DYNAMIC RIVER SYSTEMS IN THE GRAND STAIRCASE, SOUTHERN UTAH: THE ROLE OF HIGH EROSION RATES, HYDROCLIMATE VARIABILITY AND GEOMORPHIC THRESHOLDS IN HOLOCENE ARROYO DYNAMICS (Invited Presentation)

Semi-arid streams on the Colorado Plateau and across much of the southwestern US abruptly entrenched into their alluvial floodplains in the late 1800’s to early 1900’s AD, creating 5-40 m deep steep-walled arroyo channels. Careful reconstructions of unconformity-bound stratigraphic sequences revealed in arroyo walls suggest that streams experienced multiple periods of entrenchment and re-aggradation since the middle Holocene. Timing of arroyo entrenchment and re-aggradation have been found to be near-synchronous in some studies, suggesting a hydoclimatic forcing. However, despite over a century of research into the ‘Arroyo Problem’, causal relationships with specific climate conditions and the role of autogenic processes in arroyo dynamics are not agreed upon.

This presentation reports on the geomorphology and chronostratigraphy of five catchments draining the Grand Staircase in southern Utah. Age control used to constrain arroyo cut-fill sequences (n= 70 sites) comes from radiocarbon dating of charcoal (n=180) and single-grain optically stimulated luminescence dating (n=140). Catchment-averaged erosions rates come from Be-10 cosmogenic radionuclide (CRN) concentrations in modern, Holocene and Pleistocene sediments (n = 65).

Reconstructions indicate that each catchment had 3-5 periods of aggradation followed by entrenchment, with the timing of events most similar in the last 2-3 ka. CRN results indicate high bedrock erosion rates, which fueled high sediment supply and largely aggradational Holocene conditions across the region. Following aggradation, stream profiles had reduced concavity and locally over-steepened downstream reaches, which provided the necessary gradients to drive entrenchment. However, the timing of arroyo cutting was ultimately controlled by 1) sediment supply and the time it takes a catchment to aggrade and reach a concavity-slope driven threshold and 2) hydroclimatic variability, as the driving forces for entrenchment only occurred during high-discharge events once aggradation brought the systems to a threshold in slope. A conceptual model is presented to illustrate how hydroclimatic variability, geomorphic thresholds and catchment specific relaxation/recovery time combined to produce the quasi-synchronous nature of the regional arroyo cut-fill records.