Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

As the volume of a mountain torrent increases through the junction of many tributaries, the energy of its moving waters becomes sufficient to sweep away the fragments which come to its bed. Before this stage is attained the stream rarely touches the solid under rock of the mountain, the base of the current resting upon the larger loose stones which it was unable to stir. In this pebble-paved section, because the stream could not attack the foundation rock, we find no gorges—in fact, the whole of this upper section of the torrent system is peculiarly conditioned by the fact that the streams are dealing not with bed-rock, but with boulders or smaller loose fragments. If they cut a little channel, the materials from either side slip the faster, and soon repave the bed. But when the streams have by a junction gained strength, and can keep their beds clear, they soon carve down a gorge through which they descend from the upper mountain realm to the larger valleys, where their conjoined waters take on a riverlike aspect. It should be noted here that the cutting power of the water moving in the torrent or in the wave, the capacity it has for abrading rock, resides altogether in the bits of stone or cutting tools with which it is armed. Pure water, because of its fluidity, may move over or against firm-set stones for ages without wearing them; but in proportion as it moves rocky particles of any size, the larger they are, the more effective the work, it wears the rock over which it flows. A capital instance of this may be found where a stream from a hose is used in washing windows. If the water be pure, there is no effect upon the glass; but if it be turbid, containing bits of sand, in a little while the surface will appear cloudy from the multitude of line scratches which the hard bits impelled by the water have inflicted upon it. A somewhat similar case occurs where the wind bears sand against window panes or a bottle which has long lain on the shore. The glass will soon be deeply carved by the action, assuming the appearance which we term "ground." This principle is made use of in the arts. Glass vessels or sheets are prepared for carving by pasting paper cut into figures on their surfaces. The material is then exposed to a jet of air or steam-impelling sand grains; in a short time all the surface which has not been protected by paper has its polish destroyed and is no longer translucent.

The passage from the torrent to the river, though not in a geographical way distinct, is indicated to the observant eye by a simple feature—namely, the appearance of alluvial terraces, those more or less level heaps of water-borne débris which accumulate along the banks of rivers, which, indeed, constitute the difference between those streams and torrents. Where the mountain waters move swiftly, they manage to bear onward the waste which they receive. Even where the blocks of stone cling in the bed, it is only a short time before they are again set in motion or ground to pieces. If by chance the detritus accumulates rapidly, the slope is steepened and the work of the torrent made more efficient. As the torrent comes toward the base of the mountains, where it neither finds nor can create steep slopes over which to flow, its speed necessarily diminishes. With each reduction in this feature its carrying power very rapidly diminishes. Thus water flowing at the rate of ten miles an hour can urge stones four times the mass that it can move when its speed is reduced to half that rate. The result is that on the lowlands, with their relatively gentle slopes, the combined torrents, despite the increase in the volume of the stream arising from their confluence, have to lay down a large part of their load of detritus.

If we watch where a torrent enters a mountain river, we observe that the main stream in a way sorts over the waste contributed to it, bearing on only those portions which its rate of flow will permit it to carry, leaving the remainder to be built into the bank in the form of a rude terrace. This accumulation may not extend far below the point where the torrent which imported the débris joins the main stream; a little farther down, however, we are sure to find another such junction and a second accumulation of terrace material. As these contributions increase, the terrace accumulations soon become continuous, lying on one side or the other of the river, sometimes bordering both banks of the stream. In general, it can be said that so long as the rate of fall of the torrent exceeds one hundred feet to the mile it does not usually exhibit these shelves of detritus. Below that rate of descent they are apt to be formed. Much, however, depends upon the amount of detritus which the stream bears and the coarseness of it; moreover, where the water goes through a gorge in the manner of a flume with steep rocky sides, it can urge a larger amount before it than when it traverses a wide valley, through which it passes, it may be, in a winding way.

At first sight it may seem rather a fine distinction to separate torrents from rivers by the presence or absence of terraces. As we follow down the stream, however, and study its action in relation to these terraces, and the peculiar history of the detritus of which they are composed, we perceive that these latter accumulations are very important features. Beginning at first with small and imperfect alluvial plains, the river, as it descends toward the sea, gaining in store of water and in the amount of débris which comes with that water from the hills, while the rate of fall and consequent speed of the current are diminished, soon comes to a stage where it is engaged in an endless struggle with the terrace materials. In times of flood, the walls of the terraces compel the tide to flow over the tops of these accumulations. Owing to the relative thinness of the water beyond the bed, and to the growth of vegetation there, the current moves more slowly, and therefore lays down a considerable deposit of the silt and sand which it contains. This may result during a single flood in lifting the level of the terrace by some inches in height, still further serving to restrict the channel. Along the banks of the Mississippi and other large rivers the most of this detritus falls near the stream; a little of it penetrates to the farther side of the plains, which often have a width of ten miles or more. The result is that a broad elevation is constructed, a sort of natural mole or levee, in a measure damming the flood waters, which can now only enter the "back swamps" through the channels of the tributary streams. Each of these back swamps normally discharges into the main stream through a little river of its own, along the banks of which the natural levees do not develop.

