and the bed of the river elevated by the deposit of sedimentary matter, a cut-off, seasonably, judiciously and properly made, will shorten and straighten the course of the river, increase the velocity, deepen the channel, relieve the reach above from the accumulation of water kept back by the increased resistance of the bend and the deposit of sedimentary matter, lower the surface of its water, and remove all danger of overflow. Where there is a bar at the mouth of a river, as there is at the mouth of each of the passes of the Mississippi, and where the water that flows over the bar has been much diminished in volume and velocity by a number of outlets and its force still further dissipated and weakened by an undue and unnecessary expansion of the channel at the mouth, the only remedy is the closing up of these outlets and the contraction of the mouth by permanent banks within a width of channel uniform with that above. By the adoption of this course, the water being confined, its velocity increased and its power concentrated, the whole force of the river is brought to bear upon the bar, which, if it be composed of mere mud and sand, is obliged to give way before it, and gradually, but certainly, increase the depth of water over it. From these remarks it will appear evident, that the best and in fact only safe agent in the improvement of the river Mississippi, is its own water as volume and velocity combine a power, that, properly directed, is all-sufficient for the purpose. It is the volume of water in a river, and its velocity, that make the channel, and very little observation is necessary to convince any one, that, wherever the volume is dissipated or divided and the velocity diminished, there shoals or bars are generally found, and the water is kept back and prevented from discharging itself freely; while, on the other hand, wherever the water is confined within a channel of a normal breadth, the velocity is sufficient to keep it open and deep enough to discharge its water freely. Connected with the river Mississippi, in high water, is a number of bayous, lagoons, or dead-water lakes, into which the surplus water of the river flows in time of high floods, where they remain as in a reservoir, to return again when the water in the main channel begins to subside. Were the bed of the river deeper than it is, these would be no longer necessary as repositories for the surplus water, and the communication with the water of the river might be cut off without danger; but, in the present evidently elevated condition of the bed, they are highly useful in checking the too rapid and high rise, and too sudden fall, of its floods. The northern lakes may, in like manner, though on a far more extensive scale, be looked upon as reservoirs and equalizers of the St. Lawrence. Such is the influence they exercise on that river, that the difference between low and high water mark at Montreal does not exceed two feet. The idea, however, which seems to be entertained by some, that outlets or channels additional to the main one, through which portions of the water are discharged, are useful in relieving the river in high water, and preventing overflows, is altogether a mistaken one. It is true a lateral outlet, newly opened, may afford a temporary relief in time of high water; but, instead of doing a permanent benefit to the channel of the river, it only serves to increase the cause that gives rise to the evil, to prevent which it was adopted as an expedient. From the increasing frequency and destructiveness of overflows it is evident that the bed of the river is being elevated. This can only arise from the deposit of sedimentary matter, caused by a diminution of the velocity of the current wherever the elevation has taken place. Any diminution of the volume of water in the river will necessarily cause a decrease of its velocity, and that, of course, will cause an increase of deposits. An outlet, therefore, though it may afford a temporary relief this year, by relieving the river of its surplus water, yet, by diminishing the volume of water in the river, and consequently decreasing the velocity, it causes a still greater increase of the deposits, and a more rapid elevation of the river's bed—the very cause of the evil complained of-so that, next year, the bed of the river having risen still higher, the danger of overflow becomes still greater. The end and object of any system that may be adopted for the improvement of the Mississippi should be to deepen its bed. Any system that may be proposed, however it may appear to have a temporary beneficial effect, if it should in any way, directly or indirectly, immediately or remotely, have a tendency to elevate the bed of the river, should be unhesitatingly rejected. such a system, instead of being one of improvement, would end in the filling up of the present channel, and the ruin of those whose interests are involved in its continuance. The lateral branches or outlets of the Mississippi, have a shorter course to the Gulf, a greater fall and, consequently, a much greater velocity, in proportion to the volume of water in their channels. It will be found, also, on examination, that the velocity of their respective currents near the bottom is much greater than in the Mississippi. The consequence is, that, while the bed of the Mississippi is evidently becoming more elevated, the greater scouring power in the current of those outlets is deepening their channels, and that they are yearly receiving a larger portion of the water of the main river. Should this process be permitted to continue, without any effort to counteract it, the final result must be, that one of those outlets will one day become the main channel, through which the great mass of water from above will discharge itself. The velocity of water in a river diminishes from the surface toward the bottom, where the resistance is greatest. The following rule, which is founded on experiment, will enable us, when we have the velocity at the surface, to find the velocity at the bottom of a perpendicular. The velocity at the bottom of a river is equal to the velocity at the surface minus 0.008 multiplied by the velocity at the surface multiplied by the depth =v-0.008 vd. For example: If the velocity at the surface be five feet per second, and the depth one hundred feet, the velocity at the bottom is equal to 5-0.008×5X100=1 foot per second. According to experiment, the velocity of the water, on a sandy bottom, should not exceed one foot per second, to prevent it from wearing away, and the channel from deepening. Any velocity at