Outburst floods records in the Yigong River since holocene, southeastern Tibet

Background Paleofloods are critical geomorphic driving forces shaping the landform evolution of the Tibetan Plateau margin. Although paleoflood investigations have been conducted in the Yarlung Tsangpo River, including documented sedimentary records in the gorge and upstream reaches, studies on its secondary tributary, the Yigong River, remain insufficient. In particular, quantitative hydraulic simulations of prehistoric outburst floods in this basin are lacking, as existing research mainly focuses on the modern extreme flood events of 1902 and 2000, leaving Holocene paleoflood evolution poorly constrained. Methods Based on systematic field geological surveys, sedimentary records of paleofloods were identified, including valley fill deposits at the main channel edge, flood-induced giant bar deposits at the river confluence, and slack water deposits (SWDs) in the downstream main channel. Sedimentological and chronological analyses were conducted, and hydrological reconstruction was performed for different peak discharge scenarios. Results The giant bar deposits consist of massive coarse sand and gravel with scattered oversized boulders, while downstream sediments near Tongmai Bridge show an abrupt facies transition from coarse gravels to well-sorted sandy gravels. The SWDs are dominated by silty couplets and massive sand beds, representing distal flood sedimentary signatures. Integrated analyses constrain three Holocene outburst flood events at 14.8 ka BP, 8.8 ka BP, and 6.7 ka BP. Hydrological reconstruction yields a maximum peak discharge of 4.4 × 105 m³/s with an ancient dammed lake reservoir capacity of 3.5 × 1010 m³, and a minimum peak discharge of 2.3 × 105 m³/s with a reservoir capacity of 1.22 × 1010 m³. Conclusions This study confirms that the Yigong River is a high-frequency zone of Holocene outburst floods. The newly discovered paleoflood records fill a research gap on prehistoric extreme flood events in this sub-basin and provide crucial supplementary data for understanding catastrophic flood processes and geomorphic responses in the steep, tectonically active Yarlung Tsangpo River, offering valuable insights into regional paleohydrological evolution and extreme flood risk assessment.