The Geology of Chaco Canyon, New Mexico In Relation To The Life And Remains Of The Prehistoric Peoples Of Pueblo Bonito Smithsonian Miscelleanous Collections Volume 122, Number 7

From the foregoing it should be evident that in Chaco Canyon conditions were favorable for floodwater farming from the beginning of alluviation. The area available also gradually increased as the plain widened with the filling of the canyon. The canyon, therefore, afforded a locality for the initiation and development of a civilization based on agriculture. That this agriculture was precarious and that crops might fail owing either to lack of rain and consequently of floods, or to the occurrence of unusually cold seasons, does not constitute a factor that would prevent the development of a local community of relatively high culture. These hazards merely limit the number of people and fix their standards of living. In pre-Spanish times the Hopis are said to have insured themselves against crop failure from these causes by the storage of a year's supply, or more. A like necessity may have been one of the compelling causes which led to development of more houses and a house cluster more elaborate than the simple pit dwelling.

The same comments may be applied to the problem of water. Lack of a local water supply does not prevent the use of suitable floodwater fields, for at the present time the Hopi farmer may cultivate tracts 10 or even 20 miles from his home. Moenkopi, 40 miles northwest of Oraibi, was a farming community when seen by Oñate in 1604 and remained so until the Navaho forced its temporary abandonment about a century ago. An alluvial plain in the process of sedimentation always has local depressions which fill with water after rains or floods. These pools or charcos (Bryan, 1920) afford a limited water supply during the growing season. The Papagos of southern Arizona were entirely dependent on charcos during the planting and harvesting of their crops until, within the past few years, the United States Indian Service drilled wells at the fields. Before coming of the Spanish, these Indians not only cultivated the same fields while dependent on charco water but carried the crop on their backs to winter residences located at permanent, or at least longer lasting, water many miles distant. Thus lack of permanent water is not necessarily a hindrance to floodwater farming, although it may be an obstacle to permanent residence.

In Rincon del Camino one of the Navaho workmen employed by the Pueblo Bonito Expedition developed a small spring in a rock shelter or niche under a cliff. At other similar localities on the north side of Chaco Canyon the rock is damp and covered by an efflorescence of salts. Here it might be possible to develop water by systematic digging, and springs of this type may have been the principal source of domestic water to the ancient people. It is certain that a very slight increase in rainfall over the present annual average would produce springs in such localities.

Conditions of alluviation lead also to a relatively high water table. At present, water may be obtained by digging about 10 feet below the bed of the arroyo, or some 40 feet below the plain. Before the modern arroyo was cut the stream ran in a shallow channel and ground water must have saturated the valley fill to within 10 feet of the surface, a distance comparable to the present depth of water below the bed of the arroyo. As the dry season advanced, the prehistoric peoples may have scraped holes in low places and thus formed a primitive type of well. No evidence of such wells has been found but the digging of them would have been entirely feasible for the inhabitants of Pueblo Bonito even though they lacked metal tools.

The formation of an arroyo similar to the present one, or the one we have called the post-Bonito channel, confines floodwaters to a narrow belt below the level of the plain. The ground-water level is also lowered and floods from tributaries are less effective in wetting the ground. Farming by means of floodwater is consequently impossible over the whole plain and is limited to insignificant areas.

If, therefore, the geologic chronology tentatively outlined on page 37 be accepted, Chaco Canyon enjoyed a relatively long period of alluviation with conditions favorable to floodwater farming. As indicated by finds of potsherds and other relics, people of Early Pueblo culture occupied the valley at least during the time required to build the fill from a level 21 feet below the present surface to 4 feet below. The favorable conditions then existent led to development of the relatively complex civilization of the Great Pueblo period, a culture that flourished during deposition of the upper 4 feet of valley fill.

Formation of the arroyo system represented by the post-Bonito channel may be given as the approximate cause for abandonment of the valley by these Pueblo III people, although other factors such as war, invasion, disease, or gradual decrease in means of subsistence may also have had their effect. However great the changes these other factors might have produced, it seems unlikely that any one or two of them would have kept out of use for long a place so eminently suitable for floodwater farming as Chaco Canyon.