The condition of this rich valley of the Lower Mississippi, immediately after and since the war, constituted one of the disastrous effects of war most to be deplored. Fictitious property in slaves was not only righteously destroyed, but very much of the work which had depended upon the slave labor was also destroyed or greatly impaired, especially the levee system.
It might have been expected by those who have not investigated the subject, that such important improvements as the construction and maintenance of the levees would have been assumed at once by the several States. But what can the State do where the people are under subjection to rates of interest ranging from 18 to 30 per cent., and are also under the necessity of pledging their crops in advance even of planting, at these rates, for the privilege of purchasing all of their supplies at 100 per cent. profit?
It has needed but little attention to make it perfectly obvious that the control of the Mississippi River, if undertaken at all, must be undertaken by the national government, and cannot be compassed by States. The river must be treated as a unit; its control cannot be compassed under a divided or separate system of administration.
Neither are the States especially interested competent to combine among themselves for the necessary operations. The work must begin far up the river; at least as far as Cairo, if not beyond; and must be conducted upon a consistent general plan throughout the course of the river.
It does not need technical or scientific knowledge to comprehend the elements of the case if one will give a little time and attention to the subject, and when a Mississippi River commission has been constituted, as the existing commission is, of thoroughly able men of different walks in life, may it not be suggested that their verdict in the case should be accepted as conclusive, so far as any a priori theory of construction or control can be considered conclusive?
It should be remembered that upon this board are General Gilmore, General Comstock, and General Suter, of the United States Engineers; Professor Henry Mitchell (the most competent authority on the question of hydrography), of the United States Coast Survey; B. B. Harrod, the State Engineer of Louisiana; Jas. B. Eads, whose success with the jetties at New Orleans is a warrant of his competency, and Judge Taylor, of Indiana.
It would be presumption on the part of any single man, however skilled, to contest the judgment of such a board as this.
The method of improvement proposed by the commission is at once in accord with the results of engineering experience and with observations of nature where meeting our wants. As in nature the growth of trees and their proneness where undermined to fall across the slope and support the bank secures at some points a fair depth of channel and some degree of permanence, so in the project of the engineer the use of timber and brush and the encouragement of forest growth are the main features. It is proposed to reduce the width where excessive by brushwood dykes, at first low, but raised higher and higher as the mud of the river settles under their shelter, and finally slope them back at the angle upon which willows will grow freely. In this work there are many details connected with the forms of these shelter dykes, their arrangements so as to present a series of settling basins, etc., a description of which would only complicate the conception. Through the larger part of the river works of contraction will not be required, but nearly all the banks on the concave side of the beds must be held against the wear of the stream, and much of the opposite banks defended at critical points. The works having in view this conservative object may be generally designated works of revetment; and these also will be largely of brushwood, woven in continuous carpets, or twined into wire-netting. This veneering process has been successfully employed on the Missouri River; and in some cases they have so covered themselves with sediments, and have become so overgrown with willows, that they may be regarded as permanent. In securing these mats rubble-stone is to be used in small quantities, and in some instances the dressed slope between high and low river will have to be more or less paved with stone.
Any one who has been on the Rhine will have observed operations not unlike those to which we have just referred; and, indeed, most of the rivers of Europe flowing among their own alluvia have required similar treatment in the interest of navigation and agriculture.
The levee is the crowning work of bank revetment, although not necessarily in immediate connection. It may be set back a short distance from the revetted bank; but it is, in effect, the requisite parapet. The flood river and the low river cannot be brought into register, and compelled to unite in the excavation of a single permanent channel, without a complete control of all the stages; and even the abnormal rise must be provided against, because this would endanger the levee, and once in force behind the works of revetment would tear them also away.
Under the general principle that the local slope of a river is the result and measure of the resistance of its bed, it is evident that a narrow and deep stream should have less slope, because it has less frictional surface in proportion to capacity; i.e., less perimeter in proportion to area of cross section. The ultimate effect of levees and revetments confining the floods and bringing all the stages of the river into register is to deepen the channel and let down the slope. The first effect of the levees is to raise the surface; but this, by inducing greater velocity of flow, inevitably causes an enlargement of section, and if this enlargement is prevented from being made at the expense of the banks, the bottom must give way and the form of the waterway be so improved as to admit this flow with less rise. The actual experience with levees upon the Mississippi River, with no attempt to hold the banks, has been favorable, and no one can doubt, upon the evidence furnished in the reports of the commission, that if the earliest levees had been accompanied by revetment of banks, and made complete, we should have to-day a river navigable at low water, and an adjacent country safe from inundation.
Of course it would be illogical to conclude that the constrained river can ever lower its flood slope so as to make levees unnecessary, but it is believed that, by this lateral constraint, the river as a conduit may be so improved in form that even those rare floods which result from the coincident rising of many tributaries will find vent without destroying levees of ordinary height. That the actual capacity of a channel through alluvium depends upon its service during floods has been often shown, but this capacity does not include anomalous, but recurrent, floods.
It is hardly worth while to consider the projects for relieving the Mississippi River floods by creating new outlets, since these sensational propositions have commended themselves only to unthinking minds, and have no support among engineers. Were the river bed cast-iron, a resort to openings for surplus waters might be a necessity; but as the bottom is yielding, and the best form of outlet is a single deep channel, as realizing the least ratio of perimeter to area of cross section, there could not well be a more unphilosophical method of treatment than the multiplication of avenues of escape.
In the foregoing statement the attempt has been made to condense in as limited a space as the importance of the subject would permit, the general elements of the problem, and the general features of the proposed method of improvement which has been adopted by the Mississippi River Commission.
The writer cannot help feeling that it is somewhat presumptuous on his part to attempt to present the facts relating to an enterprise which calls for the highest scientific skill; but it is a matter which interests every citizen of the United States, and is one of the methods of reconstruction which ought to be approved. It is a war claim which implies no private gain, and no compensation except for one of the cases of destruction incident to war, which may well be repaired by the people of the whole country.
Boston: April 14, 1882.