OBESERVATIONS OF ARROYO CUT-FILL DYNAMICS: A PRELIMINARY CHRONOSTRATIGRAPHY FROM JOHNSON WASH, SOUTHERN UTAH

Historic arroyo entrenchment was widespread, abrupt, and nearly synchronous in the southwest US. Arroyos are characterized by an entrenched channel, near vertical channel banks composed of fine-grained cohesive alluvium, and a flat channel bottom. The cutting of arroyos at the turn of the 20th century was one of the most significant historic geomorphic events in the region, generating high sediment yields and stimulating research addressing the forcing mechanisms and necessary geomorphic and climate conditions leading up to rapid excavation of valley-fill alluvium. Historic arroyo entrenchment exposed 5-30 meters of unconformity bound packages of Holocene alluvium. Stratigraphic records indicate these fluvial systems were dominated by aggradational conditions, characterized by shallow channels and floodplain accretion, interrupted by channel entrenchment (arroyo cutting). Why do ephemeral streams in the semi-arid SW episodically and rapidly entrench and slowly refill repeatedly during the mid-late Holocene? We present a late Holocene record of cutting and filling in an arroyo system located in Johnson Wash, southern Utah. The chronostratigraphy of valley-fill is characterized using a combination of stratigraphic relationships and radiocarbon and optically stimulated luminescence (OSL) dating. Chronostratigraphies of 13 sites characterize past hydro-geomorphic change over centennial-millennial timescales. Preliminary results demonstrate that 29 out of 31 samples date to the last 2500 years. While preliminary data suggest cut-fill dynamics were not synchronous throughout Johnson Wash, complication with the initial chronology include limited preservation and exposure of sediment packages, internal sediment transport complexities within the watershed, and problems with age control such as reworking of charcoal in the fluvial system and partial bleaching of the OSL signal. However, preliminary results suggest that at least four aggradational packages are preserved in arroyo walls of Johnson Wash. These packages bracket three episodes of incision at approximately 2000, 1500, and 1000 years BP. Interestingly, these episodes of entrenchment are somewhat synchronous with other regional arroyo chronologies suggesting that climate may be important control on arroyo cut-fill dynamics.