In seasons of flood it carries loads and loads of pebbles along. As the flood goes down these pebbles are dropped and covered with the sediment that settles along its banks. Then these pebbles begin to decay and so enrich the soil. Later along comes another flood, takes the pebbles out of the bank, carries them farther along, and, as the waters go down, puts them back in the bank again. In course of time this kind of fresh food from the decaying pebbles gets carried into the sea, where it helps to furnish food and shell material for the shell-fish and raw material to be worked up by the sea's rock mills. [Illustration: WAYS OF A WANDERING RIVER] III. The Machinery of the Rivers To do all their great part in the world's work the rivers need only time, enthusiasm, patience, machinery, and tools. All these the rivers have, and the machinery they use and the engineering methods they follow are much more modern than we would suppose. Take, for example, the way in which rivers widen their banks. The current cuts with the greatest force on the outside of bends, and the motion and effect is practically that of a circular saw. This sawing is done on the largest scale where the current meanders. Swinging from side to side it cuts away both banks. And what it cuts away it spreads over the valley by its back-and-forth motion, much as men spread dirt with scrapers when they are grading a road. That's how crooked rivers make broad valleys. But they have to have the help of us pebbles, too. We're hard to get along without! Notice, the next time the river or the creek is up, the rolling, hopping motion of the pebbles as they are carried along by the rushing water. It is these pebbles grinding on the bottom and sides of the river's bed that help most in this kind of valley deepening and widening. In the same way we pebbles helped dig those grand affairs, the gorges and the canyons in the mountains. The Grand Canyon of the Colorado is a part of our work. In the widening of valleys the circular saws of crooked streams are very useful, but there are other things at work. The rains dissolve the soil and wash the banks away and slope them down; Jack Frost, with his wedges, pries out both soil and rock; the little farmers with many feet--the burrowing animals and insects--and the famous farmer with no feet at all--the angleworm--loosen soil, and so help the river to carry it away; and the ice, when the river breaks up in the spring, chisels off the banks as it passes. [Illustration: HOW RIVERS BUILD STONE BRIDGES