CAUSE OF ALTERNATE EROSION AND SEDIMENTATION
If, as outlined in the preceding pages, the alternate erosion and sedimentation in Chaco Canyon resulting in the production of a plain of alluviation suitable for farming at successive times and the dissection and partial destruction of this plain in the intervening intervals is an adequate cause for the rise and destruction of human cultures, the ultimate reason for changes in the habit of streams becomes of large importance. A river or stream deposits sediment when it has a load greater than it can carry on a given grade. It erodes when it can carry more material than is furnished to it. The quantity of water, variations in this quantity, grade, supply of sediment, size of grain of the sediment, and shape of the channel are factors which determine the habits of a stream. The complex interrelationships of these factors are difficult to determine quantitatively, and are summed up in the word "regimen." The regimen of streams in the Chaco Canyon region is such that they now erode whereas formerly they deposited material on the canyon floors.
As this change in regimen is general, general causes must be sought. A number of writers5 attribute the present epicycle of erosion to introduction of livestock. Formation of trails and destruction of vegetation by overgrazing are supposed to have concentrated floodwaters and to have allowed these floods to erode the present channels. The argument of these writers is best expressed by Duce (1918, p. 452): "We may, therefore, summarize the effect of cattle by saying that they increase the rapidity of the run-off and the rate of erosion by destroying vegetation, by compacting the soil, and by forming channels for the passage of water."
The apparent coincidence in time of the initiation of overgrazing and the beginning of dissection is significant and more data on this phase of the problem are needed. Coincidence of settlement and the deepening of the channel of Rio Puerco seems well established (Bryan, 1928a). The theory that erosion is caused by a slight uplift and increase in the gradient of streams has been generally rejected because erosion of about equal magnitude affects streams of different drainage systems that flow in all possible directions. Uplift so nicely adjusted to the drainage pattern is inconceivable.
Huntington (1914, pp. 33-34) was doubtless the first to advocate a climatic change as the cause of erosion, and Bryan (1923a, pp. 77-80) has brought together available evidence that a slightly wetter climate was characteristic of southern Arizona at the time of the first Spanish explorations.
Gregory (1917, pp. 131-132) advocates climatic change and rejects the argument for overgrazing in the following words:
Reagan (1922, 1924a, 1924b) presents the interesting hypothesis that prehistoric peoples, by means of small reservoirs, check dams, and embankments designed to spread floods, used or distributed most of the water so that floods of the main arroyos were decreased in volume and violence. Hence, sediment was deposited and the arroyos filled up. When these ancients disappeared and their structures fell into decay, erosion was resumed. Reagan also supposes that the prehistoric peoples killed off the game, and thereafter the vegetative cover increased until it gave a maximum of protection to the soil. This cover was destroyed with introduction of domestic animals after the Spanish conquest. Thus Reagan assumes that the structures built by prehistoric men were sufficient to reduce floods without complete consideration of the difficulties involved. He ignores the fact that alternation between erosion and alluviation began before entrance of the Puebloan peoples into the area. The earlier epicycles could not have been influenced by such causes as Reagan advances.
Olmsted (1919, p. 88) estimated the cost of check dams and bank structures for control of Gila River above the San Carlos dam site at a total of $6,401,029. Elaborate as these plans seem, engineers by no means agree that they are adequate to control floods on Gila River, much less to restore channel conditions to those obtaining before 1880. In recent years the Soil Conservation Service has built structures for control of erosion which exceed in magnitude anything possible to the ancients. Success has been rare and modest. That primitive man could erect barriers sufficient in number and size to accomplish this result seems improbable. If such attempts had been made, remnants should remain to be easily identified.
All the investigators mentioned above considered that they were dealing with only one period of sedimentation and one period of erosion, although Huntington thought these minor changes of the geological Recent were the latter part of a series of such changes running back into Pleistocene time. If the post-Bonito channel represents a valid cycle of erosion and sedimentation then three complete local cycles of erosion and sedimentation and part of a fourth must be explained. These events may be put in tabular form, as follows:
Cycles of erosion and sedimentation in Chaco Canyon
Presented in this form it seems evident that a postulate of climatic change has greater inherent possibilities as a true explanation of the facts. Domestic animals certainly cannot be charged with inception of the post-Bonito erosion. However great the influence of overgrazing, therefore, it must be regarded as a mere trigger pull which initiated an epicycle of erosion that was brought about by other causes.
