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Late Pleistocene and Holocene History of the Southwestern Colorado Plateau

Multidisciplinary studies focus on the geologic, hydrologic, climatic, and botanical evolution of the Grand Canyon and surrounding region over the past 15,000 years. Especially interesting and significant is a period of regional aggradation that stated at the end of the Pleistocene and continued well into the Holocene. Sedimentary products of this aggradation are locally derived and evident along the Colorado River, in large and small drainages on the Colorado Plateau, and in alcoves along some of these drainages. This is the deposit that was widely farmed by prehistoric Puebloan peoples and is known informally as the "archeological deposit." In the Grand Canyon, new data document surprisingly early agriculture: maize around 3000 BC, and cotton around 650 AD. Incision of the deposit started around 1200-1300 AD, amounting to about 10 m since that date. It is possible that arroyo cutting is the latest manifestation of this naturally occurring incision. The deposits attributed in this study to aggradation have been interpreted by others as slackwater deposits caused by immense floods. This work will continue to help resolve the controversy because the difference is not trivial: If immense floods have occurred in the past few thousand years, they could occur again today, sweeping away many human structures, including dams along the Colorado River.

For more information, contact Ivo Lucchitta, USGS, Flagstaff, AZ.