The Brook

Part IV

Earth and Sky

Teacher’s Story

“Little brook, sing a song of a leaf that sailed along,
Down the golden braided center of your current swift and strong.”

—J. W. Riley.

A brook is undoubtedly the most fascinating bit of geography which the child encounters; and yet how few children who happily play in the brook—wading, making dams, drawing out the crayfish by his own grip from his lurking place under the log, or watching schools of tiny minnows—ever dream that they are dealing with real geography. The geography lesson on the brook should not be given for the purpose of making work out of play, but to conserve all of the natural interest in the brook, and add to it by revealing other and more interesting facts concerning it. A child who thus studies the brook will master some of the fundamental facts of physical geography, so that ever after he will know and understand all streams, whether they are brooks or rivers. An interesting time to study a brook is after a rain; and May or October give attractive surroundings for the study. However, the work should be continued now and then during the entire year, for each season gives it some new features of interest.

Each brook has its own history, which can be revealed only to the eyes of those that follow it from its beginning to where it empties its water into a larger stream or pond. At its source the brook usually is a small stream with narrow banks; not until it receives water from surrounding hills does it gain enough power to cut its bed deeper into the earth, thus making its banks higher. Where it flows with swift current down a hillside, it cuts its bed deeper, because swift-moving water has more power for cutting and carrying away the soil. However, if the hillside happens to be in the woods, the roots of trees or bushes will help to keep the soil from being washed away. Unless there are obstacles, the course of the brook is likely to be more direct in flowing down a hillside than when crossing level fields. The delightful way in which brooks meander crookedly across the level areas is due to the inequalities of the surface, which interfere more with water on a plain than on a hillside, since the gravity which pulls it forever down has less chance to act upon it forcibly in these situations. After a stream has thus started its crooked course, in time of flood the current strikes with more force against the curves, and cutting them deeper, makes the course still more crooked. The places on the banks where the soil is bare and exposed to the force of the current, are the points where the banks are cut most deeply at flood time.

But the brook is not simply an object to look at and admire; it is a very busy worker, its chief labor being that of a digger and carrier. When it is not carrying anything—that is, when its waters are perfectly clear—the stream is doing the least work. The poets, as well as common people, speak of the playing of the brook when its limpid waters catch the sunbeams on their dimpling surface; but when the waters are roily the brook is working very hard. This usually occurs after a rain, which adds much more water to the volume of the brook; the action of gravity upon this larger and heavier body forces it to flow more swiftly and every drop in the stream that touches the bank or bottom, snatches up a tiny load of earth and carries it along. And every drop thus laden, when it strikes against a corner of the bank, tears more soil loose through the impact, and other drops snatch it up and carry it on down the stream. And after a time there are so many drops carrying loads and bumping along, knocking loose more earth, that the whole brook, which is made up of drops, looks muddy. In its work as a digger, every drop of water that touches the soil at the bottom or on the banks of the brook uses its own little load of earth or gravel as a crowbar or pickaxe to pry up other bits of dirt and gravel; and all of the drops hastening on, working hard together, dig the channel of the brook wider and deeper. In some steep places, so many of the drops are working together that they are able to pick up pebbles or stones, with which they batter and tear down larger pieces of the bank and scrape out greater holes in the bottom of the stream. On and on the brook flows, a gang of workmen each of which is using its own load as a tool, all in close procession and working double quick. But as soon as the brook reaches a plain or level, its activity ceases; the drops act tired and seem to have no ambition to pick up more soil, and each lets fall its own load as soon as possible, dropping the larger pieces of gravel and rock first, carrying the finer soil farther, but finally letting that down also. If we examine the sediment of a flooded brook, we find that the gravel is always dropped first, and that the fine mud is carried farthest before it is deposited.

The roar of a flooded stream is very different from the murmur of its waters when they are low. It is not to be wondered at, when we once think of all that is going on in the brook during periods of flood. There are some simple experiments to show what the force of water can do when turned against the soil. Pour water from a pitcher into a bed of soft soil, and note how quickly a hole will be made; if the pitcher is held near the soil, less of a hole will be formed than if the pitcher is held high up, which shows that the farther the water falls, the greater is its force. This explains why the banks of streams are undermined when a strong current is driven against them. The swift current, of course, breaks away more earth at bends and curves than when it is flowing in a straight line; for ordinarily, when flowing straight, the current is swiftest in the bed of the stream, and is therefore only digging at the bottom; but when it flows around curves, it is directed against the banks, and therefore has much more surface to work upon. Thus it is that bends are cut deeper and deeper. If the bare arm is thrust into a flooded brook, we find that many pieces of gravel strike against it; and if we reach the bottom, we can feel the pebbles being moved along over the brook bed.