The Romance of the Colorado River. Frederick Samuel DellenbaughЧитать онлайн книгу.
level of their surroundings. The perpendicularity of the walls of these channels, or canyons as they are called, depends on the volume and continuity of the flowing stream, on the aridity of the country through which they are cut, and on the rock-formation. A fierce and continuous torrent, where the rainfall is at the minimum, will so speedily outrival the forces of erosion that the canyon will have vertical walls. An example is seen in those frequent "mud" canyons found in arid regions, where some brook, having its source in highlands, cuts a channel through clay or dry earth with vertical sides, that stand for years. As long as the surface of the adjacent lands is undisturbed, it acts like a roof, throwing off the water that falls upon it into the main stream.* Thus the foundations of these walls are not assailed from BEHIND, which is their weakest point. If the land surface is broken up, permitting the rains to soak in and saturate the clay or earth, the whole mass becomes softened and will speedily fall and slide out into the canyon.** The sides of all canyons in an arid region are more or less protected in the same way. That is, the rains fall suddenly, rarely continuously for any length of time, and are collected and conducted away immediately, not having a chance to enter the ground. Homogeneous sandstone preserves its perpendicularity better than other rocks, one reason being that it does not invite percolation, and usually offers, for a considerable distance on each side of the canyon, barren and impervious surfaces to the rains. Where strata rest on exposed softer beds, these are undermined from the front, and in this way recession is brought about.
* Just as wheat flour getting wet on the surface protects the portion
below from dampness. The rainfall is often so slight, also, that a
surface is unchanged for years. I once saw some wagon tracks that were
made by our party three years before. From peculiar circumstances I was
able to identify them.
** Robert Brewster Stanton explained this very clearly in his
investigations for the Canadian Pacific Railway into the causes of
land-slides on that line.
In the basin of the Colorado are found in perfection all the extraordinary conditions that are needed to bring forth mammoth canyons. The headwaters of all the important tributaries are INVARIABLY IN THE HIGHEST REGIONS and at a long distance from their mouths, so that the flood waters have many miles of opportunity to run a race with the comparatively feeble erosive forces of desert lands. The main stream-courses are thus in the lower arid regions and in sedimentary formations, while their water-supply comes from far away. The deepest gorges, therefore, will be found where the rainfall is least, unless diminishing altitude interferes. Thus the greatest gorge of the whole basin, the Grand Canyon, is the one farthest from the sources of supply, and in the driest area, but one, of the whole drainage system. It ends abruptly with the termination of the high arid plateau which made it possible, but had this plateau extended farther, the Grand Canyon would also have extended a similar distance. It is plain then that the cutting of these canyons depends on the amount of water (snow is included) which may fall in the high mountains, the canyons themselves being in the drier districts. It is also clear that if, by some chance, the precipitation of the high sources should increase, the corrasion of the stream-beds in the canyons would likewise increase and outrun with still greater ease the erosion of their immediate surroundings. On the other hand, if the precipitation in the arid surroundings should increase, the wearing down of the side walls would for a time—till covered by debris and vegetation—go on more rapidly till, instead of Canyons of the Colorado River type, there would be deep, sharp valleys, or wide valleys, according to the amount of difference between the precipitation of the low lands and the high. Where the two were nearly the same, that is, a balance of precipitation,* the slopes might be rounded and verdure-clad, though this would depend on the AMOUNT of precipitation. On lower Snake River a change seems to be going on. The former canyon-cliffs are covered by debris and vegetation, but in places the old dry cliff-lines can be discerned beneath like a skeleton. The precipitation there has not been great enough to destroy the old lines—only enough to mask them.
* There could be a balance of precipitation and still very little
snow- or rainfall, or they might be very great.
The "inner gorge" of the Grand Canyon appears to have been cut far more rapidly than the outer one, and at a much later period; were this not the case there would be no inner gorge. It is a singular fact that some side canyons, the Kanab, for example, while now possessing no running water, or at best a puny rivulet, and depending for their corrasion on intermittent floods, meet on equal terms the great Colorado, the giant that never for a second ceases its ferocious attack. Admitting that the sharper declivity of the Kanab would enhance its power of corrasion, nevertheless we should expect to see it approach the Grand Canyon by leaps and bounds, like the Havasupai farther down, but, on the contrary, there are parts that appear to be at a standstill in corrasion, or even filling up, and its floor is a regular descent, except for the last three or four miles where the canyon is clogged by huge rocks that seem to have fallen from above. The maximum height of its present flood-waters is about six feet, proved by a fern-covered calcareous deposit, projecting some fifteen feet, caused by a spring (Shower-Bath Spring) on the side of the wall, seven or eight miles above the mouth, which is never permitted by the floods to build nearer the floor of the canyon. A suspicion arises, on contemplating some of these apparent discrepancies, that the prevailing conditions of corrasion are not what they were at some earlier period, when they were such that it was rendered more rapid and violent; that there was perhaps an epoch when these deep-cut tributary canyons carried perennial streams, and when the volume of the Colorado itself was many times greater, possessing a multiplied corrasive power, while the adjacent areas were about as arid as now. At such a time, perhaps, the Colorado performed the main work of the inner gorge, the Kanab, and similar affluents, their deep now rather evenly graded canyons. Such an increase of volume, if we suppose the aridity to remain as now, could have come about only by an increase of precipitation on the mountain summits. During the Glacial Epoch, the Rocky Mountain summits were considerably glaciated, the amount varying according to altitude and latitude. The general topography of the Colorado River was about as it is to-day, and the rainfall in the valleys probably nearly the same, or at least only a little greater. In other words, the conditions were those of to-day intensified. In summer, then, the amount of water seeking outlet by these drainage channels to the sea was enormously multiplied, and the corrasive power was correspondingly augmented. When the ice caps finally began to permanently diminish, the summer floods were doubtless terrific. The waters of the Colorado now rise in the Grand Canyon, on the melting of the snows in the distant mountains, from forty to one hundred feet; the rise must then have amounted to from one hundred to four hundred or more. The Kanab heads in two very high regions—the Pink Cliffs and the Kaibab. Though probably not high enough to be heavily glaciated they were high enough to receive an increased snowfall and to hold it, or a portion of it, over from one year to another. Thus the canyons having their origin on these high regions would be given perennial streams, with torrential floods each summer, compared with which anything that now comes down the Kanab would be a mere rivulet. The summit of the Kaibab is covered with peculiar pocket-like basins having no apparent outlets. These were possibly glacial sinks, conducting away some of the surplus water from the melting snow and ice by subterranean channels. It seems probable, therefore, that glacial flood-waters were an important factor in the formation of the canyons of the Colorado. If this supposition is correct it would account, at least in a measure, for that distinct impression of arrested activity one receives from the present conditions obtaining there.*
* Some canyon floors, where there is no permanent large stream,
appear to have altogether ceased descending. Dutton says of those which
drain the Terrace Plateaus: "Many of them are actually filling up,
the floods being unable to carry away all the sand and clay which the
infrequent rains wash into them."—Tertiary History, p. 50. See also pp.
196 and 228 Ib.
The drainage at the edges of most canyons is back and away from the gorge itself. The reason is that the rains cannot flow evenly over a canyon brink, owing to irregularities of surface, and once an irregular drainage is established, the water seeks the easiest