Cosmogenic Nuclides Help to Understand Relations Among Climate Change, Erosion Rates, and Landscape Response in Grand Staircase Region of Colorado Plateau

Over 81% of all streams in the southwest United States are ephemeral or intermittent. Given the delicate balance between sediment supply and discharge, these streams are sensitive monitors of landscape response to past and future climate change. At the turn of the 20th century (AD ~1880 ± 30), many streams in the SW suddenly incised up to 30 m into their floodplains forming steep-walled arroyos, entrenched into cohesive fine-grained valley-fill alluvium. Historic arroyo entrenchment has exposed unconformity-bound packages of Holocene alluvium and chronostratigraphic reconstructions indicate that these systems have undergone multiple periods of rapid episodic entrenchment followed by slow re-aggradation during the mid-to-late Holocene. Settlements in the semi-arid SW are dependent on water availability and during historic arroyo entrenchment, wide-spread channel incision and floodplain abandonment caused communities to become disconnected from water resources and led to abandonment of some towns. Importantly, climate models predict greater temperature extremes and more frequent floods and droughts in the SW US. Investigating geomorphic response to past climate change enables us to better understand how the landscape might respond to future climate change. In this study, I quantify the spatial and temporal variability of spatially integrated long-term (century to millennial-scale) erosion rates in the Grand Staircase region of Colorado Plateau using terrestrial cosmogenic nulcide Beryllum-10. These data are used to determine if and how variability in erosion rates influence the sediment supplied to the stream and arroyo cut-fill dynamics. My central hypotheses are that 1) the timing of aggradation (valley-filling) is influenced by periods of elevated erosion rates and 2) arroyos are located where erosion rates are high due to instabilities developed in stream profiles during aggradation that result in reduced concavity (i.e., change in stream gradient). Preliminary results based on a limited number of samples (n=11) suggest erosion rates in Kanab Creek and Johnson Wash are generally fast and spatial variability is related to elevation, lithology, slope, and contribution of stored sediment. For example, basins draining the Pink Cliffs, composed Tertiary lacustrine sediments of the Claron Formation, appear to be eroding faster than the lower elevation Navajo sandstone forming the White Cliffs. As expected, basins primarily draining the steep cliff topographies are eroding faster than basins draining the relatively flat bench topographies. However, lower basins that contain stored alluvium have apparent faster erosion rates than small headwater streams, with the notable exception being the headwater basin located in the Pink Cliffs