224 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I47 



as we witnessed repeatedly, and then turn abruptly aside and leave 

 the valley floor dust dry. Although the local climate has long bor- 

 dered on the arid, we have ample evidence that precipitation was 

 once more plentiful. Perennial grasses and greasewood formerly 

 flourished here; pines and cedars grew in side canyons and on the 

 mesas above. Cottonwoods and willows once traced a green ribbon 

 down midvalley, and some few were still there as late as 1920, the 

 year of the Society's Chaco Canyon reconnaissance. Vegetation 

 multiplies when the water table is high and a ground cover both 

 hinders runoff and quickens the deposition of silt and sand. Navaho 

 elders recall a time, or say they do, when water could be had any- 

 where in the valley with a little digging. 



Surface vegetation, however, has not always been lush enough to 

 hold Chaco floodwaters in check. There were times of drought or near 

 drought when shrubs and grasses withered and the runoff ran faster. 

 Below-average rainfall is recorded repeatedly in the annual growth 

 rings of timbers salvaged from Pueblo Bonito. Douglass (1935, 

 p. 49) points to a period of subnormal rains extending from A.D. 

 1005 to 1036 or even later. The 10-year drought between 1090 and 

 1101 was a severe one and may have had a more devastating effect 

 upon local plant life than is possible now to measure. Conceivably 

 it could have initiated the 12- to 15-foot-deep "buried channel" that 

 so intrigued Bryan — the channel that destroyed an annually increas- 

 ing amount of Bonitian farmland and, in so doing, probably hastened 

 migration of the Bonitians. 



DROUGHT AND THE BURIED CHANNEL 



Bryan first chanced upon this buried watercourse near the south- 

 east corner of Pueblo del Arroyo without knowing at the time that a 

 nearby exposure had previously been discovered by W. H. Jackson 

 (1878, p. 444). At this site, the section 4 of his map, Bryan (1954, 

 p. 54) described the old channel as 13 feet 5 inches deep and its 

 gravelly bottom 15 feet below the existing surface. The difference 

 in the two figures illustrates the fact that, after the 13-foot gully 

 had been completely refilled, an additional 19 inches of sand and 

 silt were spread upon the refill. Elsewhere the overburden is thicker 

 and the gully, deeper. The bottom of it lies 18 feet below the surface 

 in Test Pit No. 3, on the plain fronting Pueblo Bonito (pi. 7, upper). 



A short distance upstream from section 4 and on the opposite 

 bank, Jackson (ibid., p. 443) in 1877 observed a small ruin whose 

 foundations were "5 or 6 feet below the general level of the valley." 



