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Mississippi River at high water "a volume of water equal to, or approximating to an equality with, the surplus of flood-water over the medium flood," and allow it to flow over the alluvial lands bordering the river, and deposit its sediment upon them.

This would, in his opinion, soon elevate those low lands to a considerable extent, and at no very distant day bring them to about the level of the banks of the river, render them cultivable, and the country healthy.

For the facts and figures which I shall use in this communication, I beg leave to refer to the report upon the Mississippi River, prepared by Captain A. A. Humphreys and Lieutenant H. L. Abbot, topographical engineers, and submitted by Captain Humphreys to the Bureau of Topographical Engineers, August 5, 1861.

The knowledge of a few simple facts concerning the region in question, well known to those who live upon the alluvial lands of the Mississippi, would lead one to distrust the feasibility of such a project.

The swamps and shallow lakes of the alluvial region are filled with rain-water long before the river reaches its flood condition, and remain so filled until the river goes down. Any material additions to their volume made by crevasses cause an encroachment upon the cultivated lands, and should the breaks in the levees be extensive and the high water of long continuance, the most serious inundations occur, involving the loss of crops and stock worth millions. The explanation of this is, that the fall of rain upon the alluvial lands is excessive, and the surface so flat that the eye can detect no deviation from a level, careful instrumental measurements being necessary to ascertain the direction as well as amount the slope that exists. The lakes are allow, except those along the river, which once formed portions of it and still retain in part its great depth. The facts cited indicate that no large volume of river water can be let in upon the alluvial lands without serious injury to the cultivable portions, the highest parts of the alluvion.

General Roberts proposes to draw off from the river, during the period of high water, and spread upon the alluvial lands all the volume in excess of that of the medium flood. This would bring the surface of the river very nearly to the level of the natural bank. In other words, would restore the conditions existing before any levees were built, and subject the whole alluvial region to overflow. Perhaps he may dissent from this exhibit of his proposition, but he will not object to my using, in a discussion of the project, the quantity of sedimentary matter contained in the volume of river water indicated. Let us see, then, how much earthy matter that volume would spread upon the alluvial lands. The area of those lands is

Square miles. The St. Francis bottom ... The Yazoo bottom The Tensas and Macon bottom

4,000 The alluvial lands below the mouth of Red River

6, 300 6, 800

12, 300

Total.....

29, 400

I will take the most favorable case for the project, the great flood year of 1858. The river during that year was less than one hundred and thirty days above the natural bank. Let us assume it to have been one hundred and thirty days. The surplus volume discharged by it during that time, over and above the volume discharged by the river when just bank full, was 1,200,000,000,000 cubic feet.

Now, had this quantity escaped from the river into the alluvial lands during the period of high water of 1858, it would have flooded the whole alluvial region, cultivated as well as uncultivated, from Cairo to the Gulf, during the entire period of one hundred and thirty days.

For the quantity of earthy matter held in suspension by the river water, I will use the largest proportion found in the investigations made upon the Mississippi River under my direction. That proportion is 12oo by volume. That is, for every 1,200 cubic feet of water, there was one cubic foot of earth. This is double the amount of sedimentary matter carried by the river water during the mean flood period. The proportion of

1200

would give for the volume of water just noted 1,000,000,000 cubic feet of earth.

I should explain here that when there were no levees the water thrown off by the river into the St. Francis bottom returned to the river again by the returning bayous and the St. Francis River, having deposited its sedimentary matter upon the bottom lands. It thus protracted the duration of the flood.

The water similarly thrown off into the Yazoo bottom returned to the river by the Yazoo River. The same is to be observed of the Tensas bottom, the water returning to the Mississippi by Red River.

Again, in order to make the most favorable case possible for Ġeneral Roberts's project, I will suppose that the whole volume of water necessary to bring the river within its banks in the flood of 1858 entered each bottom land in succession, that is, the bottom lands of the St. Francis, the Yazoo, and the Tensas.

We have seen that that volume of water carried in suspension 1,000,000,000 cubic feet of earth. That bulk, when spread upon an area of 6,000 square miles, (the area of the St. Francis bottom,) would have a thickness of šo of a foot. At this rate it would require twelve years to make a deposit one inch thick upon the St. Francis bottom.

