Modelling Glacier Mass Balance and Runoff in the Kaskawulsh River Headwaters of Southwest Yukon, Canada, 1980–2022

The highly-glacierized headwaters of the Kaskawulsh River is home to 9% of all glacier ice in Yukon, Canada, and was the source of a sudden meltwater-rerouting event in 2016 that has had significant downstream consequences. We use a distributed mass-balance model driven by downscaled and bias-corrected climate reanalysis data that incorporates observations of sub-debris melt, accumulation, and transient snowline positions to estimate the 1980–2022 glacier mass balance, discharge, and water budget of the Kaskawulsh River headwaters. We estimate a catchment-wide cumulative mass loss of 18.02 Gt over 1980–2022 (−0.38 ± 0.15 m w.e. a⁻¹) and a mean annual discharge of ~60 m³ s⁻¹, 25% of which originates from non-renewable glacier wastage. The water budget is dominated by glacier ice melt, accounting for 61% of mean annual discharge, followed by snowmelt at 31%, rainfall at 6%, and melt from refrozen ice layers at 2%. Extreme negative and positive mass-balance years produce the largest perturbations in glacier ice melt contributions to the water budget, ranging from a maximum of 67% following negative years to a minimum of 53% in positive years. Trend detection using the Mann-Kendall test shows that catchment-wide annual discharge increased by 3.9 m³ s⁻¹ per decade from 1980 to 2022, with statistically significant contributions from glacier ice melt (2.8 m³ s⁻¹ per decade) and rainfall (0.5 m³ s⁻¹ per decade). Increasing air temperatures and declining spring snowfall have lead to seasonally accelerated snowline retreat, earlier ice exposure, and earlier onset of net ablation in the catchment at a rate of ~5 days per decade. Based on summer air temperatures projected by CMIP6 and the empirical sensitivities of modelled runoff we calculate for 1980–2022, we hypothesise a more than doubling of annual runoff from this catchment by 2080–2100. This result, combined with a decrease in the variability of discharge from glacier ice melt over 1980–2022, suggests that this catchment is unlikely to reach “peak water” (i.e., peak glacier contribution to catchment runoff) this century.