THE UN A WEEP CANON. 783 



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 ob- 

 struction 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 de- 

 gree 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 equi- 

 librium is established, and the cutting goes on at the same rate as the 

 range increases in elevation. When the rise ceases, the lake is gradu- 

 ally 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 dan- 

 ger 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 Pla- 

 teau. This is an inclined plateau ; its crest-line, starting at the north- 

 ern 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 gigan- 

 tic 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 

 canon-walls a few hundred feet in depth. The Grand, below its junc- 

 tion with the Gunnison, also flows close to the southwest side of the 

 valley, and is, for a part of its course, in a low caiion, fifty to one hun- 



