From St. Louis to the Ohio river the Mississippi has been im proved by spur dikes (contraction works) consisting of four rows of piles tied together, extending into the river generally perpen dicular to the bank line. The banks and bottom at the bases of the spur dikes are revetted with rip rap and timber mattress for a distance of iooft. or more above and below the centre line of the dike. The piles are driven through the mattress. Brush and stone are placed between the piles. The current is checked by the dike and the resulting silt deposits build up the structure. The contraction of the low water by these dikes causes the project depths to be scoured in the channel between the outer ends of the dikes on one side of the river and the dikes or bank on the opposite side of the river. These works are supplemented by dredging when necessary.
Below the mouth of the Ohio river dredging has been used more than contraction works for the maintenance of channels, but spur dikes and bank revetments are used in some localities. As the Mississippi flows southward it becomes larger and larger from the inflow of tributaries, the maintenance of a low-water channel 9ft. in depth becomes less and less difficult, and the places where either contraction works or dredging are necessary become farther and farther apart. From Baton Rouge to New Orleans, La., a channel depth averaging 34ft. or more over a least width of several hundred feet has obtained naturally. The existing project for this stretch of the river (132mi. long) pro vides for a channel 5ooft. wide and 35ft. deep at low water, to be maintained by dredging. Little work has been necessary so far to maintain this channel.
From New Orleans, La., to the Head of the Passes (about 94mi.) through which the Mississippi flows into the Gulf of Mex ico, the natural channel is generally sufficient for ocean-going ves sels. Of the several outlets into the gulf, two, the South Pass and the Southwest Pass, have been improved and require continuous improvement for navigation. The Southwest Pass is 20.imi. long. It is improved by contraction works and dredging. The existing project provides for a channel i,000ft. wide and 35ft. deep in the Southwest Pass. The general plan of improvement provides for Flood Control.—The lands bordering the Mississippi river below the vicinity of Rock Island, Ill., some 1,55omi. above the mouth were originally subject to overflow. From Rock Island south to Cape Girardeau, Mo., such lands are comparatively nar row, varying from 3mi. to 5mi. in width. Below Cape Girardeau the alluvial valley proper, subject to overflow in its original con dition, averages 5omi. in width for a direct line distance of 600mi. to the Gulf of Mexico.
From the earliest times protection against floods has been se cured in varying degrees by levees along the river. The first levee was constructed at New Orleans in 1717. In the beginning
each owner built his own levee with his own resources. The Gov ernment of the times, however, was much interested in the levee construction. One of the conditions of the grants from the king of France obliged plantation owners to build levees. Inhabitants within several miles of the river were required to work on the levees. In 1743 the governor of the territory ordered each in habitant to complete his portion of the levee line or forfeit his grant. Similarly early settlers on the upper river built their own embankments in an effort to protect the narrow rich bottom lands for farming.
Individual levee construction was later replaced with construc tion by combinations of owners and communities, in an effort to protect larger areas. This led to numerous levee districts being formed and chartered in each State with the power to levy taxes to raise money for levee and drainage works. For many years the U.S. Government refrained from accepting any responsibility for flood protection. Then in 1879 the Mississippi River Commis sion consisting of seven members, four of whom, including the president, are officers of the Corps of Engineers, U.S. Army, was created by Congress and charged with making surveys and stud ies of the Mississippi river. For years thereafter, however, the primary function of Federal work was the improvement of navi gation with flood control only incidental.
At first, funds of the United States could not be spent for levees unless local interests contributed a material proportion of the cost. The local contributions to the work were gradually re duced until finally after the 1927 flood, Congress adopted a plan for the alluvial valley to be constructed entirely at Federal ex pense, requiring that local interests provide only rights of way for levees on the main river. North of the alluvial valley proper and on tributaries subject to backwater this act still required that local interests contribute one-third the cost of construction as well as provide rights of way. However, more recent legisla tion has frankly acknowledged flood control throughout the United States to be a national function and has relieved local in terests north of the alluvial valley from making this one-third contribution to construction costs. A Federal project is now au thorized for practically every one of the levee districts that border the Mississippi river from Rock Island, Ill., south to Cape Girardeau, Missouri. In general, these projects involve raising and enlarging the existing levees that were originally constructed by the local levee districts but are inadequate in grade and sec tion. In addition to these levee units plans have been made for reservoirs in the upper Mississippi river basin for the control of tributary floods and the reduction of flood flows on the upper and lower Mississippi river.