Page:The New International Encyclopædia 1st ed. v. 13.djvu/687

MISSISSIPPI RIVER. latter are far too numerous to be more than barely mentioned. The natural banks of the river, in those places where the current sets against them, and is likely to wear them away, are strengthened by revetment work. The character of this revetment varies somewhat according to circumstances, but, generally speaking, is of two kinds. The first is a kind of soft bottom put on the bed of the stream at the point of greatest erosion. This consists of what are called mattresses or hurdles, which are constructed of mats of brush woven and fastened together with wire strengthened with a sort of latticework of heavy timbers placed on the banks and secured by a substantial ballast of rubble-stones. A crib of timbers is first constructed in sections, amounting in all to dimensions sufficient to cover the place which is to be protected. Over this is laid a carpet of heavy brush, with the twigs of fibre running generally in parallel lines. Over this is laid another carpet of similar construction, with its fibres crossing that of the first at an angle usually less than a right angle, and the two are knitted or sewed together with strong wire. Sometimes a third carpet may be laid, with the brush lying at still another angle. Then another crib or lattice-work of heavy timber is laid on these carpets, and the whole is bound together with wire, or another series laid on top if extra strength and weight are needed. The entire contrivance is then placed in position, covering the bottom of the river from the centre of the channel to the margin of the bank. As a matter of course it is necessarily handled in sections, but the sections are placed and fastened together so as to form a continuous carpet when laid, and they are then heavily ballasted with rubble-stone, laid systematically to form an uncemented pavement over the carpet. It is found that this practically secures the mattress in position for a considerable time under ordinary circumstances, and actually prevents the eating away of the banks by diverting the course of the current back to the desired channel. Generally it occurs that the action of the river before this treatment is applied to it has made the bank uneven. Sometimes it will even overhang the river a little, and it is necessary to smooth the surface to afford an even bed or floor on which to lay the mattress. Instead of digging this away, the familiar method of hydraulics is utilized. With a powerful engine a stream of water is pumped through a hose, and the soft earth is readily washed into shape. The ultimate purpose of these mattresses and other devices is to secure an approximately uniform width between the banks of about 3000 feet. When this is done it is believed that the bottom will be scoured out by the current, so that a depth of not less than sixteen or eighteen feet will be secured throughout the lower river. The permeable dike is a contrivance used in many places where it would be impossible to lay mattresses, where there may be, in fact, no bank to lay them on, but a wide stretch of slack water reaching out into a lagoon. These dikes are of simple construction, consisting of rows of piles driven as firmly as may be in the soft bottom. The piles are placed from two to five feet apart, and between the rows quantities of brush are placed and fastened. The water passes through these works freely at first, but being checked by the partial obstruction, it drops

the sediment with which it is so heavily charged, and itself completes the dam which confines its course. These dikes are found to be very effective substitutes for complete dams, and are put where it would be difficult, if not impossible, to build a solid dam in the first place. More substantial, or rather more compactly built dams are placed at some points where there is a tendency on the part of the river to form cut-offs. (See section Description of Natural Features.) This tendency is perhaps the thing most dreaded and most carefully guarded against.

. It has been found that, in order to meet the whole question scientifically, it would be necessary to build a double line of levees on each side of the river. The inner line is built to define the course of the channel and to prevent floods; the outer line, which is located far down the river ridge, is designed to protect the farms and plantations from the invasion of the back water in case the front levee is broken. The levee is in certain places the only possible safeguard. Cairo, Ill., affords a conspicuous instance of this. No revetments or dikes could possibly guide the current so that it would flow past the city without overflowing it, for the simple reason that the city is below high-water mark. It is of necessity entirely surrounded by a levee that rises some fifty-five or sixty feet above low-water mark, or ‘zero,’ as it is called on the Government records. The keeping of the works in repair is a matter of vital importance; constant watching and constant strengthening are the only things that can successfully combat the action of the rushing stream.

. The characteristics of the banks are so different that the various localities offer special problems in themselves, and must be handled without reference to conditions that obtain elsewhere. The bluffs are threatened in one place, while in another the opposite low bank is attacked, and the river devotes its energies toward cutting a new channel. Revetments are sometimes on the bluff side, then again on the opposite; basins are occasionally cut in the soft bottom ground where the old river bed used to be, and spur-dikes are in other places the only remedy.

The mouth of the Red River brought several vexed questions before the commission, and the practical result of their conclusions will not be known until the works now in progress are completed. It is feared that the bed of the Atchafalaya, the present main outlet of the Red River, will enlarge sufficiently to convert the entire country between it and the Mississippi into an arm of the sea. The only safeguard seems to be by a series of dikes and submerged dams to turn the low-water flow of the Red River all into the Mississippi.

Another serious problem was presented by the bars at the mouths of the delta, which have been a serious obstacle to vessels entering the river. This was solved by the celebrated engineer James B. Eads. He selected the South Pass, and by the construction of jetties which narrowed the channel at its mouth, and thus increased the velocity of the current, he made the river cut its own bar away, and obtained a depth through the bar and throughout the pass of 34 feet, with width adequate for all purposes of navigation. This improvement has resulted in