PLEISTOCENE INCISION HISTORY OF THE ANIMAS-SAN JUAN RIVER SYSTEM

Fluvial deposits preserved in terraces above modern rivers yield valuable records of paleohydrology and bedrock incision. Rates of incision can be calculated by dividing the height of an erosional strath above the modern river by the age of abandonment of the terrace. This approach has produced incision histories for many important 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 reorganization.

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 in the late Pleistocene, 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 events have had on the various parts of the watershed.

Here we report a preliminary 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 under Lake Powell. Early results compiled from existing and new age constraints generally show more rapid incision rates near the 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.