Page:Popular Science Monthly Volume 20.djvu/805

Rh rapidity of its current. Erosion is therefore more rapid below the crest. But this erosion not only deepens the bed of the stream below the crest, but also pares away the crest, from down-stream upward, so that the point where the velocity of the stream changes is constantly tending up-stream. This point, then, will always be found above, i. e., up-stream from the crest of the obstacle. The degree of obstruction which this rising mass will afford to the stream depends, not at all on the absolute height to which it may rise, but to the degree of rapidity of its rise as compared with the erosive power of the stream. If the rate of rise be greater than the erosive power at first, it forms a temporary dam, and a lake is produced above the obstacle, which increases in depth until a sufficient fall is given to the stream to enable it to cut at the same rate at which the range rises. Then equilibrium is established, and the cutting goes on at the same rate as the range increases in elevation. When the rise ceases, the lake is gradually drained in whole or in part, as the river gradually cuts away the dam by moving its crest up-stream. A diversion of the stream can only occur by reason of a new channel being made accessible by the rise of water back of the dam.

Such, in brief, is the conduct of a river when its course is in danger from the rise of a mountain-range across it. It may be added that the many examples before us show that in nearly every case the river has had little trouble in sawing its way through them. Dams have seldom risen to very great height, nor have lakes collected to great depths.

One very remarkable case has, however, come under the writer's personal observation, of a river having been diverted from its course, and forced to take a long detour, having made an unsuccessful attempt to cut through the obstacle. In the western part of Colorado, near the Utah boundary, is a great plateau, known as the Uncompahgre Plateau. This is an inclined plateau; its crest-line, starting at the northern base of the San Juan Mountains, runs off northwestward for fifty miles or more. It slopes with the dip of the strata at a low angle to the northeast, breaking off toward the southwest by a series of gigantic steps. Its crest ranges in height from 9,000 to 10,000 feet above sea-level, while the valley of the Gunnison, at its eastern base, has an elevation from 4,500 to 5,000 feet. At its eastern base lies the valley of the Uncompahgre, Gunnison, and Grand, which trends off to the northwest, and is occupied successively by the rivers above named. It is a broad, open valley, ranging in width from ten to twenty miles. In its southeastern and upper part it is traversed, near the middle, by the Uncompahgre River. Below the point where this stream joins the Gunnison, the latter hugs the base of the plateau, and is hidden by cañon-walls a few hundred feet in depth. The Grand, below its junction with the Gunnison, also flows close to the southwest side of the valley, and is, for a part of its course, in a low cañon, fifty to one