THE ARROYO PROBLEM: A NEW RECORD OF MID-LATE HOLOCENE CUT-FILL DYNAMICS IN JOHNSON WASH, SOUTHERN UTAH

Over 80% of streams in the southwest United States are ephemeral or intermittent; these streams are commonly sensitive monitors of the effects of climate change on the landscape. In the late 1800's to early 1900's, many streams throughout the semi-arid southwest experienced rapid channel entrenchment, generated high sediment yields, and were transformed into the arroyo landforms characteristic of the region today. Arroyos are an end-member, vertical-walled channel form created by relatively rapid incision into cohesive, fine-grained valley-fill. While many southwestern streams are entrenched today, stratigraphic records indicate during the mid-late Holocene they were primarily characterized by aggradational conditions with shallow channels dominated by floodplain accretion. Previous age control suggests that aggradation takes place on centennial to millennial timescales, while episodic entrenchment occurs on decadal timescales. Research questions include determining if arroyo cut-fill sequences are synchronous within a basin and between adjoining basins, as expected for a climate-driven response; or if chronologies are asynchronous, suggestive of autogenic processes and differing geomorphic thresholds. This research employs detailed sedimentologic and stratigraphic descriptions of unconformity-bound alluvial packages coupled with AMS radiocarbon and optically stimulated luminescence (OSL) dating to investigate arroyo cut-fill sequences exposed along the ~25km-long arroyo in Johnson Wash, southern Utah. Preliminary results will be used to reconstruct periods of aggradation and constrain periods of incision in a catchment that has no dated sediment records. Sediment archives from Johnson Wash valley-fill will be compared to adjacent (Kanab Creek and Kitchen Corral Wash) and regional drainages to provide spatial context on the timing of alluviation and arroyo cut-fill dynamics.