Two megafloods in the middle reach of Yarlung Tsangpo River since Last-glacial period: Evidence from giant bars

Megafloods in the Yarlung Tsangpo River have long been recognized as a major agent causing regional geomorphic change of the Namche Barwa. Here we present a well-constrained record of catastrophic outburst flood history in the middle reach of the river. The detailed sedimentological analysis of seven giant bars provides reliable evidences at least two paleofloods, which are consistent with the results obtained by optically stimulated luminescence dating and one-dimensional hydraulic simulation. Flood units with similar reconstructed peak discharges and ages have been assumed to represent the same flood. According to the timing analysis, the largest observed flood event (megaflood I) of 1.0 × 106 m3/s is dated, via Optically stimulated luminescence (OSL) dating on the three giant bars, as occurring between 12.06 ± 1.01 ka and 11.05 ± 0.90 ka. Megaflood II, dated via OSL dating of a giant bar, occurred between 4.96 ± 0.38 ka and 3.20 ± 0.24 ka, which was fitted consistently to a peak discharge of 0.3 × 106 m3/s. A number of subordinate and unrecorded large floods may also have occurred. The study revealed that failure of the Jiedexiu dammed lake causes megaflood I, and megaflood II might have influenced activity of prehistoric humans in the late Neolithic age. Our study identifies new sources for the previously studied megafloods, which could further explain the focused erosion in the steep topography of the Yarlung Tsangpo Grand Canyon.