QUANTIFYING ALPINE VALLEY WALL EROSION RATES USING COSMOGENIC NUCLIDES AND VOLUMETRIC ESTIMATES OF TALUS DEPOSITS, TETON RANGE, WYOMING

Rockfalls and other stochastic mass wasting processes challenge researchers to find effective methods for quantifying rates of valley wall erosion necessary to understand tectonic or climate-driven changes to the landscape. This study compared three methods of estimating rockfall erosion rates along alpine valley walls since glacial retreat in the Teton Range, Wyoming. The first method estimated the volume of accumulated rockfall debris by projecting the slope of a bedrock wall beneath a two-dimensional profile of the talus fan surface. The profile area multiplied by fan width resulted in a volume that was divided by the contributing bedrock area and time since glacial retreat (13.5 ka). The second method predicted the volume of talus based on a three-dimensional triangulated irregular network (TIN) model of a fan and surrounding bedrock surfaces created from a minimum of 20 GPS points shot across both talus and bedrock surfaces with a laser range finder. The third method measured cosmogenic ¹⁰Be concentrations from amalgamated samples of at least 15 cobbles collected across each talus fan surface. The rates from these three methods were also compared to in-situ cosmogenic ¹⁰Be bedrock erosion rates from ridges overlooking each talus deposit. Erosion rates from 10 two-dimensional profiles varied from 0.3-2.6 mm/yr. Four TIN volume estimates resulted in similar rates of 0.2-0.4 mm/yr. Cosmogenic ¹⁰Be erosion rates on four talus surfaces ranged from 0.1-1.2 mm/yr. All estimates of erosion rates from talus were greater than four in-situ bedrock erosion rates on ridges, which ranged from 0.01-0.08 mm/yr. Alpine erosion rates since glacial retreat calculated from the two talus volume methods produced reasonable estimates of valley wall erosion compared to amalgamated cosmogenic erosion rates for the same talus fans. All three methods showed variability in erosion rates as can be expected from the random distribution of rockfall events; however, the average rates from each method fall within uncertainty of the other methods studied here. The TIN volume method was the most efficient in terms of time and simplicity required for data collection in the field and erosion rate calculations.