As to water supply, the desert belongs in two regions, arid and semi-arid. West of the Missouri and extending to the foot of the Rocky Mountains, is a vast area of foothills and gently sloping, grassy prairies, which constitute a large part of the semi-arid or sub-humid region of the United States. This broad belt, stretching from Mexico northward into Canada, has no clear-cut boundary separating it from the humid region on the east or the arid region on the west, owing to the variance of the mean annual precipitation in many localities. A convenient and easily marked line for the eastern boundary of the arid region is one closely following the hundred and third meridian. On the north it bends away from the meridian toward the west, and on the south tends eastward north of the Rio Grande. On the west the arid region extends to the Pacific Coast in extreme southern California, but from Monterey north there is a narrow belt of semi-arid and humid country bordering the Pacific Ocean. The Coast Range is humid, especially in Oregon and Washington, where the rainfall is as heavy as in any other part of the United States. [Illustration: FORTY BUSHELS OF WHEAT PER ACRE Shoshone Project, Wyoming. This was desert land in 1915] The true desert, wherein the production of crops is wholly dependent upon the artificial application of water, lies for the most part west of the Rocky Mountains. Its estimated area, 900,000 square miles, is probably slightly greater than that of the semi-arid region. An accurate determination of the relative size of each, however, is not possible until a comprehensive hydrographic survey has been completed. Contrary to popular opinion, this region is not a vast wilderness, desolate and unpromising. Between the mountain ranges lie many beautiful valleys, through which flow numerous streams fed by melting snows. Utah and Nevada contain large areas of sage-brush desert, comprising what is known as the Great Basin, which has no outlet to the sea, and is doomed to aridity by reason of the absence of living streams. Millions of acres of desert now inhabited by the coyote and the rattlesnake await only the coming of the engineer to wake to teeming fecundity. In one important respect the arid and semi-arid regions are alike, and that is in the character of the streams. Almost without exception the rivers of the West are erratic and “flashy” in flow, are subject to long periods of drought and sudden and destructive floods. The full utilization of these streams for irrigation and power necessitates storage. In the control of floods, engineers have found their greatest problem, and one whose solution has taxed the skill, imagination and daring of twentieth-century genius. [Illustration: WHEAT ON SHOSHONE PROJECT, WYOMING This land was desert in 1908] _National Irrigation_