Buffering of interglacial landscape evolution due to reduced sediment transport during glacial periods in Bhutan

Fluvial systems provide a primary means of sediment transport in alpine environments. This transport potential is regulated by precipitation, a fundamental contributor to stream power. Here, we investigate the legacy of reduced sediment transport capacity during dryer glacial intervals to fundamentally affect the diverse long-term evolution of 2 alpine catchments in the NW of Bhutan. The landscape in this region is characterized by three distinct geomorphic domains including transport-limited alluvial plains in the Inner Valleys, detachment-limited regimes in narrow 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 Wong Chhu basin in the West and the Puna Tsang Chhu basin in the East, both traverse these geomorphic domains, yet show marked differences in their river profiles, with the alluvial plain of the Wong basin located 1000 m higher than the Puna Tsang Chhu basin. 

The characteristic difference in elevation and extent of the alluvial infill between the two main basins of NW Bhutan, points to a systematic difference in relative erosional efficacy. For the effects of baselevel fall due to differential uplift on the Himalayan rangefront to be expressed as increased erosional potential in the High Himalaya, sediment deposited in alluvial planes of the Inner Valleys during low stream power glacial periods must first be evacuated during interglacial intervals.  Through a combination of geomorphological mapping and chi analysis of river profiles, we demonstrate that while post-glacial incision today is approaching the upper limit of the alluvial plain in the Puna Tsang Chhu (and can therefore drive bedrock incision through much of a ~40 kYr interglacial interval), the present-day erosional limit in the Wang Chhu is less than half-way through the alluvial fill in the Wang Chhu. It is therefore unlikely rivers in the Wang Chhu have been able to access bedrock or propagate the effects of baselevel fall to the upper extents of the catchment since the mid-Pleistocene transition (MPT). 

The evolution of the fluvial system appears to be reflected in rock mass weathering and hillslope evolution throughout the study area. Engineering geological descriptions of rock mass properties, in particular weathering, recorded at 295 sites over a period of 8 weeks in the region closely associated with alluvial valleys demonstrates a range of weathering grades from fresh rock to residual soil. A progressive increase in weathering degree with increasing elevation above river channels is evident in both catchments, indicating the signal reflects the time since active fluvial erosion ceased (as opposed to a pre-existing rock mass property). We observe higher degrees of weathering in the Puna Tsang Chhu valley, corresponding to the more humid climate in this valley supporting more rapid bedrock weathering. This more efficient transition from rock to soil at lower elevations may hint at a positive feedback in which despite indications of an additional 1000 m of cumulative incision since the MPT, hillslopes have evolved to erode at a rate which approaches that of fluvial incision.