FLOW REGIME AND SEDIMENT LOAD AFFECTED BY 

 ALTERATIONS OF THE MISSISSIPPI RIVER 



J. R. Tuttle 



Lower Mississippi Valley Division, Corps of Engineers 



A. J. Combe, III 



U. S. Army Corps of Engineers 

 New Orleans District, New Orleans, Louisiana 



ABSTRACT 



The Mississippi River drainage 

 basin includes all or part of 31 

 states and 2 Canadian Provinces cov- 

 ering about 41 percent of the con- 

 tiguous United States. The shape of 

 the basin is much like a funnel with 

 the spout entering the Gulf of Mexico 

 in the State of Louisiana. 



Over the past several thousand 

 years the Mississippi River has occu- 

 pied and abandoned seven deltas re- 

 sulting in progradation of the shore- 

 line and development of a large low- 

 relief deltaic plain in south Louis- 

 iana. In its natural state, the 

 river channel and its vast floodplain 

 were used to convey all flows south- 

 ward to the Gulf of Mexico following 

 natural drainage patterns evolved 

 over centuries of meandering by the 

 river. Man's occupation of the val- 

 ley brought with it extensive modi- 

 fications to the river and its flood- 

 plain. These modifications, which 

 made it possible for man to survive 

 and prosper in the valley, have 

 changed the distribution of water and 

 sediment in distributaries and major 

 outlets entering the estuaries. 



Distribution of flow to the 



coastal area of south Louisiana has 

 changed significantly over the past 

 140 years. A massive log raft re- 

 moved by the State of Louisiana from 

 the Atchafalaya River in the middle 

 1800' s and subsequent natural en- 

 largement, hastened by flood control 

 and navigation works, has resulted in 

 the Atchafalaya River, a major dis- 

 tributary of the Mississippi River, 

 transporting about 25 percent of the 

 discharge of the Mississippi. On the 

 Mississippi, extension of flood pro- 

 tection levees resulted in closure of 

 three distributaries above New 

 Orleans and confined water and sedi- 

 ment to a well-defined leveed chan- 

 nel. In the Atchafalaya Basin, de- 

 velopment of basin guide levees con- 

 fined floods to a leveed floodway and 

 two outlets, Atchafalaya River below 

 Morgan City and Wax Lake Outlet. 



In the past 30 years average 

 suspended sediment loads in the Mis- 

 sissippi River Basin have been re- 

 duced about 50 percent. The natural 

 process of sediment deposition in the 

 Atchafalaya Basin has progressed from 

 near the head of the Atchafalaya 

 River in the late 1800' s to Atchafa- 

 laya Bay, materially hastened by 

 alterations in the basin. The middle 

 reach of the Atchafalaya has experi- 

 enced significant natural filling and 



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