Reconstruction of glacial lake outburst floods in northern Tien Shan: Implications for assessment

An improved understanding of these processes is essential for designing reliable disaster risk reduction strategies, particularly in the context of ongoing climate change. This study aims to reconstruct past debris-flow activity on the Aksay cone in the Kyrgyz range using a dendrogeomorphic approach. A total of 96 Picea abies trees growing on the cone and along the main channel were selected based on past debris-flow damage. The trees were then analyzed using increment borers, and the dating of past events was based on the assessment of growth disturbances (GD) in tree-ring records. A total of 324 GD were identified, including detection of damaged trees, tangential rows of resin ducts, reaction wood, and abrupt growth changes. In combination with aerial and geomorphic recognition, the tree-ring analysis allowed the reconstruction of the main spatial patterns of past events on the Aksay cone. The findings suggest that at least 27 debris flows have occurred on the site between 1877 and 2015, with at least 17 events not previously documented. High process activity was observed in the 1950s and 1960s, with major events occurring in 1950, 1966, and 1968. The spatial analyses indicate two different spatial patterns, suggesting dissimilar magnitudes of glacier lake outburst floods and rainfall-induced debris flows. The results represent the longest, annually resolved GLOF series in the region, with implications for risk assessment not only in the Ala-Archa valley but also in the entire Kyrgyz range. Debris flows are rapid movements of loose debris, air, and water, which can cause significant damage to infrastructure and loss of life. The causative factors of debris flows, including glacial lake outburst floods, have been extensively described in the literature. GLOFs are formed from the breakage or overfilling of moraine- or ice-dammed lakes, resulting in huge debris flows that can shape cones and modify depositional forms. These events can also impact environments located far downstream, causing damage to populated valleys. Recent GLOF events have been documented in various mountain ranges worldwide, including the Andes, Central Asia, and the Himalayas. The occurrence of these events is expected to increase in the coming decades due to climate change, glacier retreat, and slope instability. In the northern Tien Shan region, the IPCC projects a moderate increase in temperatures, which may contribute to glacier retreat and glacier lake formation. The widespread lack of long-term observations and robust climate change projections has hindered efforts to assess possible changes in GLOF activity in the region. This study combines an assessment of aerial pictures, historical records, field surveys, and tree-ring analyses to build the longest, annually resolved history of GLOF and debris-flow activity in northern Tien Shan. The study site, the Aksay cone, is the largest debris-flow fan in the Ala-Archa National Park, located near the capital city of Bishkek. The cone is covered by Picea abies and Betula trees, with two glaciers, the Uchitel and Aksay glaciers, contributing to the debris flow activity. The Aksay cone is characterized by a steep icefall with a slope of 52% and a drop of up to 200 m. The debris-flow cone extends from 2200 to 2350 m a.s.l. and is covered by Picea abies in areas with older sediments and Betula in areas affected by debris flows. The study aims to distinguish GLOFs from debris flows using the signs left by these events and their spatial patterns, as well as the position of trees on the cone. The hypothesis was first tested with available historical records, and the results suggest that the signs and spatial patterns of GLOFs and debris flows are different.