Quantifying periglacial erosion in the Nepal high Himalaya

Quantifying erosion rates in high mountain environments is challenging because the erosional processes are often stochastic and catastrophic. Distinguishing between periglacial and glacial rates is especially challenging. In this study we focus on determining erosion rates from a glaciated alpine landscape where the bedrock sidewalls are eroding predominantly by blockfall. We utilize a topographically well-constrained, debris-covered glacier on the north slope of the Annapurna Range, central Nepal Himalaya, to quantify an average headwall retreat rate. By measuring supraglacial debris depths from 0.1 to 2.4 m and average short-term down-valley transport rates of 17 m/yr we calculate a yearly sediment flux of 5820 ± 1990 m³/yr. The supraglacial debris originates from the steep, rocky headwall of the valley and we use the calculated flux with an estimated headwall contributing area to determine a slope-perpendicular headwall retreat rate of 1.3 ± 0.5 mm/yr, which corresponds to a vertical lowering rate of 0.42 ± 0.16 mm/yr and a horizontal retreat rate of 1.2 ± 0.5 mm/yr. This rate is significantly greater than the steady-state erosion rates that we determine for the valley ridge crests and sidewalls (0.08 ± 0.03 mm/yr and 0.20 ± 0.05 mm/yr, respectively) using concentrations of the in situ produced cosmogenic radionuclide, ¹⁰Be. Additionally, ¹⁰Be exposure ages of the lateral moraine crests and supraglacial debris suggest a glacial advance about 470 yr ago and a long-term supraglacial transport rate of about 10 m/yr. Using this long-term transport rate reduces our inferred headwall retreat and suggests that the up-glacial bedrock slopes are eroding at roughly the same rate. While our results do not quantify the rate of glacial erosion, the absence of a glacial gorge suggests that there may be a rough balance between glacial downcutting and the erosion of the valley sidewalls. Rates quantified here are almost an order of magnitude lower than fluvial incision rates of the upper Marsyandi River that drains the study area, suggesting that even glaciated catchments are not keeping pace with the rapidly down-cutting rivers of the Himalaya.