The India-Eurasia continental collision zone provides a unique example of active mountain building, plateau development and climatic forcing. Tectonic deformation and climatic influences exerted by mountain building extend well beyond the Himalaya; for example, the elevated Tibetan plateau initiates, controls and forces the Asian monsoon. While the Tibetan plateau today remains arid, the southern Himalayan front receives several meters of rainfall during the Indian summer monsoon season between June and September. The southern Himalayan mountain front represents an ideal location to study monsoonal oscillations and their effects on geomorphic and hydrologic processes.

In order to quantify the important process of mass removal, I analyzed spatial and temporal precipitation patterns of the oscillating monsoon system and their geomorphic imprints. Along the humid southern Himalayan mountain front, the interplay between topography and Indian summer monsoon circulation profoundly controls precipitation distribution, erosion, sediment transport, and river discharge. I processed passive microwave satellite data to derive high-resolution rainfall estimates for the last decade and identified an abnormal monsoon year in 2002. During this year, precipitation migrated far into the Sutlej Valley in the northwestern part of the Himalaya and reached regions behind orographic barriers that are normally arid. There, sediment flux, mean basin denudation rates, and channel-forming processes such as erosion by debris-flows increased significantly. Similarly, during the late Pleistocene and early Holocene, solar forcing increased the strength of the Indian summer monsoon for several millennia and presumably lead to analogous precipitation distribution as were observed during 2002. However, the persistent humid conditions in the steep, high-elevation parts of the Sutlej River resulted in different erosional landscape responses, such as deep-seated landsliding. Landslides were exceptionally large, mainly due to two processes that I infer for this time: At the onset of the intensified monsoon at 9.7 ka BP heavy rainfall and high river discharge remobilized aggragated the alluviated material, and lowered the baselevel. At this point, rivers were able to erode into bedrock again. During the intensified monsoon phase, enhanced discharge, sediment flux, and increased pore-water pressures along the hillslopes eventually lead to exceptionally large landslides that have not been observed in other periods.

The excess sediments that were removed from the upstream parts of the Sutlej Valley were rapidly deposited in the low-gradient sectors of the lower Sutlej River. There, a massive fluvial infill has been incised episodically during the Holocene. Timing of downcutting correlates with centennial-long weaker monsoon periods that were characterized by lower rainfall. I explain this relationship by taking sediment flux and rainfall dynamics into account: High sediment flux derived from the upstream parts of the Sutlej River during strong monsoon phases prevents fluvial incision due to oversaturation of the fluvial sediment-transport capacity. In contrast, weaker monsoons result in a lower sediment flux that allows incision in the low-elevation parts of the Sutlej River.

These results demonstrate that the landscape in the Himalaya is very sensitive to the strength of the monsoon. Associated with this climate change, is a fundamental shift in hillslope and hydrological processes. This in turn has a profound impact on the ecology and agriculture of this region.