THE INFLUENCE OF SEDIMENT SUPPLY ON ARROYO CUT-FILL DYNAMICS: A PRELIMINARY DATASET OF CATCHMENT AVERAGED EROSION RATES CALCULATED FROM IN-SITU 10BE

Abstract:

Widespread and near-synchronous post-settlement stream entrenchment (arroyo cutting) in the southwest US stimulated research addressing forcing mechanisms and necessary geomorphic and climate conditions leading to episodic evacuations of valley-fill alluvium. Arroyos are an end-member channel form associated with ephemeral streams entrenched into cohesive, fine-grained, valley-fill. Historic arroyo entrenchment exposed 5-30 m of unconformity-bound packages of different aged Holocene alluvium. Chronostratigraphic reconstructions indicate that during the mid-late Holocene these systems underwent multiple periods of rapid episodic entrenchment followed by slow re-aggradation. Previous and ongoing work has developed alluvial chronostratigraphies of Kanab Creek, Johnson Wash, and surrounding streams in southern UT using a combination of stratigraphic relationships, radiocarbon, and single-grain OSL dating. This research investigates the role of allogenic forcing (climate change) and autogenic processes on cut-fill dynamics. This study tests if temporal or spatial variations in sediment supply have influenced the timing and location of arroyo aggradation and entrenchment. We measured in-situ 10-Be in quartz from alluvial and colluvial sediment in Kanab Creek and Johnson Wash to quantify catchment-average erosion rates. Samples were collected from modern channels throughout the watershed and from dated alluvial packages preserved in arroyo walls. Results quantify spatial and temporal variability in sediment supply throughout the two watersheds as a function of lithology, slope, elevation, contribution of sediment stored in valley-fill, and time. Moreover, 10-Be results from dated Holocene alluvium will be used to evaluate if climate change has influenced sediment supply and arroyo cut-fill dynamics.