« AnteriorContinuar »
bend, it seldom does so with anything like uniformity. The cut position of the bank, and consequently its resistance to erosion differs at different places, as does also the intensity of the currepi attack. Owing to these causes, caving is usually much more rap in some parts of a bend than in others. At the locality of this rapid caving the curvature may increase and the shape of the ben] change so that the current is detected away from the concave bank below, so that for some distance it follows the middle of the rive or even attacks the convex bank opposite. As it must, from the shape of the bend, return to the concave bank not far below, there are thus formed two crossings with the attendant bad navigation. Noticeable examples of this are found in the bend of Island 33 (190) (Plate LXXI) and in Cow Island Bend (250) (PlatLXXII), in both of which the channel leaves the concave bank, crosses to the convex and then returns, while between these crossings there is a bar against the concave bank.
The effect of this irregularity in the rate of caving is also spe. cially noticeable near the lower ends of bends, for on these irregularities depend the shape of the concave bank at these places, and on these the direction and locality of the crossing and the point of maximum impingement of the current in the bend below. Slight changes in the shape of the bank at the foot of a bend may cause large changes in the other functions just mentioned, and it is largely due to this and to the consequent shifting of the crossings that the river at the crossings is usually so wide. Generally, as the bank caves away the crossing and the point of attack on the opposite bank move downstream, but if the bank at the foot of the bend resists erosion much better than the bank a little farther up, just the opposite is the result.
It has been mentioned above that straight reaches of considerable length are sometimes found on the river. In such reaches the banks do not serve as fixed guides to the channel, and it is apt to wander here and there, sometimes cutting away one bank, sometimes the other, with a resultant widening of the river. At high water such a wide place causes a reduction in velocity and a deposit of sediment, usually in the form of one or more middle bars. Among these bars, composed of fairly coarse material, the low water has to make its channel, and it is frequently deflected by these bars against and outs away the main bank, sometimes one and sometimes the other, sometimes both. This but intensifies the trouble, making the river
ng is usually mai
- At the line trave and the stage Indaray from the
View taken near water surface.
pornite. As it mo are bank not far la
the attendant bader ad in the bend of - Island Bend 3
nel leaves the con es, while between the
hank e rate of caving is a f bends, for on the e bank at these plas ne crossing and the pe in the bend below
foot of a bend may ust mentioned, and t shifting of the ens o wide. Generally, nt of attack on the k at the foot of the a little farther up
No. 5. Shows caving bank at Caruthersville, 1898.
reaches of considera
such reaches the best and it is apt to wani e bank, sometimes 1
At high water suel a deposit of sedima
bars. Among the OW water has to mali hese bars against 30 sometimes the other -le, making the rive
still wider at high water, causing the bars to increase in size & height, and so on.
Such straight reaches are apt from these causes to be the wi.. portions and the worst points on the river, as far as navigation concerned. As examples of this, we have Plum Point Reach, ale which much is to be related hereafter, and the reach from X: Madrid (70 R) to Point Pleasant (80 R). This reach is almen straight for 10 miles, and in it the action above described went is until, in 1898, the river was over 2 miles wide, with three large an. high bars in it, dividing up the channel into four bad chutes,
In general, when we have from any cause a widening of the river there will be deposits made, and these usually take the form of mid dle bars. These middle bars are frequent in straight reaches, ani at times in bends, where the caving takes place too rapidly for the making banks to keep up with it. These bars thus made may, by : change of conditions, be soon cut away again, but they may, as has been described, act so as to intensify the widening and therefore their own growth. In such cases they grow higher and higher, bcome covered with willows, and eventually with timber, and becom: islands. Many of the islands of the river were probably made chis way, and others are growing up now. One of these, Elmot Bar or Island (160), has grown up since about 1870, and its growth from time to time may be traced on the maps of Plum Point Reach (Plate IV and Plates LVII to LXIX.) One thing specially notierable about this island is that instead of being cut away slowly at its head and growing at its foot, as is the general rule, it has grown largely at its head, which is now much farther upstream than in the early eighties.
Islands in the river are very numerous and exist of all sizes. Most of them were probably made gradually by growth from a sand bar, somewhat as above described, and some possibly by cut-offs Nearly all of the islands are so situated that the main current of the river is thrown rather to one side of them, and on this side is found the main channel of the river, the passage on the other side being usually only a high-water chute, dry or not navigable at low water. Into these chutes there is carried during the higher stages large quantities of sand or gravel, and usually considerable drift, and in this way many such chutes have in the past been completely filled and the islands thus joined on to the main bank of the river. Some such chutes are so situated that the current through them at very high stages is sufficient to keep them open or even scour them out
pt from these causes to
on the river, as far a is, we have Plum Post
hereafter, and the mai ant (80 R). This
the action above descend r 2 miles wide, with the channel into four bed it n any cause a widening these usually take the
frequent in straight
These bars thus mal
maps of Plum Pois :) One thing specially of being cut away slow he general rule, it has ch farther upstream 1
rous and exist of all mally by growth from 7 some possibly by a hat the main currento - and on this side is : ge on the other side 1 not navigable at low g the higher stages considerable drift a ast been completely
bank of the river. nt through them at or even scour them
No. 6. Shows caving bank near Caruthersville, April, 1899.
slightly, while at slightly lower stages deposits are made in It and the chute, therefore, remains for years in about the samu :: eral condition; other chutes, after having been almost closed, à been by a change of conditions scoured out and have becomtime the main channel of the river.
When these chutes behind islands are open below mid-stage discharge passing through them as the main river falls after a da reduces the amount passing through the main river, and therelin reduces the power of the river to cut a proper low-water chalu: through the sand bars made during the food. In such cases it sometimes desirable that the chutes be closed by artificial meas: and the problem of how to do this is, in the case of large and lot chutes, one of the most difficult connected with the regulation the river.
One noticeable feature of the river bottom as shown on the mas; is the large number of horseshoe-shaped lakes in the botton, These were formed by cut-offs.
As the banks in the bend of the river cave away and the poin. opposite grow up, it at times happens that these points will becom long and in places narrow. If the conditions are such that cavia: takes place on both sides of such a neck, it will get narrower au narrower, until in some high water it will be cut in two by surfa scour or by the washing out by hydraulic pressure of a pervious sub-stratum. The first result of a cut-off is to shorten the rive? and thus increase its slope both in the immediate vicinity and for miles above and below. This increased slope means increased velor ity and increased scour and bank destruction and rapid changes o regimen. This bank destruction will usually be along the concara banks of bends and tends, therefore, to the gradual lengthening of the river and thus in time recovering the length lost by the cut-off
. After a cut-off has taken place, the old bed of the river begins to silt up. This
process goes on most rapidly next to the new channel, the two ends of the old river fill up first, and there is thus formed a horseshoe-shaped lake.
On account of the rapid changes in regimen due to cut-offs, they are considered objectionable, and at some places considerable work has been done by the Commission to prevent them from happening. However, as all such cases have happened outside the limits of these districts, no further reference to them will be made.
In 1876 there occurred about 75 miles above Memphis (205 miles below Cairo) a cut-off that shortened the river by about 15 miles.