Many of my readers who have lived out-of-doors a great deal have learned by common observation the simple problem of how the forest regulates stream flow. Any one who has been in a treeless region after a heavy rainstorm can recall how suddenly the streams swell and flood their banks, and how soon these same streams return to their former flow. On the other hand, a severe rainstorm in a forested region will hardly have an appreciable effect upon the streams. The difference is not very hard to explain. In a treeless region there are no natural obstacles which might delay or prevent the raindrops from reaching the ground. The soil is usually hard and dry, and the water runs off as though from a gable roof. In a forest, we well know, the crowns of the trees intercept most of the rain that falls; very little strikes the ground directly. The rain that strikes the crown is dissipated on the leaves or needles, on the twigs and branches, and on the trunk. It must travel a long way before it reaches the ground, and all this delay helps in preventing a rapid run-off or flood. The soil in the forest is covered by a living ground cover of flowers, shrubs and young trees, and by a dead cover composed of leaves, twigs, dead branches, fallen trees, all of which interrupt the raindrop's journey to the ground. Even after the rain reaches the ground, only a small part of it goes off as surface run-off. The soil in the forest is loose and full of holes and channels made by decaying roots, earth worms, etc., so that the water is absorbed as fast as it reaches the soil. Also the soil in the forest contains a large amount of organic matter, resulting from decaying leaves and branches, and this organic matter acts as a great sponge, because it is capable of holding several times its own weight of water. As a result of the living and dead ground cover, the crown cover, and the organic matter in the soil, the rainfall is fed to the streams gradually through weeks and months, instead of a few hours, and the nearby rivers have a steady, equable flow, instead of alternate stages of floods and low water. Closely bound up with the protection of watersheds is the erosion problem. Without a forest cover, rain runs off mountain slopes very rapidly, often carrying with it silt and sand, and, in severe floods, even rocks and bowlders. A well known physical law states that the carrying capacity of a stream increases as the sixth power of its velocity. In other words, double the velocity of a stream and you have multiplied its carrying power by 64; increase its velocity ten times, and you multiply its carrying power by a million. The delay caused by the forest cover in each raindrop's journey down a mountain side not only prevents floods, but also preserves the fertility of the fields in the valleys below.