But the time that the flood of 1858 was above the natural bank of the river was more than double that of the average floods, and we should have, for an average effect of flooding yearly all the St. Francis alluvion, less than one inch of deposit for twenty-four years of overflow. The mean difference of level of that swamp and the bank of the river is ten feet. To bring up the swamp to the level of the river bank would require more than two thousand eight hundred and eighty years. If the smaller quantity of sedimentary matter were used, the number of years would be about doubled.

If the sedimentary matter could be concentrated instead of being spread over the whole bottom, the depth of deposit would of course be increased. But the shape of the country is not adapted to this process. Moreover, the project of General Roberts comprises the whole area of the alluvion.

Here let me remark, that the project is not new to me; it is probably as old as the levee system, and is a fruitful subject of discussion with persons living on the alluvion, especially those who have noticed the deposits made by crevasse water at the edge of the swamp in the immediate vicinity of the crevasse, when the break in the levee was large and the high water continued. A notable example of it was given by the great Bonnet Carré crevasse of 1850, which, though only six miles from Lake Pontchartrain, and having therefore comparatively free flow to the Gulf, flooded an extensive district and destroyed a large amount of property. Such notable deposits are made only when the crevasse is so large that immense damage to the plantations on the alluvion is incurred.

It seems to me unnecessary to illustrate the subject further, or apply figures to the other bottom lands. So long as there are vast districts of the higher portions of the alluvial land along the Mississippi River that are unoccupied, and will remain so until the river is effectually leveed, it appears to me unnecessary to set investigations on foot to ascertain whether some limited localities of the lower portions of the alluvion can be raised by letting in upon it the turbid river water, especially as the features of the country are not adapted to the economical use of such processes.

The figures exhibited show that such a process upon a large scale is impracticable. The only practicable mode of reclaiming the swamp lands is to levee the river banks securely, and as cultivation extends inward, to establish a proper system of drainage.

The second view presented by General Roberts is, that by spreading a portion of the sedimentary matter of the river upon the swamp lands, there will be less of it deposited in the Gulf at the mouths of the river. In his opinion the bars will not then extend so rapidly into the Gulf as now, and, as a consequence, the surface of the river in its lower course, or near the sea, will not be raised as rapidly as it is now, (the rise of surface due to the extension of the mouth of the river into the Gulf,) and the height of the levees on the lower plantations will not have to be increased as frequently as now. Further, he is of opinion that there will then be a greater depth of water upon the bars at the mouth of the river than there is now.

Respecting the increase of height to be given to the levees in the lower course of the river, owing to the progress of the mouths into the Gulf, I beg leave to refer to pages 435 and 436, “report upon the Mississippi River," &c., where it is shown that it will require an extension of the mouths of the river twenty-five miles into the Gulf to raise the surface of the river one foot at Fort St. Philip, and that according to the present rate of progress, five centuries will elapse before the river accomplishes that extension.

Owing to the great depth of the Gulf, where the mouths of the river now lie, the rate of progress into the Gulf will be slower in future than it has been in past days.

As to increasing the depth of water upon the bars by reducing the quantity of sedimentary matter brought to the Gulf, I beg leave to remark that the depth upon those bars depends upon the quantities of water discharged over them, and not upon the quantity of supsended sedimentary matter brought to the Gulf by the river water. Further, the bars are not formed by the deposit of the sedimentary matter of the river, but by the deposit of the earthy matter pushed or moved along the bottom of the river. Hence, a reduction of the sedimentary matter of the river will not diminish the magnitude nor affect the form of the bars.

Should any furtherinformation or views concerning the bars be desired, reference can be made to the last chapter of the report already mentioned.

Having thus shown the impracticability of attaining the ends proposed by General Roberts, I trust I may be excused from presenting a view of the cost necessary to carry out his plans.

The popular impression that the floods of the Nile are allowed to spread upon its alluvion, has been sometimes referred to by persons ignorant

of the totally different conditions of the two rivers, as a reason for allowing the floods of the Mississippi to flow over its alluvion.

