THE BENEFITS AND DIFFICULTIES OF USING THE STRATIGRAPHY OF LARGE RIVERS TO IDENTIFY REGIONAL PALEOCLIMATE EVENTS: CASE STUDY LOWER COLORADO RIVER

Rivers aggrade and degrade as a result of perturbations that affect the supply of sediment and water from their watersheds. Streams draining small watersheds may be subject to relatively frequent aggradation and degradation cycles in response to relatively local perturbations. By contrast, large trunk streams draining continental-scale watersheds are less sensitive to local perturbations, and aggradation and degradation tend to occur mainly in response to major, regional climatic and tectonic events. Interpretation of fluvial deposits along major rivers is a challenge, due to the existence of unconformities as well as the possible complicating influences of variable sediment source area, regional tectonics, and differential preservation of deposits with different lithologies (eg, gravel is more likely to be preserved than silt and clay). In spite of the difficulties in dating and interpreting these deposits, understanding the stratigraphy of large rivers can provide valuable information about important climatic and geologic events. Large rivers respond to major terrestrial paleoclimatic signals that affect much larger regions than those recorded in more localized terrestrial records, such as in lakes, caves, middens, and dunefields. In addition, large drainage systems are relatively long-lived features, so their deposits can contain long records of climate events. A case in point is the stratigraphic record of the lower Colorado River, the trunk stream of southwestern North America, which has aggraded and incised hundreds of meters multiple times in the past several million years. The remnants of the penultimate aggradational cycle provide evidence that a major regional event, likely climatic in nature, caused rapid aggradation and degradation of the lower Colorado River. This sequence of events occurred relatively rapidly during Oxygen Isotope Stage 3 and/or 4 (between about 50-100 ka). We will present new topographic, tephrochronologic, and luminescence data that document this episode and constrain its age. We hypothesize that the trigger for this alluvial cycle could have been an important, regional climatic shift from wetter to more arid conditions on the Colorado Plateau, and a previously unknown interval in which the North American monsoon system may have become active during late Pleistocene time.