Bedload-to-suspended load ratio and rapid bedrock incision from Himalayan landslide-dam lake record

About 5400 cal yr BP, a large landslide formed a > 400-m-tall dam in the upper Marsyandi River, central Nepal. The resulting lacustrine and deltaic deposits stretched > 7 km upstream, reaching a thickness of 120 in. C-14 dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly (similar to 500 yr). Reconstructed volumes of lacustrine (similar to 0. 16 km(3)) and deltaic (similar to 0.09 km(3)) deposits indicate a bedload-to-suspended load ratio of 1:2, considerably higher than the <= 1: 10 that is commonly assumed. At the downstream end of the landslide dam, the river incised a new channel through >= 70 m of Greater Himalayan gneiss, requiring a minimum bedrock incision rate of 13 mm/ yr over last 5400 yr. The majority of incision presumably occurred over a fraction of this time, suggesting much higher rates. The high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments. (c) 2007 University of Washington. All rights reserved.