distribution system was periodically 

 supplemented during high flows by 

 various crevasses and breaks in the 

 natural and later man-made levee 

 systems of the lower Red and Missis- 

 sippi rivers (Elliott 1932). 



NATURAL FLOW REGIME 



For discharges up to bankfull 

 the Mississippi River and four major 

 distributaries, delivered flow to the 

 estuaries. Estimated bankfull capa- 

 city of the Mississippi River in 1851 

 was about 1,000,000 cf s . The com- 

 bined peak discharge of Bayous Pla- 

 quemine and Lafourche in the 1858 

 flood was 45,000 cubic feet per sec- 

 ond (cfs), with the peak of Bayou 

 Plaquemine being three times greater 

 than Bayou Lafourche. No measure- 

 ments were available for Bayou Man- 

 chac or Atchafalaya River, however, 

 based on early descriptions, Bayou 

 Manchac had less capacity than Bayou 

 Lafourche and the Atchafalaya River 

 was choked by a massive log raft 

 probably rendering it ineffective ex- 

 cept during flood overflow. In times 

 of flood, the Atchafalaya Basin (zone 

 3) served as the major outlet for ex- 

 cess flood waters. Flows entered the 

 Basin by overtopping banks, through 

 the Atchafalaya River and Bayou Pla- 

 quemine with some contribution from 

 Bayou Lafourche, and through cre- 

 vasses in natural levees on the west 

 bank of the Mississippi from Red Riv- 

 er Landing, Louisiana, to Donaldson- 

 ville, Louisiana. Zone 1 and zone 2 

 received excess flood water through 

 crevassing of natural levees which 

 occurred frequently but not neces- 

 sarily in every flood. 



NATURAL SEDIMENT DISTRIBUTION 



There is no reliable data upon 

 which to make a determination of the 

 magnitude of sediment loads trans- 

 ported by the Mississippi River in 

 its natural state. Measurements of 

 suspended sediment were taken in the 

 1800 's at various locations on the 

 river; however, the equipment used 

 and the random sampling procedures 

 followed make the results of those 

 measurements of little practical use 

 (Paper H 1930). It is assumed that 

 the distribution of sediments was 

 about in proportion to the distribu- 

 tion of flows previously described. 

 It is very probable that only a minor 

 portion of the sediment loads entered 

 distributaries, and that accompanying 

 excess flood flows ever reached the 

 vicinity of the coastline due to the 

 low topographical features of zones 

 1 , 2 , and 3. 



ALTERATIONS OF THE MISSISSIPPI 



In recent history numerous al- 

 terations of the Mississippi have 

 been accomplished, each designed to 

 fulfill a specific objective and ne- 

 cessary for man to continue to sur- 

 vive and prosper in the Mississippi 

 River Valley. Not all alterations 

 affect the flow regime and sediment 

 load of the river, at least not sig- 

 nificantly, and some alterations af- 

 fect only the distribution (the route 

 of discharge and sediment) rather 

 than the magnitude of discharge and 

 sediment loads. Those alterations 

 considered significant include: 

 levees, reservoirs, bank stabiliza- 

 tion, and removal of Atchafalaya Riv- 

 er log raft. 



341 



