LEVEES 



The primary effect of levees on 

 flow regime and sediment loads enter- 

 ing estuaries has been confinement of 

 discharges and sediment loads to 

 three specific all stage outlets: 

 Head of Passes and vicinity; Atcha- 

 falaya River, south of Morgan City, 

 Louisiana; and Wax Lake Outlet, 

 west of Berwick Bay, Louisiana and 

 one flood relief spillway, Bonnet 

 Carre Spillway located just north of 

 New Orleans, Louisiana (Figure 6). 

 Levee extensions closed Bayous Man- 

 chac, Plaquemine, and Lafourche in 

 1828, 1866-1867, and 1903, respec- 

 tively. 



BANK STABILIZATION 



It has been estimated that cav- 

 ing banks in the lower Mississippi 

 River, prior to stabilization, yield- 

 ed annually about 1,000,000 cubic 

 yards of material per mile of river 

 (Shen 1971). The program of bank 

 stabilization in the lower Missis- 

 sippi River steming from the 1928 

 Flood Control Act is about 76 percent 

 complete. Recently, estimated vol- 

 umes of material caving into the riv- 

 er annually in the Vicksburg Dis- 

 trict, are a fraction of the previous 

 estimate, therefore, revetments are 

 probably responsible for a substan- 

 tial portion of the reductions in 

 suspended sediment loads experienced 

 on the Mississippi River and tribu- 

 taries . 



RESERVOIR REGULATION 



Over the past 50 years several 

 hundred single and multipurpose re- 

 servoirs have been constructed in the 

 headwaters of the major tributaries 

 of the Mississippi. Currently these 

 reservoirs control the runoff from 

 about 58 percent of the basin area. 

 Reservoirs trap large percentages of 



incoming sediment loads, therefore, 

 it is possible that reservoirs have 

 influenced reduction of suspended 

 sediment loads on the Mississippi. 



REMOVAL OF ATCHAFALAYA 

 RIVER LOG RAFT 



In 1831 Captain Shreve made 

 a cutoff in the Mississippi River 

 across the neck of Turnbull Island 

 (to aid navigation) which left the 

 mouth of the Red River and the head 

 of the Atchafalaya River in an ox- 

 bow lake with a two-way connection 

 to the Mississippi River (Figure 7). 

 The Atchafalaya, at this time, was 

 an ineffective distributary of the 

 Mississippi, choked by a massive log 

 raft covering 20 miles of its length. 

 A few years after Shreve' s cut off, 

 local interest, and later the State 

 of Louisaina, undertook removal of 

 the raft for the purpose of develop- 

 ing navigation on the Atchafalaya. 

 Their efforts were eventually suc- 

 cessful and the Atchafalaya was re- 

 portedly open by 1855 (Latimer 1951). 

 Because of a distinct gradient ad- 

 vantage, the Atchafalaya enlarged 

 rapidly near its mouth causing lands 

 previously exempt from overflow to be 

 submerged annually by the increasing 

 volume from above. Local interest 

 responded by building levees, con- 

 fining flows, and closing outlet 

 channels, causing the Atchafalaya to 

 scour its bed, thus, hastening the 

 inevitable natural enlargement of the 

 river. While the upper Atchafalaya 

 was rapidly enlarging, the middle and 

 lower reaches of the Atchafalaya 

 Basin were experiencing rapid and ex- 

 cessive sedimentation, signaling the 

 beginning of a deltaic process. In 

 1932 efforts to hasten the develop- 

 ment of an efficient well-defined 

 single channel through the deteri- 

 orating reach were undertaken. The 

 present, channel is a culmination of 

 those efforts. 



342 



