The Kangri Karpo Mountain Range on the Qinghai-Tibet Plateau frequently experiences glacial lake outburst floods (GLOFs). This study assessed the risk of outburst floods for Guangxieco Proglacial Lake (GPL) in this Mountain Range as a typical case to reveal the effects of rapid glacial change. The area of Gongzo Glacier behind GPL decreased by 7.39 +/- 0.10% from 1987 to 2019, while this glacier advanced by 32.45 m from 5 June to 27 October in 1988. Guangxieco Proglacial Lake decreased from 0.42 +/- 0.03 km(2) in 1987 to 0.19 +/- 0.03 km(2) in 1988 and then continuously expanded to 0.43 +/- 0.04 km(2) in 2019. Heavy precipitation occurred before 15 July 1988, when no supraglacial lake existed. Meanwhile, sustained abnormally high air temperature caused accelerated glacier and snow melting. Since 1988, a larger volume of rainfall and meltwater impounded by the ice wall caused an increase in the basal water pressure in the glacier. A significant increase in winter mass balance has caused a further increase in the downward gravity component of glacier sliding. As a result, the glacier advanced rapidly while reopening previously blocked subglacial drainage systems. The accumulating subglacial water rapidly drained into the Proglacial Lake causing an elevated lake level and a GLOF event. However, the current area of the glacial lake has recovered to the scale present before the outburst in 1988. Therefore, local government agencies and the local community should improve early warning systems and take measures designed to prevent a new GLOF and to minimize the risk of a recurrence of a GPL outburst.

Glaciers in the Himalayan region have been receding rapidly in recent decades, drawing increasing concerns about the release of legacy pollutants (e.g., mercury (Hg)). To investigate the distribution, transport and controlling factors of Hg in glacier-fed runoff, from June 2019 to July 2020, a continuous monitoring and an intensive sampling campaign were conducted in the Rongbuk Glacier-fed basin (RGB) on the north slope of Mt. Everest in the middle Himalayas. The total Hg (THg) and methyl Hg (MeHg) concentrations were 1.56 +/- 0.85 and 0.057 +/- 0.025 ng/L, respectively, which were comparable to the global background levels and were mainly affected by the total suspended particulate matter (TSP). In addition, THg and MeHg showed significant diurnal variations, with peak values appearing at approximately 17:00 (upstream) and 19:00 (downstream). Based on the annual runoff and average Hg concentration, the annual export fluxes of THg and MeHg were estimated to be 441 g and 16 g, respectively. The yields of THg and MeHg in the RGB were 1.6 and 0.06 mu g/m(2)/year, respectively. Currently, the annual Hg export of meltwater runoff in the Himalayan region is approximately 337 kg/year. When flowing through the proglacial lake, the THg concentrations decreased by 32% and 15% in the proglacial lake water and in the outlet, respectively, indicating that proglacial lakes had a sedimentation effect on the Hg transport. The Hg export from meltwater runoff in the Himalayas will likely increase considering the meltwater runoff has been projected to increase in the future. Nonetheless, emerging proglacial lakes may exert ambiguous effects on the glacier exported Hg under changing climate. Proglacial lakes could lower the levels and amounts of Hg in the glacier runoff, whereas the outburst of proglacial lakes could lead to an instantaneous release of Hg stored in lake waters and sediments. Our analysis shed light on the environmental impact of glacier retreat in the Himalayas and highlighted the need for integrated monitoring and study of Hg in glacier runoff and glacial lakes.