LATE HOLOCENE BEHAVIORS OF STREAMS ON THE COLORADO PLATEAU: VARIATIONS IN CHACO AND MCELMO CANYONS BASED ON ARCHAEOLOGIC AGE CONTROLS

In two late Holocene sequences in somewhat similar drainages, Chaco Canyon in NW New Mexico and McElmo Canyon in SW Colorado, mostly-sandy deposits 4 to 8 m thick are well exposed in the walls of modern arroyos formed in the 1800’s. Studied stretches of the west-flowing drainages are ~8 km long. Both sequences are dated by numerous Anasazi remains, commonly to ~25 years resolution between ~AD 500 and 1300 (~1450 to 650 BP). Dating and other archaeologic advice was from Tom Windes and Gwinn Vivian at Chaco and Wayne Howell and the Crow Canyon Center at McElmo.

McElmo Canyon deposits can be taken as an end member demonstrating complex response (in the sense of Schumm). Two aggradational pulses separated by ~ 200 years migrated upstream, each requiring ~200 years to do so, thus bracketing a diachronous entrenchment episode between ~AD 750 and 1100.

At Chaco Canyon in NW New Mexico, the valley floor is perched, apparently behind an eolian dune. It entrenched to meet the rest of the system at grade when the dam was breached (as it is currently). Arroyo cutting was much faster than at McElmo, within the resolution of the age data, consistent with the mechanical, on-off nature of base-level control. The post-Bonito sequence of Bryan and Judd proves to have formed from AD 1025 to ~1100, i.e. decidedly syn-Bonito in age. Entrenchment in each case approximately coincides with the earliest intensive use by man, as it did in the 1800’s.

Adjustment of inhabitants to entrenchment varied with entrenchment style. At McElmo, ak chin farmers were always able to find favorable stretches due to diachronous entrenchment. At Chaco, more synchronous entrenchment may have forced the development of elaborate water-control systems.

Entrenchment at McElmo apparently began earlier and finished later than at Chaco. A comparison of these alluvial chronologies with others suggests the generally diachronous nature of Late Holocene sequences. For example at both Chaco and McElmo the younger sequence strongly overlaps ages of the older (Tsegi) sequences in Black Mesa sections.