In the alluvial valley proper the great flood of 1927 produced unprecedented flood heights at many points. The destruction wrought by this flood attracted national attention and emphasized the magnitude of the problem. This interest resulted in the adop tion in 1928 of a project for flood control of the Mississippi river in its alluvial valley and for its improvement from the Head of the Passes to Cape Girardeau, Mo., in accordance with the engi neering plan recommended by the Chief of Engineers, U.S. Army. The sum of $325,000,000 was authorized to be expended on this project. By an act approved June 15, 1936, this project was ex tended and the authorization increased by $272,000,000. After the Ohio river flood of 1937, Congress added to the project and authorized an additional expenditure of $40,000,000.
The flood control plan as amended proposes to protect the al luvial valley of the Mississippi river from Cape Girardeau, Mo., to the Gulf of Mexico (including the St. Francis, Yazoo, Tensas, and Atchafalaya basins) as well as the alluvial lands around Lake Pontchartrain, against the maximum predicted flood. North of the Arkansas river, floods are confined generally to the main leveed channel, but a limited area in the St. Francis basin, in southeast Missouri, and the city of Cairo, Ill., are given addi tional protection by the New Madrid floodway, which, during extraordinary floods only, will become an adjunct of the main channel to widen the river and hold down stages. South of the Arkansas river flood waters, in excess of what the leveed channel of the main river will carry safely, are to find their way to the gulf through floodways located in the lowlands of the basins west of the river, which lowlands have always carried extraordinary flood waters. The city of New Orleans is given additional pro tection by the Bonnet Carre spillway, which will divert flood wa ters from the Mississippi river into Lake Pontchartrain. In addi tion to levee work, the amended project includes improvement and regularization dredging of the Mississippi and Atchafalaya rivers to increase their discharge capacities ; the construction of an additional outlet to the Gulf of Mexico, via Wax Lake, La.; and headwater flood control projects for the St. Francis and Yazoo rivers.
During the 6o years the Federal Government has been engaged in improving the Mississippi river, many theories have been ad vanced for its improvement. The theory which has had the most advocates, and which has been actually tried to the greatest ex tent is that of "levees only." At first it was believed that its con finement by levees would cause the flood waters to scour out a channel large enough to accommodate flood flows. The confine ment of the waters of the Mississippi by levees has substantially raised the flood heights but has not caused the river bed to be enlarged by scouring or, vice versa, to be built up by deposits.
The theory that the river bed is being raised due to its confine ment by levees is a fundamental misconception. Several thou sand cross-sections of the river measured from time to time do not show any cumulative changes in the elevation of the river bed itself. The bed and natural banks of the river are continually undergoing the local changes found in any alluvial stream subject to a widely varying discharge, but the gross effect of these changes on the discharge capacity of any considerable section of the river proper, since the construction of levees, is so small as to be less than the limits of accuracy of measurement.
In recent years the capacity of the main river between the mouth of the Arkansas river and Red river has been materially increased by cutting off a number of the circuitous bends. A total of 13 cut-offs made in this section of the river have reduced the former river length of 33omi. to about 2 10 miles. This river straightening has increased the slope and velocity and conse quently the discharge capacity is greater.
Flood relief by means of reservoirs has received thorough con sideration in connection with flood control plans for the alluvial valley of the Mississippi river. It is not practicable to depend upon reservoirs for any great degree of protection of the alluvial valley. However, reservoirs have been planned and authorized for certain tributaries and though primarily for tributary flood control they will be of incidental benefit to flood control on the main river. The minor reduction of flood flows on the lower Mississippi river, that will result from such reservoirs, will in crease the factor of safety of the levees required in the alluvial valley and will tend to lessen the frequency of use of the side floodways and diversion channels constructed there.
The practical way to secure flood control of the Mississippi river is to provide complete levee protection where such protec tion is practicable and elsewhere to permit the flood waters of excessive floods to spill out of the leveed channel at selected points when stages reach the danger point. This water must be allowed to flow to the Gulf of Mexico through the most efficient natural drainage basins. The overflows into these basins must be limited by natural ridges or by secondary levees, the location of which is determined on the economic basis of the value of prop erty they protect. The water within the river channel does no damage and flows to the gulf with the utmost efficiency because of its high velocities. It should, therefore, be kept within the channel as long as possible, but the excess above the safe-carrying capacity of the leveed channel must be permitted to escape into floodways.