Abstract
Valleys in the northern Zailiiyskiy Alatau range, Tien Shan Mountains, Kazakhstan, are especially prone to large-scale debris flows due to increasing temperatures, high-intensity weather events, glacier retreat and sediment availability. To assess the area's hazard status this study uses dendrogeomorphological techniques and sedimentological analysis together with archival records to date debris flows in the Bolshaya Almatinka river drainage basin, an area that affects the city of Almaty (population 2 million). Our first study site in the lower Almatinka valley included terraces beside Kumbelsu creek and beside the Almatinka River in its middle reaches. Progressively younger trees on successively lower terraces indicate numerous but decreasing flood magnitudes since the 1920s. Our second site on the Ozernaya valley floor in the upper drainage basin is divided into nine distinct sedimentological assemblages. Their dating, derived from tree rings, highlights two major debris flow periods: the first between AD 1607 and 1633 and the second between AD 1702 and 1769. Documentary evidence shows that, since the mid-19th century approximately two-thirds of the large-scale debris flows in the region, including 14 in the study area, have been the result of moraine-dammed glacial lake outbursts, with the remainder due to intense summer rainstorms. The findings suggest that, since 1921, while the frequency of debris flows may have increased their hazard levels have declined; however, this declining trend could be reversed by seismic events. Future work should be directed to assessment of vulnerable moraine-dammed lakes, head slopes and sediment supply areas.