Frequency-magnitude analysis of megafloods: Quantifying sediment transport and geomorphic effectiveness in the Tsangpo Gorge, eastern Himalaya

Extreme outburst floods are among the most powerful agents of landscape evolution in steep, tectonically active terrains. While evidence of such megafloods is widespread across the eastern Himalaya, their frequency-magnitude relationship remains poorly constrained, limiting our understanding of their cumulative role in long-term landscape evolution. Here, we integrate paleoflood chronostratigraphy record, 2D hydraulic simulations, and sediment transport analysis to reconstruct the timing, magnitude, and geomorphic effectiveness of megafloods (1–5 × 10⁶ m³/s) in the Yarlung Tsangpo Gorge since the Last Glacial period. A meta-analysis of 85 reported outburst flood deposits reveals at least nine discrete megaflood events, which cluster during intervals of global climate warming and intensified monsoon precipitation. Our results demonstrate an inverse correlation between flood magnitude and recurrence frequency. Intermediate-magnitude events (discharge of 2–3 × 10⁶ m³/s; n = 5) were the most geomorphologically effective due to their higher recurrence rate, accounting for ∼64% of the total sediment volume (∼13 km³) transported by megafloods (∼13 km³). In comparison, the largest recorded event (discharge of 4 × 10⁶ m³/s) mobilized a substantial volume of sediment (∼3.4 km³), which comprises only ∼25% of the cumulative megaflood sediment transport in this gorge. These findings emphasize the critical role of relatively frequent, intermediate-magnitude mega-events in driving long-term landscape evolution in high-energy mountainous settings. Furthermore, we show that post-Last Glacial Maximum (LGM), the recurrence interval of megafloods shortened from 1-in-10,000 to 1-in-4,000 years, indicating an accelerated fluvial geomorphic processes in the eastern Himalaya during the late Quaternary. Our findings highlight the frequency-magnitude dynamics of megafloods and their nonlinear control on landscape evolution, providing a quantitative framework for assessing future outburst flood hazards and informing risk mitigation strategies in vulnerable mountainous regions.