EPISODIC ARROYO ENTRENCHMENT AND AGGRADATION IN KANAB CREEK, SOUTHERN UTAH

Many alluvial valleys in the American Southwest are entrenched with arroyos, and stratigraphic evidence exposed in modern arroyo walls indicates that these fluvial systems experienced repeated periods of entrenchment and re-aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally synchronous, implying that climate fluctuations are the dominant drivers. However, many of these interpretations rely on records with limited and problematic age control, as well as broad correlations across the southwest. While hydroclimatic fluctuations must play a role, intrinsic reach- or catchment-specific geomorphic thresholds to entrenchment are hypothesized to partially control the timing of arroyo processes.

This study focuses on Kanab Creek, southern Utah. Kanab Creek is entrenched within two arroyos that are separated by a bedrock knickzone, indicating that arroyo headscarp retreat cannot propagate from the lower reach into the upper and suggesting potentially different alluvial histories. Episodes of prehistoric arroyo cutting and filling are reconstructed from 17 sites by recognition of buttress unconformable contacts in the arroyo-wall stratigraphy and age control derived from optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) radiocarbon dating. Results indicate at least five periods of aggradation occurred since ~6.0 ka in the lower reach, each interrupted by an episode of arroyo entrenchment. Although the upper reach record is not yet well-constrained, evidence suggests that the earliest period of entrenchment in this reach occurred within dating error of the lower reach downstream. Comparison of these records to recently completed chronologies from arroyo systems in the region indicates near-synchronous arroyo processes over the last ~1.5 ka; however, beyond 1.5 ka correlations are less clear. Broadly contemporaneous alluviation suggests a climatic driver, and comparison to paleoclimate records suggests that arroyo entrenchment events may be driven by transitions from periods of multi-year drought to wetter periods. However, not all such transitions are associated with arroyo entrenchment, suggesting the importance of geomorphic thresholds in modulating the timing of climate-driven arroyo processes.