Glacial Retreat and Associated Glacial Lake Hazards in the High Tien Shan

A number of studies have identified glacial retreat throughout the greater Himalayan region over the past few decades, but the Karakorum region remains an anomaly with large stagnating or advancing glaciers. The glacial behavior in the Tien Shan is still unclear, as few studies have investigated mass balances in the region. This study focuses on the highest peaks of the Tien Shan mountain range, in the region of Jengish Chokusu along the Kyrgyzstan-China-Kazakhstan border. In a first step, a 30-year time series of Landsat imagery (n=27) and ASTER imagery (n=10) was developed to track glacial growth and retreat in the region. Using a combination of spectral and topographic information, glacial outlines are automatically delineated. As several important glaciers in the study region contain medium to high levels of debris cover, our algorithm also improves upon current methods of detecting debris-covered glaciers by using topography, distance weighting methods, river networks, and additional spectral data. Linked to glacial retreat are glacial lake outburst floods (GLOFs) that have become increasingly common in High Mountain Asia over the last few decades. As glaciers retreat, their melt water is often trapped by weakly bonded moraines. These moraines have been known to fail due to overtopping caused by surge waves created by avalanches, rockslides, or glacial calving. A suite of studies throughout High Mountain Asia have used remotely-sensed data to monitor the formation and growth of glacial lakes. In a second step of the work, lake-area changes over the past 15 years were tracked monthly and seasonally using dense Landsat/ASTER coverage (n=30) with an automatic procedure based on spectral and topographic information. Previous work has identified GLOFs as a significant process for infrastructural damage in the southern Tien Shan/northern Pamir, as well as in the better studied Himalaya region. Lake identification and quantification of lake-growth rates is a valuable first step to identifying potential flooding hazards in the region. In a third step, ASTER imagery is used for mass balance estimates from stereogrammetry over the last 10 years (n=10). These mass balance estimates are some of the first in the region, and provide valuable context on the rate of retreat identified using Landsat imagery. By combining glacial retreat rates with lake-change rates, a preliminary hydrological budget was derived for the region. This budget provides a basis for estimating GLOF risk in the region, by combining risk factors such as lake-growth rate and glacial retreat rate with glacier slope and other near-lake risk factors, such as propensity for landslides, likely to cause overtopping in lakes and downstream flooding. Following the identification of dangerous lakes, elevation models were used for flow routing, to assess total downstream area that would be affected by a GLOF. In combination with infrastructure and demographic information, the algorithm can estimate potential downstream impacts on local communities. This work provides an integrated picture of glacial lake hazards in a previously unstudied region of the Tien Shan, as well as valuable mass balance and glacial retreat rate estimates.