Ch. 18] LOWER MISSISSIPPI RIVER 325 



shape of the bed usually reflects mainly the flood-stage influence. At 

 low water the stream consists of alternating deep pools of slight slope, 

 and steeper slopes over the crossing bars where the depths become 

 critical for navigation purposes, and dredging is often necessary unless 

 the water depth can be increased by confining the width of low-water 

 flow. Dikes or other channel-training or -contraction works lose their 

 value if the channel shifts, and hence maintenance of effective channel 

 improvements for navigation requires that banks be stabilized suffi- 

 ciently to maintain the general channel alignment. Experience indi- 

 cates that a channel of gradual curvature, with the size and radius of 

 bends determined for each stream by experiment but often shaped to 

 smaller radius toward their lower ends, and the concave banks pro- 

 tected by revetment, generally will maintain itself most satisfactorily. 

 Channel regulation for navigation purposes is usually directed toward 

 this objective. 



IMPROVEMENTS OF RIVERS FOR FLOOD CONTROL 

 AND NAVIGATION 



Lowee Mississippi River 



Flood protection is the most important problem in the Lower Mis- 

 sissippi Valley, below the mouth of the Ohio River. Low levees were 

 built by the early colonial settlers and gradually extended and raised 

 by local interests. After establishment of the Mississippi River Com- 

 mission, in 1879, river regulation was attempted by contraction with 

 permeable dikes and closure of secondary channels, supplemented by 

 dredging where necessary for navigation, and construction of levees 

 that were expected to aid navigation by confining flood waters and 

 thus inducing scour of the channel. The dikes and closure dams were 

 generally inadequate, however, and dredging came to be the accepted 

 method of improvement for navigation. The levees were also inade- 

 quate to withstand major floods and were topped or broken from time 

 to time. 



A more comprehensive federal flood-control project was adopted 

 after the great flood of 1927. The levees have been greatly improved, 

 so that they have withstood all floods since 1927; auxiliary leveed 

 floodways have been provided to carry part of major floods past the 

 city of Cairo and along the Atchafalaya River, which follows an old 

 abandoned course of the Mississippi from the mouth of Red River 

 to the Gulf at a point about 150 miles west of the present Mississippi 

 mouth; the Bonnet Carre spillway has been constructed to divert part 



