Page:Popular Science Monthly Volume 20.djvu/804

782 and Utah, where the river has apparently preserved even its minor sinuosities in a cañon thousands of feet in depth. The Colorado River system is a magnificent example of the persistence of rivers. Established ages ago, when only the great general contour of the region was outlined, it antedated all the ranges of mountains and the plateaus which now diversify the surface. Nearly all these uplifts are at right angles, or nearly so, to the courses of the main streams; yet, in all cases save one, the rivers have preserved their courses, by cutting gorges as the mountains rose. Grand River, which is one of the two largest branches of the Colorado, presents us with a fine succession of these cases. Indeed, it may be said that from its head, in Middle Park, to its mouth, the river is almost continually in trouble; its course is nothing but a succession of gorges and of transverse valleys. In Middle Park it cuts several minor ranges; at its point of exit from the park it encounters the Park ranges, which it cleaves from summit to base, making a cañon two to three thousand feet in depth. Then follow many miles of precipitous canon, of great depth, which the river has carved in a high plateau. Emerging from this it meets a barrier, in what is known as the Hogback range, through which it levels a passage. Then follows for many miles a deep and narrow valley, between the Book Cliffs on the north and the Battlement Mesa on the south, which looks down on the river from a height of fully four thousand feet. Next, the Little Book Cliffs dispute its passage. These face the west, and toward the east, in which direction the river approaches, have a long and gentle slope downward. The river, holding steadily its course, enters the plateau, and rapidly eats its way below the surface. For many miles its cañon is so deep, narrow, and tortuous, that it can with the utmost difficulty be traced. At the face of the cliffs it emerges suddenly to daylight, in the broad, desert valley of the Gunnison. It holds its normal course across this valley, meeting the Gunnison on the west side. Then, right against the bluffs which border the Uncompahgre Plateau, it turns sharply at right angles, and flows off northwest, then west, then southwest again, and south, hugging closely the northern end of this great plateau, while on the right stretches away the desert expanse of the Grand River Valley to the base of the Book Cliffs.

It may be interesting to trace the behavior of a stream under these trying circumstances, when a mountain-range rises to dispute its path. We are not here concerned with those mountains which have arisen suddenly, by catastrophic action, but only with such as have been slowly evolved. In the former case, rivers, like all other natural features, share in the general overturning and destruction. When an elevation commences gradually across the course of a river, its first effect is to lessen the rapidity of the current above the crest of the elevation and to increase it below that point. The erosive power of a stream is proportional, other things being equal, to the