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.