Repeated failures of the giant Beshkiol Landslide and their impact on the long-term Naryn Basin floodings, Kyrgyz Tien Shan

Landslides are major hazards that lead to cataclysmic changes in regional physiography. Their consequences are particularly significant when they affect a river system, forming dammed-lake upstream that represents a high flood threat for the downstream region. The Naryn River is the largest river in the Kyrgyz Tien Shan and is of great economic importance. The Beshkiol Landslide, the largest one in Central Asia but of unknown age, has most likely blocked the Naryn River in the past during the Late Pleistocene, with evidence of thick lacustrine deposits as well as numerous paleo-shorelines preserved upstream. In this study, a detailed geomorphological and sedimentological analysis combined with luminescence and ¹⁴C dating provides a strong chronological framework to refine the dynamics between the Beshkiol landslides and dammed-lakes in the Naryn Basin. We propose that the Beshkiol Landslide was first triggered 51.9 ± 4.4 kyrs ago, with a 410 m-high dam that blocked the Naryn River. A first lake with a total volume of 121 ± 50 km³ lasted for >37.0 ± 5.1 kyrs, one of the longest landslide-dammed lake residence time ever documented in the world. Our sedimentological observations highlight a catastrophic lake outburst flood between 15.6 and 14.1 kyrs cal BP, likely related to a landslide dam breach. A short-lived phase of fluvial erosion impacted the whole Naryn Basin followed by a second landslide activation (280 m- high dam) and subsequent flooding by a second lake of 27 ± 10 km³. This second lake had a minimum residence time of 7.7 ± 1.3 kyrs before its final gradual drainage that was followed by a fluvial erosional phase still active today in the Naryn Basin. We also suggest that the distal unconsolidated part of the Beshkiol Landslide could be remobilized in the event of an earthquake and/or extreme rain episode, causing a potentially dam of the Naryn River, which would have strong regional economic impacts.