Modern erosion rates in the High Himalayas of Nepal

Current theories regarding the connections and feedbacks between surface and tectonic processes are predicated on the assumption that higher rainfall causes more rapid erosion. To test this assumption in a tectonically active landscape, a network of 10 river monitoring stations was established in the High Himalayas of central Nepal across a steep rainfall gradient. Suspended sediment flux was calculated from sampled suspended sediment concentrations and discharge rating curves. Accounting for solute and bedload contributions, the suspended sediment fluxes were used to calculate watershed-scale erosion rates that were then compared to monsoon precipitation and specific discharge. We find that, in individual watersheds, annual erosion rates increase with runoff. In addition, our data suggest average erosion rate increases with discharge and precipitation across the entire field site such that the wetter southern watersheds are eroding faster than the drier northern watersheds. The spatially non-uniform contemporary erosion rates documented here are at odds with other studies that have found spatially uniform long-term rates (105–106 yr) across the pronounced rainfall gradient observed in the region. The discrepancy between the modern rates measured here and the long-term rates may be reconciled by considering the high erosional efficiency of glaciers. The northern catchments were much more extensively glacierized during the Pleistocene, and therefore, they likely experienced erosion rates that were significantly higher than the modern rates. We propose that, in the northern watersheds, the high rates of erosion during periods of glaciation compensate for the low rates during interglacials to produce a time-averaged rate comparable to the landslide-dominated southern catchments.