Of these varieties the first is found more abundantly than all the others combined. In many places it has fallen down and strews the country to such an extent as to be absolutely impassable on horseback. There is very little timber of marketable value, and at first thought it would seem that nature has here lavished her energies in a most wasteful manner. But the great value of these forest growths, is their agency in the conservation of a water supply for the surrounding country. A glance at the map will show that the Park is in the midst of a vast arid region extending far into the surrounding states. The reclamation of these desert wastes, and their conversion into productive lands, can be accomplished by irrigation alone, and for this purpose the abundant streams which descend from the mountains are the indispensable water supply. From the summit of the Grand Teton, the range of vision covers probably the most remarkable group of river sources upon the earth. To the north rises the Missouri which flows three thousand miles through Montana, the Dakotas, Iowa, Nebraska, Kansas and Missouri. To the east rises the Yellowstone, which, after leaving the Park, flows four hundred miles through southern and eastern Montana until it unites with the Missouri. From the eastern foot-hills of the Absaroka and Shoshone Ranges flow the Wind and Big Horn Rivers through the extensive valleys of the same names in Wyoming and Montana. Southwardly, across the Wind River Range rises the Platte which flows eastward eight hundred miles through Wyoming and Nebraska. From the west flank of these mountains issue the tributaries of the Green River (afterward the Colorado) which flows through Wyoming, Utah and Arizona into the Gulf of California. Finally, interlaced with the sources of the Yellowstone, and the Missouri, are those of the Snake River which flows through Wyoming, Idaho and Washington into the Columbia, and thus reaches the Pacific. Not only do these streams rise in this limited area; they derive from it most of their waters. In the arid lowlands they receive but slight accessions, and often actually shrink under evaporation. It is therefore from a relatively small tract of country that the future water supply must come for portions of ten states in the great arid belt of the west.