the gravity of the greatest volume and momentum of water falls upon the bed of the river, there is described the thread of the channel, and all submerged space outside of this, though in the river, acts as a kind of reservoir, where eddies the surplus water until taken up by the current. And it always happens, where the channel takes one bend of the bed, a corresponding tongue of shallow water faces the indenture. Where the river, by some inexplicable cause, has been thrown from its regular channel, or its volume of water embarrassed by some difficulties along the banks, the effect is immediately perceived upon the neighboring bank. The column of water thus impinged against it at once acts upon the bank, and, singularly enough, exerts its strongest abrasive action at the bottom, undermining the bank, which soon gives way, and instead of toppling forward, it noiselessly slides beneath the water and disappears. Acres of land have thus been carried away in an incredibly short time, and without the slightest disruption of the serene flow of the mighty current.
This carrying away of the banks, immense as is the amount of earth thrown into the waters of the river, has no sensible effect in blocking or directing the current, though it imperceptibly raises the channel. The force of the water does not permit its entire settlement in quantities at any one place, but distributes it along the bottom and shores below. Were this not the case, it is easily to be seen, the abrasion of the river banks would be greatly increased, and the destruction of the bordering lands immense.
A singular feature resulting from the above may here be mentioned. By pursuing the course of the river, a short distance below, on the opposite bank, it will be seen that a large quantity of the earth introduced into the current by the falling of the banks, has been thrown up in large masses, forming new land, which, in a few seasons, becomes arable. That which is not thus deposited, as already stated, is transported below, dropping here and there on the way, until what is left reaches the Gulf, and is precipitated upon the 'bars' and 'delta,' at the mouth. It not unfrequently happens that planters along the river find themselves suddenly deprived of some of their acres, while one almost opposite finds himself as unexpectedly blessed with a bountiful increase of his domain.
From causes almost similar to those given to explain the sudden and disastrous changes of the channel of the river, are also produced those singular shortenings, known as 'cut-offs,' which are so frequently met with on the Mississippi. At a certain point the force of the current is turned out of its path and impinged against a neck of land, that has, after years of resistance, been worn down to an exceedingly small breadth. Possibly the river has merely worn an arm in its side, leaving an extensive bulge standing out 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.
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