Hydrodynamics of late Quaternary outburst floods along the Yigong River,Eastern Himalayan Syntaxis

Glacier-induced damming of major Tibetan rivers has been hypothesized to reduce fluvial incision rates and modulate erosion processes, thereby influencing geomorphic stability along the margins of the Tibetan Plateau. The Yigong River, located in the Eastern Himalayan Syntaxis, serves as a critical test site for evaluating this hypothesis due to its documented history of episodic damming. We identify fve paleo-dam events from the last glacial period, focusing on one event at the confluence of the Xiaqu and Yigong Rivers through employing feld investigations, optically stimulated luminescence (OSL) dating, and geophysical surveys. Dam formation is dated to between 12.68 ± 1.17 ka and 10.48 ± 1.02 ka, linked to glacial moraine deposits from the Xiaqu River obstructing the Yigong River channel. Hydrodynamic modeling using HEC-RAS with a 30-m grid resolution and a Manning’s n of 0.045 indicates a maximum flow depth of approximately 60 m through the Tsangpo Gorge. Model results estimate a peak discharge of 4.36 × 104 m3/s following the dam breach, well below previously suggested high-magnitude flood thresholds (~106 m3/s). These fndings imply that the reconstructed paleoflood likely did not cause the extensive erosion noted in earlier geomorphic studies. However, similar paleoflood events likely played a signifcant role in downsteam transport of fne sediment through the Tsangpo Gorge.