HOLOCENE Aggradation of the Colorado River IN the Grand Canyon and Upstream Tributaries ON the Colorado Plateau.

HOLOCENE AGGRADATION OF THE COLORADO RIVER IN GRAND CANYON AND UPSTREAM TRIBUTARIES ON THE COLORADO PLATEAU Abstract Holocene deposits of fine sand and silt outcrop frequently along the river corridor in Grand Canyon, to a height of ~10 m above present river level. Known as the Archeological Unit (AU) for the prehistoric artifacts they contain, the surfaces of these deposits were farmed extensively by Ancestral Puebloans in the late Holocene. These deposits are devoid of the coarse quartzite and porphyry gravels so typical of both older and younger river deposits and point to a Colorado River (CR) in the mid-to-late Holocene in a low-energy, depositional regime that filled the present channel to overflowing with fine sand and silt. Tributary fans and terraces also aggrade to the top of and interfinger with the AU. In eastern Grand Canyon, previously published radiometric ages of charcoal in hearths and buried soils date to 4460 yr BP at ~2 m below the top of the remnant surfaces. Similar materials of greater but unknown age extend to below present river grade. Many tools and pottery fragments scattered on top of the surfaces are identified as artifacts associated with the Pueblo II cultures (1,000 yr BP). However, radiometric ages associated with buried soils containing maize pollen are contemporaneous only with late-Archaic (>3,000 yr BP) through Basketmaker II and III cultures (~3,000 – 1200 yr BP). Ancestral Puebloan people abandoned the Grand Canyon by ~ 700 yr BP. Our data suggest Colorado River incision likely began ~1200 to 1000 yr BP, as evidence for Pueblo II farming along the main stem is lacking. This does not preclude the possibility of occasional overbank flood deposits that post-date the beginning of incision, but diversion of CR water onto corn fields appears to have ceased prior to Pueblo II – time. Events that triggered the aggradation were likely climatically induced and probably began in the late-Pleistocene or early-Holocene. Fills of similar texture, geomorphic position and age have been identified on upstream tributaries across the Colorado Plateau.