Late Pleistocene outburst floods from Issyk Kul, Kyrgyzstan?

Abstract

Elevated shorelines and lake sediments surrounding Issyk Kul, the world's second largest mountain lake, record fluctuating lake levels during Quaternary times. Together with bathymetric and geochemical data, these markers document alternating phases of lake closure and external drainage. The uppermost level of lake sediments requires a former damming of the lake's western outlet through the Boam gorge. We test previous hypothesised ice or landslide dam failures by exploring possible links between late Quaternary lake levels and outbursts. We review and recompile the chronology of reported changes in lake site, and offer new ages of abandoned shorelines using 14C in bivalve and gastropod shells, and plant detritus, as well as sand lenses in delta and river sediments using Infrared Stimulated Luminescence. Our dates are consistent with elevated lake levels between ~45 ka and 22 ka. Cosmogenic 10Be and 26Al exposure ages of fan terraces containing erratic boulders (>3 m) downstream of the gorge constrain the timing of floods to 20.5–18.5 ka, postdating a highstand of Issyk Kul. A flow‐competence analysis gives a peak discharge of >104 m3 s–1 for entraining and transporting these boulders. Palaeoflood modelling, however, shows that naturally dammed lakes unconnected to Issyk Kul could have produced such high discharges upon sudden emptying. Hence, although our data are consistent with hypotheses of catastrophic outburst floods, average lake‐level changes of up to 90 mm yr–1 in the past 150 years were highly variable without any outbursts, so that linking lake‐level drops to catastrophic dam breaks remains ambiguous using sedimentary archives alone. This constraint may readily apply to other Quaternary lakes of that size elsewhere. Nonetheless, our reconstructed Pleistocene floods are among the largest reported worldwide, and motivate further research into the palaeoflood hydrology of Central Asia. Copyright © 2017 John Wiley & Sons, Ltd.

Timing of exotic, far-travelled boulder emplacement and paleo-outburst flooding in the central Himalaya

“…All three approaches take advantage of the incipient motion principle and compute velocities for turbulent, Newtonian fluid flow when a sediment grain of given intermediate grain diameter initiates motion on the stream's bed. The Gauckler-Manning formula (Gauckler, 1867;Manning, 1891) was then used to estimate peak discharge from the peak flow velocity determined above and the valley cross-sectional geometry using the numerical optimization scheme of Rosenwinkel et al (2017). Peak discharges and maximum flood heights were calculated from flow velocities using the average of two valley cross-sections and bed slope for each boulder considered (S2).…”

Records of hydrological change and environmental disasters in sediments from deep Lake Issyk‐Kul

“…It is worth noting that under the influence of the AMO, changes in the lake water level do not exhibit cyclical fluctuations (Figure 7), which suggests that, apart from the effects of climate, lake water levels have also been affected by the Chu River separation, regional anthropogenic disturbance, and lake basin subsidence (Abuduwaili et al, 2019; Romanovsky, 2002; Rosenwinkel et al, 2017; Salamat et al, 2015). Before 1860 AD, the C/N and MS contents were relatively high (Figure 3), the replenishment and drainage effects of the Chu River and other exogenous rivers caused the TOC contents to be influenced only slightly, and the lake water level was high and relatively stable.…”

“…After 1860 AD, the values of C/N and MS obviously decreased (Figure 3), suggesting that exogenous rivers may have migrated, that the Chu River may have separated from Lake Issyk-Kul and that the TOC contents fluctuated notably as the lake level declined significantly (Figure 6; Podrezov et al, 2020;Romanovsky, 2002;Rosenwinkel et al, 2017). Some studies have indicated that Lake Issyk-Kul has become an endorheic lake due to the separation of the Chu River over the past 150 years (Giralt et al, 2004).…”

“…Lake Issyk‐Kul, called ‘the pearl of Central Asia’, is located in the western Tianshan Mountains and is the deepest and second largest alpine lake in the world (Li et al, 2018; Podrezov et al, 2020), which makes it an ideal place to research climate and environmental variations and responses in an alpine region. Currently, research in the study area mainly focuses on the utilization and protection of lake water resources (Abuduwaili et al, 2019), relationships between lake level fluctuations and regional climate and runoff (Romanovsky, 2002; Salamat et al, 2015), tree‐ring width‐based climate reconstruction (Zhang et al, 2015), earthquakes (Havenith & Bourdeau, 2010), palaeoclimatic and environmental events (Michael, 2011; Ricketts et al, 2001; Rosenwinkel et al, 2017), and sediment pollution (Li et al, 2018; Ma et al, 2018). However, to date, there is very little research on recent hydrological changes and driving mechanisms using a high‐resolution depositional and geochemical record in Lake Issyk‐Kul, which could provide a better and more complete understanding of the hydrological and environmental processes and their driving factors in Central Asia under global changes.…”

Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

“…1.3 significant GLOF events per year (Veh et al, 2019). A modern inventory of LLOFs has not been compiled as systematically, but recent reviews also suggest widespread occurrences along the Himalayan range (Ruiz-Villanueva et al, 2017). Since most of the existing information about LOFs in the Himalayas is derived from observational or historical records, limited insight is available with respect to the maximum magnitudes that can be expected for these events or the evolution of their occurrence frequency through time, for example in response to climatic change.…”