2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I47 



25 percent of the formerly arable land and thereby has largely de- 

 stroyed the most conspicuous plant on the valley floor, greasewood. 

 That 1850 arroyo is still in process of formation, but its predecessor, 

 about half as large, came into being while Pueblo Bonito was in- 

 habited and may have been a factor in compelling its abandonment. 



Fig. 1.— The northwest quarter of New Mexico, showing location of Pueblo Bonito. 



Since an arroyo promptly collects to itself all surface runoff, 

 vegetation is deprived of needed moisture and denudation follows. 

 Without grasses, trees, and shrubs to prevent erosion; and without 

 widespreading fioodwaters to irrigate and replenish its fields, a 

 population dependent upon agriculture for a livelihood, as were the 

 Bonitians, could not long survive in a waterless valley. 



