EROSION RATES INFLUENCE ARROYO CUT-FILL DYNAMICS IN SEMI-ARID CATCHMENTS DRAINING THE GRAND STAIRCASE REGION OF COLORADO PLATEAU

At the turn of the 20th century, many streams in the semi-arid southwestern U.S. rapidly entrenched 5-30 meters into their sediment-filled valleys. This entrenchment exposed discrete unconformity-bound packages of alluvium that represent prehistoric cut and fill events. An ‘arroyo’ is a geomorphic channel form of an entrenched stream with near-vertical channel banks. Stratigraphic evidence suggests that many semi-arid streams transition between two end-member channel forms; one end-member being an entrenched arroyo characteristic of the modern landscape and the other being an aggradated state with an unconfined channel on a broad floodplain. Age control suggests that stream conditions have been largely aggradational during the last 6000 yrs, interrupted by decadal- to centennial-scale episodes of arroyo entrenchment. This study examines how temporal and spatial changes in sediment supply influence arroyo entrenchment and aggradation in semi-arid streams draining the Grand Staircase region of the Colorado Plateau in south-central Utah. Preliminary results suggest that millennial-scale catchment average erosion rates are generally fast (3.45 cm/kyr), which is faster than the global average and significantly faster than the global median catchment average erosion rate. Results also suggest high spatial variability (.40- 8.50 cm/kyr) that is related to elevation, lithology, slope, and contribution of stored sediment. Basins primarily draining the steeper topography dominated by cliffs are eroding faster than basins draining regions dominated by relatively flat benchs. Moreover, basins draining the Pink Cliffs, composed of Tertiary lacustrine sediments of the Claron Formation, are eroding the fastest; faster than the lower elevation Navajo sandstone forming the White Cliffs. We hypothesize that these fast erosion rates provide for rapid valley aggradation and steam-profile geomorphic conditions that are needed to drive arroyo cut-fill dynamics.