The floods of the Nile are regular in their recurrence, the greatest height being attained usually in September; the planting and sowing season follows the subsidence of the flood. Egypt is in the rainless region, and the overflow of the Nile fills, periodically, all the reservoirs, tanks, and canals from which the fields are irrigated, and supplies of water for every purpose are furnished. The best authorities state that its floods are not permitted to spread over its banks.

The floods of the Mississippi are irregular in their period, height, and duration, but on the average may be said to reach their height about the 1st of April. The river then remains in high-water condition, falling and rising until about the middle of July; and there are no means of predicting whether it may not be above the natural bank during all that time. There are, indeed, two maximum high-water points reached each year, the one about the 1st of April, the other about the 1st of June.

The planting and sowing season on the Mississippi begins just as the river reaches its height, and the high-water condition so late into the summer that no extensive crops can be gathered from any planting done after the river has begun to sink it to its low-water condition. Wherever its floods spread, thick-growing willow and cottonwood spring up, destroying the cotton and sugar plants, and requiring years for their eradication. Very respectfully, your obedient servant,

A. A. HUMPHREYS,

Major General Volunteers. Brig. and Bvt. Maj. Gen. RICHARD DELAFIELD,

Chief of Corps Engineers U. S. A.

M 4.

HEADQUARTERS CORPS OF ENGINEERS,

Washington, D. O., February 19, 1869. GENERAL: In reply to the communication of the chairman of the Committee on Commerce, of the Senate, dated January 26, 1869, asking for information relative to the probable cost of building and repairing the levees in the States of Louisiana, Mississippi, and Arkansas, as designated in Senate bill, No. 795, of the present session," I transmit a report upon the subject by Brevet Brigadier General H. L. Abbot, corps of engineers, to whom, on account of his connection with the surveys and investigations made in 1857–61, and with later examinations of those levees in 1865–66, and of his thorough knowledge of the subject, this matter was referred.

The report treats, first, of the repairs required to bring the existing breaks in the levees to the dimensions now adopted by the State authorities; second, of giving increased thickness to the present levees, without increasing their height; and third, of the increased height and thickness to be given to the levees, in order to protect the alluvion of those States against overflow when the whole flood volume of the Mississippi is confined within its banks, by a levee systein perfected from the head of the alluvial region to the Gulf of Mexico.

The estimates of cost for, first, repairing the existing breaks; second, for increasing the thickness of the existing levees with the present height; and third, for perfecting levees to a height sufficient to guard against all probable contingencies, are summed up by him as follows:

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NOTE.—The third item, under the State of Louisiana, is to "exclude water escaping above the boundary;" and the amount being included in the estimates under the head of Arkansas, it is not carried out in the “grand total for the three States."

I fully concur in the views of General Abbot. As this report is a continuation of the subject treated in the report of 1861, which was recently printed by Congress, and as it is a subject of great importance, I would respectfully recommend that this also may be printed by Congress. Very respectfully, your obedient servant,

A. A HUMPHREYS,

Brigadier General of Engineers, Commanding. Bvt. Maj. Gen. J. M. SCHOFIELD,

Secretary of War.

Report on repairing and perfecting the levees of the States of Louisiana,

Mississippi, and Arkansas, including an analysis of the floods of 1862, 1865, and 1867, with an appendix containing an abstract of facts collected respecting the principal tributaries of the Mississippi River during these great inundations. Submitted to Major General A. A. Humphreys, Chief of Engineers, by Brevet Brigadier General Henry L. Abbot, major corps of engineers. WILLETT'S POINT, NEW YORK HARBOR,

February 16, 1869. GENERAL: In accordance with instructions from Headquarters Corps of Engineers, dated December 23, 1868, I have the honor to submit the following report upon the repairs required to bring the existing breaks in the levees of Louisiana, Mississippi, and Arkansas, to the grade line now adopted by the State authorities; and also upon the more general question of protecting those States against overflow when the whole flood volume of the Mississippi is confined within its banks, by a levee system perfected from the head of the alluvial region, to the mouth. These two questions are quite distinct, and they will be considered separately..

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