the bottom of those outlets, exceeding this, must, necessarily, enlarge the area of their cross-sections, and increase the discharge, in a given time-while the velocity in the Mississippi will continue to decrease and the bed become elevated. In order to show, by calculation, the effect of the division of a river into two separate and equal branches: Let the breadth of the undivided river be two thousand feet, the depth eighty feet, and the fall ten fect, in the length of five hundred and twenty-eight thousand feet: the mean velocity, by the foregoing formula, will be 3.6 feet per second, and the discharge, five hundred and seventy-six thousand cubic feet per second. If the division be so arranged, according to the laws which govern the uniform motion of the water in open channels, that the breadth of each of the branches be three-fourths of that of the undivided river, or 2,000×0.75=1,500 feet, the depth of water sixty-one feet, the perimeter one thousand six hundred and twenty-two feet, and the fall to remain as above, ten feet, in the length of five hundred and twentyeight thousand feet. The mean velocity, by the foregoing formula, will be 3.148, and the discharge, two hundred and eighty-eight thousand and forty-two cubic feet per second, or half the quantity discharged by the undivided river. By the former rule laid down, the velocity, at the bottom of the undivided river, will be 3.6-0.008X3.6X80=1.3 feet per second, while the velocity, at the bottom of the branches, will be 3.148-0.008 X 3.148X61 1.6 feet per second. This ought to be sufficient to satisfy any one of the pernicious effects of the division of the water of a river into separate branches, or outlets. In the first place, the two channels occupy one thousand feet more breadth, and present so much the more resistance to the water, which, on that account, and because their volume, in each channel, is less than when united, and the depth decreased, suffer a corresponding loss in velocity. This shows, conclusively, that the whole volume of water in the river is much better able to deepen its channel, or keep it clear, than when it is divided into branches; and that, if it cannot do either in the former case, it is far less likely to do it in the latter. In order to render more clear the principles laid down in these views, I will give a table containing, in feet, the length of the Mississippi, from the Gulf to the bayou La Fourche, bayou Plaquemine, bayou Manchac and the Atchafalaya, with the fall, mean breadth, mean depth, mean velocity, velocity at the bottom, and its discharge in cubic feet. In the table will also be found the above mentioned outlets, with the length in feet from the commencement of their mouths, with their fall, mean breadth, mean depth, mean velocity, velocity at the bottom, and also their discharge in cubic feet. The length of the river to the different points named, as well as that of the outlets, is taken from Latourette's map of Louisiana. The cross-sections of the river and outlets are assumed. I also assume an outlet from New Orleans to lake Pontchartrain, and subject to the same calculations: tom, in ft., per sec.. Discharge in cubic feet per second. 1 800,000 1 800,000 1 800,000 1 800,000 Bayou Plaquemine, from the Gulf to the Mississippi, 60 miles,..... Bayou Manchac, from the Gulf to the Mississippi, 50 miles,... Atchafalaya, from the Gulf to the Mississippi, 110 miles, 528,000 4.2 3.8 120,000 4.7 4.3 141,000 Outlet, from the Lake to the Mississippi, above From the above table, it must be evident, at a glance, that, though the mean velocity of the water in the Mississippi may differ but little from that of the water in the outlets, yet, that the velocity at the bottom of the latter is much greater than that of the former, and that a much stronger scouring power is at work, gradually clearing out and deepening their channels, which must render them capable of discharg ing, every year, a larger quantity of water from the main channel, which, of course, must continue to diminish its volume, decrease its velocity, increase its deposits, elevate its bed and augment the danger of overflows. The assumed outlet from the Mississippi, above New Orleans, into lake Pontchartrain, shows, conclusively, the danger attendant upon outlets. An outlet made in the place assumed would have so great a fall, in proportion to the length of its course, that the result would necessarily be a velocity that would soon open for itself a channel, deep enough and large enough to divert the entire volume of water in the Mississippi, from its present channel, into the lake Pontchar train. Let us assume the breadth of the Mississippi, opposite New Orleans, to be two thousand feet, the mean depth one hundred feet, and the fall, at high water, fourteen feet, in the length of 100 miles, or five hundred and twenty-eight thousand feet, and we have the mean velocity 4.73 feet per second, and the discharge nine hundred and forty-six thousand cubic feet per second. Assuming that the Mississippi and Atchafalaya have each of them a breadth of two thousand feet, a mean velocity of 4.73 feet per second and a length of five hundred and twenty-eight thousand feet-but that the mean depth of the Mississippi is one hundred feet, while that of the Atchafalaya is only sixty feet-the required fall for the former will be fourteen feet, and for the latter 22.5 feet. From this it is evident that a greater depth of channel in the Mississippi would enable it to discharge its water with less fall, even though the velocity were the same; and also, that, at present, the former affords a better vent than the latter for the immense volume of water that flows through its valley, and, consequently, that much the largest portion of it passes through its channel on its way to the Gulf. How long this preference may continue, depends upon the means that may be used to prevent the former from gradually filling up its bed, while the latter is deepening and enlarging its channel. If, in addition to what we have already stated, it be assumed that the levees contribute to the elevation of the bed of the river, and consequent decrease of its ability to discharge its water, no dependence can be placed on the continuance of the present condition of the river. It is, therefore, advisable, that measures should be adopted, and that speedily, to improve the course of the river by lowering its bed and reducing the vertical rise of the surface of its water. In expressing these views I have only to regret that I am not in possession of the necessary levels, and geometrical and hydrometrical |