Popular Science Monthly/Volume 20/April 1882/The Unaweep Cañon
|THE UNAWEEP CAÑON.|
By HENRY GANNETT.
OF all the physical features of the earth, the courses of rivers are among the most unchangeable. Once outlined, they are adhered to with a wonderful tenacity. Only a general change in the slope of their basins will usually suffice to divert them from their original courses. Mountains and plateaus may rise across their paths, but, like a saw, the river cuts its way through the obstacle. It is very rare to find a case where a river has been diverted from its course by the rising of a mountain-range or other minor elevation across it, while numberless instances of rivers having overcome such obstacles are to be seen in all mountainous regions. The Cordilleran region of the West presents us with many such examples. Many are familiar with the gorge by which Green River passes the Uintah range, in Wyoming 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 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 hundred feet in height. The last two rivers, entering the valley from the east, cross it immediately and at once take a general northwest course. The Grand flows on toward the northwest until it rounds the end of the Uncompahgre Plateau, when it sweeps slowly around to the southwest, and resumes its normal direction, still hugging closely the left side of the valley.
Southwest of the Uncompahgre Plateau the eye looks out over a stretch of low, arid plateaus, traversed, in deep canons, by the Rio Dolores and its tributaries. This stream is an important branch of the Grand, entering it at the foot of the great valley, after that river has passed around the end of the Uncompahgre Plateau, and just as it plunges into the course of canons by which it passes the Sierra la Sal.
Traveling along the crest-line of the Uncompahgre Plateau, one is interested in observing how sharply the crest-line serves as a water-parting. The drainage toward the west heads in the crest, without cutting it at all. The bluff-wall is everywhere continuous. But in the midst of these observations one is astonished by riding suddenly to the verge of a tremendous gorge, thousands of feet in depth, which apparently comes in from the west, and extends up into the plateau to the northeastward, against the slope and dip of the strata, as far as the eye can reach. Down in the depths he sees a small stream flowing westward. Descending into this cañon, an operation not easily performed at any point, and traversing its bed northeastward, he comes in a few miles to a divide in the canon, and beyond the divide he finds a small stream flowing northeast into the Gunnison. This is the Unaweep Cañon.
The Unaweep Cañon is cut across the Uncompahgre Plateau, from the great valley of the Gunnison and Grand on the northeast, to the low, desert plateaus on the southwest. Its course is nearly southwest, and almost precisely at right-angles to the crest of the plateau. It joins the Gunnison at a point about six miles above the mouth of the latter stream, at an elevation above the sea of 4,600 feet. Tracing its course southwestward, its bed is seen to rise slowly, but not so rapidly as the level of the plateau, so that the cañon increases gradually in depth. The bottom rises to a divide, 7,000 feet above sea-level, and several miles east of the crest of the plateau. The walls at the divide have a height of 1,200 feet. West of the divide, the slope of the bed of the cañon changes slowly, and its descent to the westward does not become very rapid until the crest of the plateau is passed. At the crest, the height of the cañon-walls is 3,000 feet. At the point of junction with the Rio Dolores, the elevation is 4,618 feet above the sea, or practically the same as at its junction with the Gunnison.
The first few miles from the Gunnison the cañon is very narrow, has no great depth, and is cut in soft, recent, sedimentary rocks. This portion might easily have been cut by the small stream now occupying it—a theory which is supported by the fact that the strata here on the margin of the plateau have been very much disturbed, which would easily have obliterated all traces of the original cañon. Entering the body of the plateau, the cañon suddenly assumes a greater breadth. As it increases in depth, it exposes a series of several hundred feet of stratified rocks, below which the granite comes to the surface. At the crest of the plateau the lower two thirds of the walls are of granite, the upper 1,000 feet being sedimentaries. From its northeastern end the cañon gradually widens from a bed-breadth of a hundred yards to one of at least a mile; at the crest, the cañon suddenly narrows to a mere stream-way, with rugged, vertical walls.
It is purely a cañon of erosion. There is no sign whatever of any local disturbance which could account for its existence. The strata on the two sides are perfectly conformable.
To attribute this cañon to the streams now occupying it is manifestly absurd. Not only are they utterly insignificant in comparison with the amount of erosion which has taken place, but no similar streams, of any magnitude whatever, could have cut the cañon down at the divide; neither could the western one have cut back into the plateau to any such extent. There is no lateral slope toward this cañon to determine the drainage of any considerable area of the plateau in this direction. It is plainly the scene of the defeat of a large stream, in its struggle to maintain its ancient course—of a victory of volcanic over aqueous forces. Another thing is equally apparent—that the stream here diverted came from the eastward and not from the westward, and that it was the Grand River. This is shown by the following considerations: (1) The general slope of the country, disregarding local accidents of topography, is from the northeast toward the southwest; (2) the direction of the course of Grand River above this cañon, which is almost precisely in line with (3) the character of its course, which shows that it antedated all other uplifts, and, as the Uncompahgre Plateau, from its trend and association, must have been coexistent with the rest, it must have antedated this also; (4) the features of the cañon itself afford the strongest possible evidence of a stream flowing southwest through it. The profile, with the summit east of the crest of the plateau, the slow descent west of the summit to the crest of the plateau, and the rapid descent beyond the crest, point unmistakably to this conclusion. The plan of the cañon is no less clear in its indications. It is well known that a rapid stream erodes its bed downward; a sluggish one, on the other hand, erodes laterally, thus broadening its bed. Here we have precisely these phenomena. Beyond the crest of the plateau, where the slope is great, the cañon is very narrow; while east of the crest, where the velocity of the stream must have been very much lessened, it widens rapidly, and then gradually diminishes in width.
At what stage in the rise of the Uncompahgre Plateau the river abandoned the unequal contest and took its present course around the end of the plateau, we have no means of knowing definitely. Present relative elevations can not be relied upon as safe evidence. It is probable, however, that it was not far from the close of the rise, as other-wise the bed of the cañon, under the crest of the plateau, rising more rapidly than at any other point, would be the highest point, and the divide would be there instead of being several miles farther east.
Altogether, this phenomenon affords a very interesting study for the structural geologist and the geographer.