Reagan (1924a, p. 285) has carried the factor of overgrazing into prehistoric times and suggests that incoming and increasing hordes of herbivorous animals may have overgrazed the country and thus caused formation of arroyos. Thereafter, the animals having starved to death or left the region, vegetation would again spring up and sufficiently protect the land surface so that streams would again begin to aggrade. Thus he postulates recurrent overgrazing with consequent cycles of erosion and aggradation. That animals in a state of nature would overgraze an area is an assumption without proof. It seems best to pass this interesting postulate since there exists ample evidence that at least one period of sedimentation in the not-distant past was wetter than the present.
Douglass6 has described briefly the valley deposits of the Rio de Flag, a small creek near Flagstaff, Ariz. Here an arroyo dating from 1890 to 1892 has dissected a valley fill; standing stumps of pine trees are found from 4 to 16 feet below the surface and prostrate logs in the upper 4 feet. Both stumps and logs belong to the living species, Pinus ponderosa, now growing on the adjacent hillside. The stumps, however, have wide growth rings similar to those found in trees of humid lands. The prostrate logs have narrow rings like living trees of the region. It seems evident, therefore, that the zone from 4 to 16 feet below the surface was deposited under a climate much wetter than now. In addition, human relics have been found in the fill at depths from 4 to 9 feet but their relation to the stumps is uncertain. Evidence in the valley fill of the presence of man seems to indicate that the humid period demonstrated by the stumps is not very ancient. It may represent one of the cycles of sedimentation disclosed in Chaco Canyon or an older cycle not yet identified there.
Study of the Chaco Canyon deposits has not produced incontestable evidence that wetter climates prevailed there in the past. From the main valley fill2d cycle of the table, page 49we collected a few fresh-water shells but similar shells were also found in the post-Bonito channel. In some of the sandy and silty beds of the main valley fill impressions that resemble rushes were noted. The adobes are dark brown from included organic matter, but this might have derived from a heavy growth of grass or other vegetation.
There is, however, nothing in the character of these sediments that precludes their deposition under a slightly wetter climate. It is possible there may have been a sufficiently greater rainfall so that pine trees grew in favored places on the hills, cottonwood and willows may have bordered the river, the valley floor may have had large areas of perennial-grass and in a few places there may even have been marshy ground with cattails. Currently existing damp places under the north cliffs may have been small springs in times past. Such an environment seems compatible with the type of alluvium deposited and yet sufficiently favorable to have provided an adequate food supply for the peoples of the thirteen villages of Chaco Canyon. Such a climate can not be inferred from the nature of the sediments themselves but, on the other hand, those sediments present no evidence that a relatively slight modification of climate did not exist when they were being deposited.
[NOTE.Bryan's speculations in the foregoing paragraphs have been substantiated in large measure by data from our excavations and from further exploration. Our old Navaho neighbors reported pine stumps at various places about the valley; dead and prostrate pines were photographed on the south cliff and a couple of dozen trees and stumps were seen at the head of the canyon, 16 miles to the east (Douglass, 1935, p. 46). These last few remnants of former forests suggest that the annual rainfall in their time was considerably greater than our postulated 10-inch average for the present. Even more suggestive is the fact that thousands of logs, large and small, went into construction of Pueblo Bonito and its neighboring villages between A.D. 900 and 1100. The forests that furnished those logs must have been close at hand since none of the timbers we uncovered was scarred in transportation and such forests, at 6,500 feet, could have flourished as they did only in a climate somewhat wetter than that of today. Indeed, many of the old ceiling timbers from Pueblo Bonito exhibited growth rings so uniform in width they obviously had grown where moisture was fairly constant year after year. In addition, we know that rushes were then abundant and readily accessible, for quantities were utilized in the building of Pueblo Bonito.N. M. J.]
Last Updated: --2008