NEW CONSTRAINTS ON THE LATE PLEISTOCENE INCISION HISTORY OF THE ANIMAS - SAN JUAN RIVER SYSTEM

Fluvial deposits preserved in terraces above modern rivers yield records of paleohydrologic conditions and rates and patterns of bedrock incision. Rates of incision are derived by dividing the height difference between two erosional straths by the difference in age of abandonment of the terraces. This approach has been used to help reconstruct incision histories for river systems worldwide, showing that they record the effects of forcing factors like climate change, tectonic uplift, and transient adjustments of river longitudinal profiles to river piracy and base level changes.

Incision rates along the Colorado River system in the southwest U.S. show considerable spatial and temporal variability. Major late Cenozoic events that could affect incision rates in the Colorado River system include integration of the river off the Colorado Plateau in the late Miocene (and the associated upstream-migrating wave of incision), the onset of Rocky Mountain glaciation, Cenozoic differential uplift, and lithologic strength contrasts that promote or resist incision. More constraints on bedrock incision are required from a greater number of tributaries in order to evaluate the relative impact these factors have on the various parts of this key watershed.

Here we report a partial late Pleistocene incision history for the Animas-San Juan River system in the east-central Colorado Plateau. Draining the San Juan mountains, the Animas River feeds into the San Juan River, which continues across the Colorado Plateau to a confluence with the Colorado River at Glen Canyon. Early results compiled from existing and new age constraints on terrace gravels generally show more rapid incision rates near the mountainous headwaters relative to the plateau reaches downstream, with the noteworthy exception of a recently- accelerated incision rate in the downstream reach near Bluff, UT.

Our preliminary interpretation is that the onset of glaciation in the headwaters of the San Juan River caused an episode of canyon-cutting downstream, including cutting the modern bedrock gorge near Bluff, UT. More constraints will be required to further test that hypothesis and evaluate whether the more rapid incision in the headwaters could be related to hypothesized ongoing differential uplift of the San Juan Mountains relative to the central Colorado Plateau.