Evolution of the Bonneville shoreline scarp in west-central Utah: Comparison of scarp-analysis methods and implications for the diffusion model of hillslope evolution J.D. Pelletier * , S.B. DeLong, A.H. Al-Suwaidi, M. Cline, Y. Lewis, J.L. Psillas, B. Yanites Department of Geosciences, The University of Arizona, 1040 E. Fourth St., Tucson, AZ 85721, USA Received 12 April 2005; received in revised form 17 August 2005; accepted 18 August 2005 Available online 11 October 2005 Abstract Wave-cut pluvial shoreline scarps are ideal natural experiments in hillslope evolution because the ages of these scarps are often precisely known and because they form with a range of heights, alluvial textures, and microclimates (i.e., orientation). Previous work using midpoint-slope methods on pluvial scarps in the Basin and Range concluded that scarp evolution is nonlinear and micro- climatically controlled. The purpose of this study was to further examine the influence of scarp height, texture and microclimate in an attempt to calibrate a nonlinear model of scarp evolution. To do this, over 150 profiles of the Bonneville shoreline in the adjacent Snake and Tule Valleys, west-central Utah were collected and analyzed by fitting the entire scarp profile to diffusion-equation solutions, taking into account uncertainty in the initial scarp angle. In contrast to previous studies, this analysis revealed no evidence for nonlinearity or microclimatic control. To understand the reason for this discrepancy, we undertook a systematic study of the accuracy of each scarp-analysis method. The midpoint-slope-inverse method was found to yield biased results, with systematically higher diffusion ages for young, tall scarps. The slope-offset method is unbiased but has limited resolution because it requires many scarp profiles to yield a single diffusion age. A method that incorporates the full scarp profile and uncertainty in the initial scarp angle was found to be the most accurate technique. The application of the full-scarp method to the Bonneville shoreline supports the use of a linear diffusion model for scarps up to 20 m in height. Scarp orientation had no discernable effect on diffusivity values. Soil texture was found to have a weak but significant inverse relationship with diffusivity values. D 2005 Elsevier B.V. All rights reserved. Keywords: Hillslope; Pluvial; Diffusion model; Basin and range; Scarp evolution 1. Introduction Pluvial scarps have played a central role in research on hillslope geomorphology in arid regions throughout the world (Enzel et al., 1994; Hanks, 2000). Pluvial shoreline scarps form by wave-cut action during a prolonged period of lake-level high stand. Following lake-level fall, scarps formed in unconsolidated alluvi- um evolve to an bangle of reposeQ by mass movements and then further evolve by diffusive hillslope processes (Wallace, 1977). If a linear relationship between sedi- ment flux ( F) and slope is assumed: F ¼ j Bh Bx ð1Þ where h is elevation, j is diffusivity, and x is distance along a profile, then the scarp evolves from the angle 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.08.008 * Corresponding author. E-mail address: jon@geo.arizona.edu (J.D. Pelletier). Geomorphology 74 (2006) 257 – 270 www.elsevier.com/locate/geomorph