Puslapio vaizdai
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this alluvial terrace is but a narrow belt on either side of the stream, which, swollen by its floodwaters, often breaks new channels through this bench of detrital matter. In fact, all this marginal accumulation is of temporary duration, for the stream is as yet wild, and in its annual floods is apt to undo the construction-work of the previous years.

When the stream comes to have a distinct and somewhat enduring alluvial belt on either side of its path, it has entered on the stage of a river. It is indeed on the presence of this marginal accumulation that we most rest the distinction between a torrent and a river.

Cañon of the Via Mala, Switzerland.

(Showing the work done by a large torrent on rocks of close texture which are readily eroded by the stream.)

From the place where the terraces begin to form, downward to the mouth of the

stream, the conditions of its flow are vastly affected by its reactions upon this

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(Showing extent of erosion in the surrounding plateau; the sharp hills are the necks of old volcanoes, the cones of which have been worn away by the river action.)

vails by fits and starts, under the action of a curious law which causes its current to rebound from bank to bank. The nature of this principle of rebounding

alluvium from the grasp of the roots, and will then cut under the trees, causing considerable areas of forests to be precipitated into the waters and borne

away to the sea. From the point of impact, the current will again rebound in a manner which will cause it, at a certain distance below, to strike against the opposite bank, where it will again make swift encroachment against the forestprotection. After this second assault, it will swing across to a lower point on the shore against which it first impinged, and so the oscillations from side to side will be propagated down stream, it may be for a hundred miles or more. A single jetty of this description, as it has been observed in the rivers of India, will

natural jetty or bar at its mouth, thus gradually forcing the current of the larger stream against the opposite side, creating a bar there. It is furthermore to be noted, as is shown in the diagram, that between the points where the river impinges against the bank there is a space of dead water or eddying currents in which the forests find it easy to make head against the river and to extend the alluvial plain.

Thus, in the process of nature, it comes about that our rivers tend to build channels in their alluvial plains which are ex

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(Showing mountain wall, talus leading to valley, and stream embarrassed by débris.)

affect the oscillations of the current for an indefinite distance downward in its course. That which is accomplished by artifice in an immediate manner is more slowly brought about by natural causes. Each tributary stream which enters the main channel commonly has a greater swiftness of current than the larger stream into which it flows. It therefore bears in a mass of pebbles and builds a

tremely devious in their course. If the alluvial plains be wide, the river is constantly forming great ox-bow-like curves, isthmuses with narrow peninsulas such as are often seen in the lower portions of the Mississippi Valley. Finally the narrow places which connected these promontories on the shore are cut through in some time of flood, the river finding a shorter way downward to the

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