Dam and megafloods at the First Bend of the Yangtze River since the Last Glacial Maximum

Abstract The Yangtze River is the longest watercourse in China, where it flows through the most densely populated and richest regions. The upper course of the River (Jinsha River) has been recurrently blocked over the course of time. The dam-outburst flood process has exerted a significant impact on the river's drainage area. However, the frequency of such events along the course of the Yangtze River remains poorly investigated. In this study, damming chain sediments were identified in the First Bend area of the Yangtze River, and were dated using optically stimulated luminescence (OSL) and 14C techniques. The obtained ages would suggest that the river was dammed by a debris flow during the ~20–7 ka period, with a resulting paleolake covering an area of 40.79 km2 and having a water volume of 0.63 km3. Two outburst megafloods occurred at ~18–20 and ~ 6.0 ka, respectively. The river was re-dammed again during 1.5–0.4 ka, with a lake coverage of 28.20 km2 and water volume of 0.28 km3. The dammed lake may have been gradually breached after ~0.4 ka. The damming events were caused by debris flows triggered by glacial advances on Yulong Mountain.