We have now to note a curious swinging movement of rivers which was first well observed by the skilful engineers of British India. This movement can best be illustrated by its effects. If on any river which winds through alluvial plains a jetty is so constructed as to deflect the stream at any point, the course which it follows will be altered during its subsequent flow, it may be, for the distance of hundreds of miles. It will be perceived that in its movements a river normally strikes first against one shore and then against the other. Its water in a general way moves as does a billiard ball when it flies from one cushion to another. It is true that in a torrent we have the same conditions of motion; but there the banks are either of hard rock or, if of detritus, they are continually moving into the stream in the manner before described. In the case of the river, however, its points of collision are often on soft banks, which are readily undermined by the washing action of the stream. In the ordinary course of events, the river beginning, we may imagine, with a straight channel, had its current deflected by some obstacle, it may be even by the slight pressure of a tributary stream, is driven against one bank; thence it rebounds and strikes the other. At each point of impinge it cuts the alluvium away. It can bear on only a small portion of that which it thus obtains; the greater part of the material is deposited on the opposite side of the stream, but a little lower down, where it makes a shallow. On these shallows water-loving plants and even certain trees, such as the willows and poplars, find a foothold. When the stream rises, the sediment settles in this tangle, and soon extends the alluvial plain from the neighbouring bank, or in rarer cases the river comes to flow on either side of an island of its own construction. The natural result of this billiard-ball movement of the waters is that the path of the stream is sinuous. The less its rate of fall and the greater the amount of silt it obtains from its tributaries, the more winding its course becomes. This gain in those parts of the river's curvings where deposition tends to take place may be accelerated by tree-planting. Thus a skilful owner of a tract of land on the south bank of the Ohio River, by assiduously planting willow trees on the front of his property, gained in the course of thirty years more than an acre in the width of his arable land. When told by the present writer that he was robbing his neighbours on the other side of the stream, he claimed that their ignorance of the laws of river motion was sufficient evidence that they did not deserve to own land.

In the primitive state of a country the water-loving plants, particularly the trees which flourish in excessively humid conditions, generally make a certain defence against these incursions of the streams. But when a river has gained an opening in the bank it can, during a flood, extend its width often to the distance of hundreds of feet. During the inundations of the Mississippi the river may at times be seen to eat away acres of land in a single day along one of the outcurves of its banks. The undermined forests falling into the flood join the great procession of drift timber, composed of trees which have been similarly uprooted, which occupies the middle part of the stream. This driftwood belt often has a width of three or four hundred feet, the entangled stems and branches making it difficult for a boat to pass from one side of the river to the other.

Fig. 11.—Oxbows and cut-off. Showing the changes in the course of a river in its alluvial plain.

When the curves of a river have been developed to a certain point (see Fig. 11), when they have attained what is called the "oxbow" form, it often happens that the stream breaks through the isthmus which connects one of the peninsulas with the mainland. Where, as is not infrequently the case, the bend has a length of ten miles or more, the water just above and below the new-made opening is apt to differ in height by some feet. Plunging down the declivity, the stream, flowing with great velocity, soon enlarges the channel so that its whole tide may take the easier way. When this result is accomplished, the old curve is deserted, sand bars are formed across their mouths, which may gradually grow to broad alluvial plains, so that the long-surviving, crescent-shaped lake, the remnant of the river bed, may be seen far from the present course of the ever-changing stream. Gradually the accumulations of vegetable matter and the silt brought in by floods efface this moat or oxbow cut-off, as it is so commonly termed.

As soon as the river breaks through the neck of a peninsula in the manner above described, the current of the stream becomes much swifter for many miles below and above the opening. Slowly, however, the slopes are rearranged throughout its whole course, yet for a time the stream near the seat of the change becomes straighter than before, and this for the reason that its swifter current is better able to dispose of the débris which is supplied to it. The effect of a change in the current produced by such new channels as we have described as forming across the isthmuses of bends is to perturb the course of the stream in all its subsequent downward length. Thus an oxbow cut-off formed near the junction of the Ohio and Mississippi may tend more or less to alter the swings of the Mississippi all the way to the Gulf of Mexico.