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

Bhutan is a landlocked country straddling the Himalayan Arc, with elevations ranging from approximately 170 to 7,600 m a.s.l. Despite these high elevations, large portions of the do not show glacial overprint and are thought to have remained relatively stable in terms of erosion during recent geological history. This unique geomorphological setting is characterized by a quasi-stationary fluvial system and low hillslope erosion, as evidenced by well-preserved sedimentary records and deeply weathered rock horizons along adjacent hillslopes.

This study focuses on two large drainage basins in northwestern Bhutan, which encompass three distinct geomorphic domains: (1) broad alluvial plains with detachment-limited regimes, (2) transport-limited fluvial valleys with steep hillslopes and high relief, and (3) glacially overprinted low-relief landscapes at the base of the High Himalayan peaks. During multiple field campaigns, we extensively mapped the extent and type of sedimentary deposits across numerous outcrops, enabling the creation of a 3D inventory of sedimentary distributions throughout the valleys. These field data are integrated with geomorphological analyses of river profiles and a comprehensive inventory of rock mass weathering degrees to reconstruct the sequence of geomorphic events shaping the contemporary landscape.

Our observations reveal notable contrasts between the two basins. In the Wang Chhu Valley (western basin), broad alluvial plains exhibit minimal terracing, with terrace steps measuring only a few meters in height. In contrast, the Punatsangchhu Basin (eastern basin) features much narrower valleys distinguished by well-defined terraces with elevation differences of several tens of meters. Furthermore, although both basins lie at comparable distances from the range front, the central valleys in the eastern basin are approximately 1,000 m lower in elevation than corresponding locations in the western basin.

The sedimentary deposits also display distinctive characteristics. In the western basin, fluvial sediments are often interbedded with chaotic, sub-angular blocky deposits indicative of gravitational mass movements, such as debris flows. While similar deposits are present in the eastern basin, they are accompanied by lake sediments and thick accumulations of fine-grained, unstructured material containing suspended angular clasts. These deposits are likely associated with a glacial lake outburst flood (GLOF) previously documented in the region.

Combining the spatial sedimentary distribution of the two basins with topographic analyses, we propose a sequence of geomorphic events marked by extensive periods of erosional quiescence forming large fluvial deposits, which are regularly interrupted by phases of heightened hillslope sediment production or even catastrophic events like glacial lake outburst floods.