The Chautauquan, Vol. 04, December 1883

by Chautauqua Literary and Scientific Circle; Chautauqua Institution

III.—RIVERS AND GLACIERS.

We have found that the water of the river is largely derived from springs, and that all spring-water contains more or less mineral materials dissolved out of the brooks. Every river, therefore, is carrying not merely water, but large quantities of mineral matters into the sea. It has been calculated, for instance, that the Rhine in one year carries into the North Sea lime enough to make three hundred and thirty-two thousand millions of oyster shells. This chemically-dissolved material is not visible to the eye, and in no way affects the color of the water. At all times of the year, as long as the water flows, this invisible transport of some of the materials of rocks must be going on.

But let us now again watch the same river in flood. The water is no longer clear, but dull and dirty. You ascertained that this discoloration arises from mud and sand suspended in the water. You see that over and above the mineral matter in chemical solution, the river is hurrying seaward with vast quantities of other and visible materials. And thus it is clear that at least one great part of the work of rivers must be to transport the mouldered parts of the land which are carried into them by springs or by rain.

But the rivers, too, help in the general destruction of the surface of the land. Of this you may readily be assured, by looking at the sides or bed of a stream when the water is low. Where the stream flows over hard rock, you find the rock all smoothed and ground away; and the stones lying in the water-course are all more or less rounded and smoothed. When these stones were originally broken by frosts or otherwise, from crags and cliffs, they were sharp-edged, as you can prove by looking at the heaps of blocks lying at the foot of any precipice, or steep bank of rock. But when they fell, or were washed into the river, they began to get rolled and rubbed, until their sharp edges were ground away, and they came to wear the smooth rounded forms which we see in the ordinary gravel.

While the stones are ground down, they, at the same time, grind down the rocks which form the sides and bottom of the river-channel over which they are driven. You can even see in some of the eddies of the stream how the stones are kept moving round until they actually excavate deep round cavities, called pot-holes, in the solid rock.

Now, it is clear that two results must follow from this ceaseless wear and tear of rocks and stones in the channel of a stream. In the first place, a great deal of mud and sand must be produced; and, in the second place, the bed of the river must be ground down so as to become deeper and wider. The sand and mud are added to the other similar material washed into the streams by rain from the mouldering surface of the land. By the deepening and widening of the water-courses, such picturesque features as gorges and ravines are excavated out of the solid rock.

Look, again, at the channel of a river in summer. You see it covered with sheets of gravel in one place, beds of sand in another, while here and there a piece of hard rock sticks up through these different kinds of river-stuff. Note some portion of the loose materials, and you find it to be continually shifting. A patch of gravel or sand may remain for a time, but the little stones and grains of which it is made up are always changing as the water covers and moves them. In fact, the loose materials over which the river flows are somewhat like the river itself. You come back to its banks after many years, and you find the river there still, with the same ripples, and eddies, and gentle murmuring sound. But though the river has been there constantly all the time, its water has been changing every minute, as you can watch it changing still. So, although the channel is always more or less covered with loose materials, these are not always the same. They are perpetually being pushed onward, and others, from higher up the stream, come behind to take their place.

It is not in the bottoms of the rivers, then, that the material worn away from the surface of the land can find any lasting rest. And yet the rivers do get rid of a good deal of this material as they roll along. You have, perhaps, noticed that a river is often bordered with a strip of flat plain, the surface of which is only a few feet above the level of the water. Most of our rivers have such margins, and, indeed, seem each to wind to and fro through a long, level, meadow-like plain. Now this plain is really made up from the finer particles of decomposed rocks which the river has carried along. During floods, the river, swollen and muddy, rises above its banks, and spreads over the low ground on either side. Whenever this takes place, the overflowing water moves more slowly over the flats; and, as its current is thus checked, it can not hold so much mud and sand, but allows some of these materials to settle down to the bottom. In this way the overflowed tracts get a coating of soil laid over them by the river, and when the waters retire this coating adds a little to the height of the plain. The same thing takes place year after year, until by degrees the plain gets so far raised that the river, which all this while is also busy deepening its channel, can not overflow it even at the highest floods. In course of time the river, as it winds from side to side, cuts away slices of the plain and forms a newer one at a lower level. And thus a series of terraces is gradually made, rising step by step above the river.

Still the laying down of its sand and mud by a river to form one or more such river-terraces is, after all, only a temporary disposal of these materials. They are still liable to be carried away, and in truth they are carried off continually as the river eats away its banks.