow of 

 122 million acre-feet. For an equi- 

 valent volume of flow, the average 

 annual load for the period (1966- 

 1976) was 70 million tons, a reduc- 

 tion of about 50 percent. Based on 

 records available, channel stabiliza- 

 tion and other features have reduced 

 average annual suspended sediment 

 loads delivered to south Louisiana by 

 50 percent. 



The Atchafalaya Basin is unique 

 and very complex relative to sediment 

 loads and distribution. As previous- 

 ly mentioned, removal of the raft at 

 the head of the fledgling Atchafalaya 

 initiated the natural developments of 

 that stream as a major distributary 

 of the Mississippi. As scour en- 

 larged the upper channel, rapid sedi- 

 mentation took place in the middle 

 reach of the basin. Development of a 

 reasonably well-defined channel in 

 the middle reach (1932-1968) de- 

 creased the rate of sedimentation in 

 that reach, causing more sediments to 

 be transported to Atchafalaya Bay. 



By the 1950' s significant influx 

 of sediments to the Atchafalaya Bay 

 begin to occur (Roberts et al. 1980) 

 indicating that it took approximately 

 100 years for the initial channel 

 development to push through to the 

 coast of Louisiana, even though the 

 process was materially hastened by 

 alterations in the basin. Since 1950 

 the influx of sediment to the bay has 

 increased dramatically. The refer- 

 ence cited above indicated that for 

 the period 1973-1975 only 17 percent 

 of the total average annual suspended 

 sediment was retained in the Atcha- 

 falaya Basin with 65 percent carried 



by the lower Atchafalaya River and 19 

 percent carried by Wax Lake Outlet, 

 resulting in the rapid filling of 

 Atchafalaya Bay. 



SUMMARY AND CONCLUSIONS 



Under natural conditions water 

 and sediment, contained within the 

 channel and including the Red River 

 which was a tributary of the Missis- 

 sippi, flowed to the gulf by way of 

 the Mississippi River and four pri- 

 mary distributaries: Bayou Manchac, 

 Bayou Plaquemine, Bayou Lafourche, 

 and the Atchafalaya River. In time of 

 flood, excess water and sediment 

 flowed from the Mississippi and Red 

 Rivers southward to and through nu- 

 merous bayous into the lakes and 

 swamps along the coastline of Louis- 

 iana . 



Alterations of the Mississippi 

 and Atchafalaya Rivers over the past 

 140 years have significantly affected 

 the distribution of flow and sediment 

 loads entering the estuaries of south 

 Louisiana. The most significant al- 

 terations were: removal of the Atcha- 

 falaya River log raft and subsequent 

 alterations which hastened natural 

 processes; confinement of flood flows 

 by levees limiting discharges to 

 three specific outlets; and those al- 

 terations that affected a 50 percent 

 reduction in average annual suspended 

 sediment loads transported by the 

 Mississippi and Atchafalaya rivers. 



Review of alterations and their 

 affect on flow regime and sediment 

 loads entering estuaries has led to 

 the following conclusions: 



a. The average volume of flow 

 delivered annually to south Louisiana 

 has not been affected by alterations. 



347 



