It is impossible to grow a sound and thrifty forest for future generations if there are unhealthful conditions in the forest that are a constant menace to the trees. The first step in this hygienic work is close observation on the part of the Forest officers. The next important step is to prevent the infection and infestation of sound trees by getting rid of all diseased and insect-infested living and dying trees. By means of timber sales and free use, Forest officers very materially help in establishing healthy conditions on the National Forests. There is a clause in most timber sale contracts which requires the cutting by the purchaser of all snags and other unhealthy trees on the area. This measure not only eliminates undesirable trees from a hygienic standpoint, but it also makes it possible to utilize the merchantable timber left in undesirable trees, which would otherwise go to waste. On timber sales, Forest officers who do the marking leave for reproduction only such trees as are perfectly sound and healthy. Mistletoe infested trees, especially, are marked for cutting, for neither in plant nor in animal life can healthy offspring be expected to develop under unhealthful conditions. [Illustration: Figure 49. Wrecked farm buildings due to flood of May 21, 1901, Nolichucky River, near Erwin, Tenn. This is one result of denuding the Appalachian Mountains of their forest cover.] [Illustration: Figure 50. When steep hillsides are stripped of their forest growth, erosion results. Erosion has been especially serious in the Appalachian Mountains. View taken in Madison County, North Carolina.] _Water Supply._ Undoubtedly the greatest value of the mountain forests of the West, most of which are within the National Forests, lies in their influence upon the regularity of the water supply. In many States these mountains afford the only water supply for domestic use, for irrigation, and for the development of power. The future development of the entire region depends, therefore, upon a regular water supply. It is not so much the amount of water as the manner in which it flows from the mountains that is important. To insure this regularity, the vegetative covering is an important factor. For this reason, Congress made the preservation of conditions favorable to stream flow one of the principal objects in the establishment and administration of the National Forests.