Exceptionally low activities of ¹⁰Be and ²⁶Al in quartz separated from fluvial sediments in the Sunkosh drainage of Bhutan indicate some of the highest mean basin denudation rates yet measured cosmogenically, 0.8 to >12.2 mm/yr. These high rates of denudation are consistent with the topography and climate of Bhutan, which is characterized by the greatest concentration of deeply-incised, steep-gradient channels and the highest rainfall in the Himalayan region. The nuclide data support fission track and incision rate measurements made elsewhere in the region.

We sampled the active channel at two sites in the same basin, split the sand and gravel into 3 grain sizes, and analyzed the fractions separately. One site (X09) is near the downstream end of a drainage network covering a substantial fraction of western Bhutan (8000 km², elevation 300 to >7000 m). The second site (1BAL0) is in a small, partly glacierized (<10%) sub-basin far upstream (73 km², elevation 4000 to 6500 m). Nuclide activity in five samples ranges from 80,000 to <3000 atoms/g ¹⁰Be and 450,000 to <24,000 atoms/g ²⁶Al. The equivalent surface exposure periods are extremely short, ranging from 750 to <50 years (Nishiizumi et al. production rates convolved using basin hypsometry). In samples where measured ratios were finite, nuclide activities are well correlated, with an average ²⁶Al/¹⁰Be ratio of 6.0 +/- 0.5.

Interestingly, there appears to be a grain-size dependence. Sand size grains (0.25 to 0.85 mm) gave ¹⁰Be and ²⁶Al average model erosion rates of 1.1 and 2.3 mm/yr at X09 and 1BAL0, respectively; clasts >2 mm gave higher rates, >12.2 and >9.6 mm/yr, for two samples from X09 but lower rates for 1BAL0 (0.85 mm/yr). The grain size dependence may indicate differences in sediment delivery mechanisms (e.g., soil creep vs. landsliding), or differences in source proximity with coarser material being more locally derived and hence dosed at elevations where nuclide production rates are different. Alternatively, the grain size dependence may indicate different rates of erosion for areas supplying the coarse fraction.