DISTINCT RESPONSES OF LOCAL CATCHMENTS AND THE COLORADO RIVER TO QUATERNARY GLACIAL-INTERGLACIAL FLUCTUATIONS IN EASTERN GRAND CANYON – AUGMENTING A WORKING MODEL FOR DRYLANDS

A model for geomorphic response in dry environments to glacial-interglacial climate change has developed from the work of several researchers. Glacial periods are characterized by sediment storage on transport-limited hillslopes, glacial-interglacial transitions are distinguished by a sediment pulse from hillslopes, due to vegetation disturbances and a change in the precipitation regime, that causes stream aggradation, and interglacials are periods of relative stability and weathering-limited hillslopes. Based on the results of this research, this model requires modification to account for stream aggradation in some areas during glacial conditions and differences in the timing of response by different landscape components.

The tributary drainages and Colorado River corridor of eastern Grand Canyon contain a suite of well-preserved and exposed Quaternary hillslope deposits and alluvial fill terraces. Remnant colluvial mantles are laterally continuous with some fill terraces in tributary drainages, but correlative mantles prograde over mainstem terraces. In the tributary-mainstem transition zone, tributary gravels interfinger at their base with Colorado River deposits, but prograde over them in higher sections. The critical age control component of this research was completed using U-series, surface-exposure, and OSL dating.

Stratigraphic and geochronologic data reveal aggradation and incision by tributary streams and the Colorado River occurred asynchronously during glacials and roughly synchronously during glacial-interglacial transitions and interglacials. Stream aggradation in the study area during glacial conditions was likely caused by climate-induced increased sediment production not being matched by vegetation density and sediment storage on slopes. On a larger scale, we hypothesize latitudinal variation in climate forcing across central to southern North America caused the documented differences in the timing of aggradation. The Colorado River aggraded 75-65 ka due to climate conditions in its headwaters in the Rocky Mountains and tributary streams aggraded 50-40 ka due to local climate conditions on the southern edge of the Colorado Plateau.