VARIABLE SYSTEM STABILITIES IN FLUVIAL CHRONOLOGY, AND INFLUENCE ON OCCUPATION HISTORIES, CHACO AND MCELMO CANYONS (NW NM AND SW CO)

Chaco and McElmo canyons, both sites of Anasazi (AD 500-1300) habitation, share many geomorphic attributes and late Holocene sediment facies, but their rates of geomorphic change differ sharply. Entrenchment periods overlapped, but required three times longer at McElmo than Chaco. The difference can be attributed to a metastable base level at Chaco, governed by a dam that was at least partly eolian. When this dam failed, response was rapid. In contrast, response at McElmo was immediate, but in the style of slow complex response. Anthropogenic influences of several types are possible, but climate change is generally thought to drive such change at this time in the region.

If climate change affected the two canyons simultaneously, it produced two quite different records, possibly because of the difference in system stabilities. In Chaco, a shift in the balance between eolian and fluvial processes was apparently the main effect of climate change. But the system could not respond until a threshold was exceeded, i.e. the dam was breached, producing a lag of about 100 years.

Thresholds and lag times can produce markedly different fluvial chronologies, even where external change is simultaneous and of the same magnitude. It is not clear whether such effects resulted in the markedly diachronous fluvial chronologies documented in recent literature.

How do such differences impact human habitation? The slow response at McElmo allowed puebloan farmers to respond moderately, moving along drainages following loci of aggradation. No fancy adjustments were required. The delayed response at Chaco allowed colonization of a static landscape, then brief local abandonment in response to rapid entrenchment, and then re-occupation with an entirely new set of habitation techniques, involving large pueblos, labor-intensive irrigation, and extensive trade networks. This is the period at Chaco that produced the most complex culture that the Anasazi attained, and nothing less would have sufficed to reclaim Chaco Canyon as a puebloan settlement site. Human rates of change thus tracked fluvial rates of change rather closely.