Loukili Y. and Pomeroy J.W. (2018). The Changing Hydrology of Kluane Lake under Past and Future Climates and Glacial Retreat Centre for Hydrology - Report #15.

The diversion of the Slims River, Ä’äy Chù, headwaters due to the Kaskawulsh Glacier retreat is one of the most extraordinary and dramatic hydrological changes due to climate change observed in Canada in the 21st C. The Slims River flows north from the terminus of the Kaskawulsh Glacier to Kluane Lake, Lhù’ààn Mǟn – the largest lake in Yukon – and receives most of its inflow from the glacier meltwaters. The Kaskawulsh Glacier has been retreating rapidly since at least the 1950s. In May 2016, this retreat permitted ponded meltwaters at its terminus to erode a new channel through an ice dam at the valley fork and flow eastwards through a 30-metre tall canyon towards the Kaskawulsh River. Since then, Kluane Lake has experienced lower peak summer water levels. This event was widely covered in the news and described by some as “river piracy”, in that the meltwaters that used to flow northward into the White and Yukon Rivers towards the Bering Sea, were redirected eastward to feed the Alsek River, which discharges southward in the Gulf of Alaska.

This is not the first time that this diversion has happened. Partial and transient diversions of glacial meltwater from the Kaskawulsh Glacier into the Kaskawulsh River rather than to the Slims River occurred in 1953, 1967, 1970, and 1989, due to a combination of ice dynamics and glacial melt hydrology and hydraulics around the terminus. Bryan (1972) asserted that “ If these diversions continue to happen, and if the headward erosion of the Kaskawulsh River is sufficient to pirate the Slims River system, then it is possible that the entire drainage system could be redirected in a manner described by Bostock”. Shugar et al. (2017) estimated a 99.5% probability that the Kaskawulsh Glacier retreat, which triggered the piracy, can be attributed to human-caused global warming.

The goal of this report is to estimate the variability and changes in the lake levels of Kluane Lake over the historical period and into the future climates of the 21 st C, with and without the Kaskawulsh Glacier contribution. The study diagnoses the causes of variability of lake levels in the past and evaluates the impact of deglaciation on lake levels in the future in the context of climate change. The methods use a combination of weather data from observations and global climate models to drive a detailed glacio-hydrological prediction model, which calculates streamflows in the Slims River and other inflows to Kluane Lake, lake evaporation and outflows and then the lake level. Historical Kluane Lake levels during the 20 th C and future lake levels under global warming projections for the rest of the 21 st C were predicted - with and without the Kaskawulsh Glacier contribution to the Slims River. The Canadian glacio-hydrological water prediction model MESH, which couples the Canadian Land Surface Scheme with both surface and subsurface runoff on slopes and river routing, was used to model the hydrology of the Kluane Lake Basin for these predictions. The adjacent gauged Duke River Basin was also included in the model to provide opportunities to evaluate the model performance in this region against gauged streamflows. Model parameterisations of topography, land cover, glacier cover, soil type and runoff directions were made and used to set up the model on various sub-basins flowing into Kluane Lake, including the Slims River Basin.

In order to reproduce historical conditions back to the early 20 th C, meteorological forcing inputs from the European Union “WATCH” Project meteorological dataset were used to drive 6 model runs for 1901-2001. Simulated snow regimes, lake levels and Kluane River flows were compared and calibrated to observations available from 1953 onwards. To compare lake levels and hydrology between recent climates and those expected from future climate change, MESH was driven by outputs from the Weather Research and Forecasting (WRF) atmospheric weather model at 4-km resolution, under 2000-2015 conditions and the RCP8.5 “business as usual” greenhouse gas emission scenario for 2085-2100. In all model runs, the lake levels and basin hydrology were calculated with and without the Kaskawulsh Glacier contribution.

Analysis of the modelling results shows that for all periods examined, winter/spring/fall lake levels are not strongly affected by diversion of the glacier meltwaters, but summer peak lake levels are reduced by 1.6 m on average, from the observed median 781.2 m a.s.l. (above sea level) to the predicted median 779.6 m a.s.l. This is consistent with recent lake level observations by Environment and Climate Change Canada. Model analysis for the previous century documents the natural variability of the lake, including a few short-term temporary diversions of glacier outflow from or to the Slims River caused by glacier hydrodynamics at its terminus.

Results show that lake levels are very sensitive to conditions at the outflow of the lake into the Kluane River as represented by the rating curve of the river. From 1995 to 2015 the estimated rating curve changed such that average lake levels dropped 0.25 m during open water conditions. This drop in water levels is due to degradation of the outflow channel of Kluane Lake at Kluane River. It is strongly recommended that regular measurement of this rating curve be re-established in the Kluane River so that future changes can be quantified.

MESH modelling scenarios for the 20 th C show a substantial seasonal drop in Kluane Lake levels from June to October when the glacier discharge is excluded, reaching a maximum difference of 1.7 m during August from those lake levels calculated with the glacier outflows. In the absence of the glacier, median inflows to Kluane Lake via the Slims River drop from more than 350 m 3 s -1 to around 60 m 3 s -1 during the month of July. Without the glacier inputs, the modelled summer peaks in lake levels are lower and summer median levels reach barely 779.4 m using the most recent rating curve. MESH results for the early 21 st C without the Kaskawulsh Glacier inputs are realistic for the current lake level regime, with minimum, median and maximum peak levels of 779.4, 779.65 and 780.5 m respectively using the most recent rating curve. Until a modern, regularly measured rating curve for Kluane Lake is produced and maintained, these results can be used as guidance for the expected levels and flows by local design and hydrology projects. Model results for the late 21 st C under substantial climate change, provide similar Kluane Lake levels without the glacier contributions. The future projections predict a forward shift in timing of peak levels from July to early June but are otherwise not notably higher or lower than the current projections. Model analysis for the late 21 st C shows that lake levels are not further reduced or increased by anticipated shifts in the climate of the region. However, as the future rating curve on the Kluane River is unknown, there is uncertainty in these results that could be reduced by resumption of streamflow discharge measurements and measurement of new rating curves on the Kluane River. There is no indication whatsoever from the modelling scenarios of the Kluane River going dry or the flow reversing from Kluane Lake up the Slims River and down 7 the Kaskawulsh River – under current and foreseeable conditions such events are highly improbable.

The results drawn from this study are intended to answer important questions posed by Kluane First Nation of Burwash Landing, residents of Destruction Bay and surrounding areas and Yukon Government on the history and the future of Kluane Lake levels. Furthermore, the study will help inform water management and infrastructure design around Kluane Lake, and other environmental and aquatic conservation and adaptation efforts in the region. While the models employed here represent the “state-of-the-art”, there is uncertainty in the predictions. This uncertainty could be reduced in future prediction efforts by resuming Kluane River discharge measurements, which were discontinued in 1994.

https://research-groups.usask.ca/hydrology/documents/reports/chrpt15_kluane-lake_jan19.pdf