Continental Monthly , Vol. 5, No. 6, June, 1864. VariousЧитать онлайн книгу.
in the river, and connected with the mainland by an isthmus. The river striking in this arm, and not having sufficient scope to rebound toward the other bank, is thrown into a rotary motion, forming almost a whirlpool. The action of this motion upon the banks soon reduces the connecting neck, which separates and blocks the waters, until, at last, no longer able to cope with the great weight resting against it, it gives way, and the river divides itself between this new and the old channel.
Nor do these remarkable instances of abrasive action constitute the entire washing from the banks. The whole length of the river is subject to a continual deposit and taking up of the silt, according to the buoyant capacity of the water. This, too, is so well regulated that the quantity of earthy matter held in solution is very nearly the same, being proportioned to the force of the current. For instance, if the river receive more earth than it can sustain, the surplus sediment drops upon the bottom or is forced up upon the sides. If the river be subject to a rise, a proportionate quantity of the dropped sediment is again taken up, and carried along or deposited again, according to the capacity of the water. By this means a well-established average of silt is at all times found buoyant in the river.
Having briefly examined the sedimentary character of the Mississippi, some investigations as to the proportion of sediment to water may be of interest. And it is well to state here that a mean stage of flow is taken as the basis upon which to start the experiments. The experiments and analysis of the water were made by Professor Riddell, at intervals of three days, from May 21st to August 13, 1846, and reported to the Association of American Geologists and Naturalists.
The water was taken in a pail from the river in front of the city of New Orleans, where the current is rather swift. That portion of the river contains a fair average of sedimentary matter, and it is sufficiently distant from the embouchure of the last principal tributary to allow its water to mix well with that of the Mississippi.
'The temperature,' says the Professor, 'was observed at the time, and the height of the river determined. Some minutes after, the pail of water was agitated, and two samples of one pint each measured out. The measure graduated by weighing at 60 degrees Fahrenheit 7,295.581 grains of distilled water. After standing a day or two, the matter mechanically suspended would subside to the bottom. Nearly two thirds of the clear supernatant liquid was next decanted, while the remaining water, along with the sediment, was in each instance poured upon a double filter, the two parts of which had previously been agitated, to be of equal weight. The filters were numbered and laid aside, and ultimately dried in the sunshine, under like circumstances, in two parcels, one embracing the experiments from May 22 to July 15, the other from July 17 to August 13. The difference in weight between the two parts of each double filter was then carefully ascertained, and as to the inner filter alone the sediment was attached, its excess of weight indicated the amount of sediment.'
As the table may be interesting, showing the height and temperature of the water as well as the result of the experiments at the different times, we introduce it complete:
The mean average of column A. is 6.32.
The mean average of column B. is 6.30.
Transcriber's Note: Data in the above table is as in the original.
'By comparison with distilled water,' says the same, 'the specific gravity of the filtered river water we found to be 1.823; pint of such water at 60° weighs 7,297.40.' Engineer Forehay says the sediment is 1 to 1,800 by weight, or 1 in 3,000 by volume.
Professor Riddell also comes to the following conclusions, after an analytic investigation of the sediment. He took one hundred grains from the river margin, dried it at 212° Fahrenheit, before weighing, and found it to contain:
The existence of so large a quantity of sediment in the water of the Mississippi, leads to divers formations in its bed. These formations are principally 'bars' and 'battures.' The banks are also much affected.
When the water of the river, aided by the current, has attained its full capacity of buoyant earth, as we have already said, the excess falls to the bottom. Instead, however, of remaining permanently where it first lodged, which would soon fill up the channel and cause the river to overflow, the scouring of the water on the bottom forces a large portion along with the current, though it be not suspended. Pursuing its course for a while, some irregularity or obstruction falls in the way—a sunken log, perhaps. This obstacle checks the progress of the moving earth—it accumulates; the next wave brings down more—the accumulation becomes greater; until, in the course of a few years, there is a vast field of deposit, and a 'bar' is formed. These 'bars' often divert the channel, and occasion the immense washings before alluded to.
Bars are generally found close to the banks, though there are examples in which they extend in a transverse direction to the current. Bars of this kind very much embarrass and endanger navigation in low water. At Helena, Arkansas, there is an instance of a transverse bar, upon which, in October, the water is less than six feet. These bars are formed of sand, which seems to have been the heavier and less buoyant of the components of the earth thrown into the current by abrasion, the lighter portions having been separated by the water and carried off.
It will not be necessary to consider further the subject of bars in the river, but those at its mouth deserve some attention. The subject is one that has led to much theorizing, study, and fear—the latter particularly, from an ill-founded supposition that they threaten to cut off navigation into the Gulf.
Near its entrance into the Gulf, the Mississippi distributes its waters through five outlets, termed passes, and consequently has as many mouths. These are termed Pass à l'Outre, Northeast, Southeast, South, and Southwest. They differ in length, ranging from three to nine miles. They also all afford sufficient depth of water for commercial purposes, except at their mouths, which are obstructed by bars. The depth of water upon one of these is sufficient to pass large vessels; a second, vessels of less size; and the rest are not navigable at all, as regards sea-going vessels. These bars, too, are continually changing, according to the winds or the currents of the river. It is a rather singular fact that when one of the navigable passes becomes blocked, the river is certain to force a channel of navigable depth through one of the others, previously not in use; so that at no one time are all the passes closed.
In looking into the past, and noticing the changes, it is recorded that in 1720, of all the passes the South Pass was the only one navigable. In 1730, there was a depth of from twelve to fifteen feet, according to the winds, and at another time even seventeen feet was known. In 1804, upon the statement of Major Stoddard, written at that date, the East Pass, called the Balize, had then about seventeen feet of water on the bar, and was the one usually navigated. The South Pass was formerly of equal depth, but was then gradually filling up. (This pass, at present, 1864, is not at all navigated.) The Southwest Pass had from eleven to twelve feet of water. The Northeast and Southeast Passes were traversed only by small craft. Since 1830 the Southwest Pass has been gaining depth. This and Pass à l'Outre are now the only two out of the five of sufficient depth to admit the crossing of the larger class of vessels. The former, however, is the one in most general use. All the other passes, with the exception of the two mentioned, have been abandoned.
In regard to the changes and numerous singular formations at the mouths of the Mississippi, we give a statement made by William Talbot, for twenty-five years a resident of the Balize. He says:
'The bars at the various passes change very often. The channel sometimes changes two and three times in a season. Occasionally one gale of wind will change the channel. The bars make to the seaward every year. The Southwest Pass is now the main outlet used. It has been so only for three years, as at that time there was as much water in the Northeast Pass as in it. The Southeast Pass was the main ship channel twenty years ago; there is only about six feet of water in that pass now; and where it was deepest then, there are only a few inches of water at this time. The visible shores of the river have made out into the Gulf two or three miles within my memory. Besides the deposits of mud and sand, which form the bars, there frequently rise up bumps, or mounds, near the channel,