FROM BEDROCK CANYON TO ALLUVIAL VALLEY: LINKING THE HOLOCENE PALEOFLOOD HISTORY OF THE LOWER COLORADO RIVER BETWEEN GRAND CANYON, ARIZONA, AND NEEDLES, CALIFORNIA

Large floods on the Colorado River are a critical process in shaping the geomorphology and ecology of the river corridor and pose hazards to human life and infrastructure in areas proximal to the river. Knowledge of the magnitude and frequency of large floods prior to systematic streamflow measurements—termed paleofloods—aids quantitative understanding of large-flood processes and hazards. Paleoflood records also provide empirical information on how floods may respond to changing climate and further refine the tails of flood frequency distributions. Over the last several years, we’ve renewed efforts to develop a paleoflood record for the Colorado River in Grand Canyon and downstream. Our analysis has focused on sand and silt deposited in slackwater environments by large Colorado River floods back-flooding up three tributary canyons in Grand Canyon (North Canyon, RM 20.7; Saddle Canyon, RM 47.5; and Blacktail Canyon, RM 120.8) and a sequence of overbank deposits preserved within Colorado River floodplain stratigraphy near Needles, California, along the lower Colorado River. At each site we’ve found multiple Colorado River flood deposits for which we’ve described the stratigraphy, surveyed deposit elevations for hydraulic modeling to estimate flood discharge, and sampled for OSL and radiocarbon geochronology to determine flood timing. Preliminary results reveal that the lower Colorado River floodplain record extends back more than 3,000 years, similar to the duration of the stratigraphic record at Axehandle Alcove (Grand Canyon, RM 2; O’Connor and others, 1994, https://doi.org/10.1086/629644). A high alcove in North Canyon preserves evidence of an exceptionally large Colorado River flood overlying rodent fecal pellets dating to 1,290–1,176 cal yr BP (two-sigma uncertainty, Beta 638371), likely corresponding to a similarly dated high deposit at Axehandle Alcove. Discharge estimates corresponding to the stage of these high paleoflood deposits were nearly twice that of the largest historical Colorado River floods of 1884 and 1921. Continued research aims to further characterize the paleoflood record on the Colorado River through Grand Canyon and elucidate connections between stratigraphic records in bedrock canyons and alluvial settings.