Positive feedback between rates of rock mass weathering and landscape lowering through fluvial and hillslope erosion in the High Himalaya of Bhutan

River systems provide a primary means of sediment production and transport in mountain environments. While sediment transport is mainly governed by stream power, the rate of rock mass weathering is a function of rock mechanical properties and the local climatic conditions. Here, we investigate the interaction between fluvial incision and weathering rates in two alpine catchments in the NW of Bhutan. The study region is characterized by three distinct geomorphic domains including broad alluvial plains, with detachment-limited regimes, transport-limited fluvial valleys with steep hillslopes and high relief, and glacially overprinted low-relief landscapes at the foot of the High Himalayan peaks. The two major drainage basins, the Wang basin in the West and the Punatshang basin in the East, both traverse these geomorphic domains, yet show marked differences in profile view, with the alluvial plain of the Wang basin located 1000 m higher than the Punatshang basin.

In this study we combine river profile analyses with records of rock mass weathering and mapped morphological markers regarding the spatial extent of erosional regimes (past and present extent of alluvial infill and maximum glacial extent) to reconstruct the relative sequence of erosional events generating the present-day topography of the two basins. We find characteristic gradients in the degree of rock mass weathering throughout the hillslopes with fresh outcrops close to the river network and weathering grade increasing with elevation above the river channel. We interpret the higher degrees of weathering to represent the additional time available for rock mass weathering in parts of the landscape distal to the ongoing fluvial incision. While the general pattern of rock mass weathering is similar between the two basins, we observe on average higher degrees of weathering in the eastern basin (Punatshang). Although the difference in elevation of 1000 m between the two basins suggests a larger volume of sediment having been eroded in the Punatshang basin, we suggest the topographic lowering of this basin and the subsequent climatic changes (increases in mean annual temperature and total annual precipitation) to result in increased rates of rock mass weathering, which outpace those of fluvial incision.