Quantifying erosion susceptibility along a lithologic and trail type gradient (Colorado Front Range, USA) with implications for management

Abstract

Differential erosion rates generate relief and shape topography in mountain landscapes. The Colorado Front Range (USA), with its wide range of lithologies exposed in steep terrain across a small geographic area, provides an ideal place to examine differential erosion across lithologies at both human and millennial timescales, with implications for hillslope landscape evolution and land management (i.e., trails). We sought to determine how erosion rates/styles vary by lithologic unit and trail type near Boulder, Colorado (USA). We hypothesized a) modern erosion rates are correlated to long-term erosion rates; and alternatively, b) modern erosion rates may be influenced by trail construction attributes. We present short-term erosion rates from topographic differencing and longer-term bedrock beryllium-10 (¹⁰Be) erosion rates of sedimentary units, including resistant ridge-forming sandstones and conglomerates

and less resistant, fine-grained valley/slope-forming units. In general, there were lower ¹⁰Be rates in ridge-forming units and faster rates in valley/slope-forming units. Specific additional controls included slope and physiography, with resistant ridge-forming units having lower-than expected erosion rates and unique physiography (determined from principal component analysis). Comparison of rates across time scales showed the influence of episodic adjustments, such as earthflow erosion and deposition. Despite the distinct pattern of erosion across scales and lithologies, the primary influence on trail sustainability appears to be trail planning, with trails designated for use often lacking implementation of best practices. Together, this analysis provides quantitative constraints on erosion rates across lithologies at both millennial and annual timescales, informing both long-term landscape evolution and landuse management.