Katherine M Robinson , Gwenn Elizabeth Flowers, Michel Baraër, David Robert Rounce

The highly-glacierized headwaters of the Kaskawulsh River are home to 9% of all glacier ice in Yukon, Canada, have been losing glacier mass at regionally representative rates, and were the source of a sudden meltwater-rerouting event in 2016 that has had significant downstream consequences. We use an enhanced temperature-index melt model driven by downscaled and bias-corrected climate reanalysis data 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) and a mean annual discharge of ~60 m3/s, 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. Catchment-wide discharge increased by 3.90 m3/s per decade from 1980-2022, with statistically significant contributions from glacier ice melt (2.80 m3/s per decade) and rainfall (0.47 m3/s per decade). Rising 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 hypothesize 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.