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As a result of global climate change, glacial melt occurs worldwide. Major impacts are expected on the dynamics of aquifers and rivers in and downstream of mountain ranges. This study aims at quantifying the melt water input fluxes into the watersheds draining the Canadian Rocky Mountains and improving our knowledge about the fate of meltwater within the hydrological cycle. To this end, we use (1) time-variable gravity data from GRACE satellites that are decomposed into water storage compartments; (2) an ensemble of glacier information: in situ observations, geodetic measurements, and a mass balance model; and (3) in situ surface water and groundwater level observations. The glacier mass balance model estimates a total ice mass change of similar to 43 Gt for the period 2002-2015, corresponding to an average of -3,056 (+/- 2,275) MCM/yr (million cubic meters per year). 78% of the meltwater total flows west of the continental divide (to the Pacific Ocean), while 22% flows east of the continental divide (to the Arctic Ocean and Hudson Bay). However, the GRACE-derived total water storage increases, suggesting that groundwater storage compensates for the glacial melt with an increase of 3,976 (+/- 2,819) MCM/yr. A plausible explanation is that meltwater is not immediately flowing down in rivers but rather stored locally in aquifers. This hypothesis is supported by in situ river base flow observations, showing base flow increase in basins draining the ice melt, mostly west of the continental divide. Direct in situ evidences such as well water level time series are not sufficiently available to fully support this hypothesis.

Plain Language Summary This study discusses glacial melt and its impacts on water resources in the Canadian Rocky Mountains. First, we quantify glacial melt inflows into the hydrological cycle flowing to the draining watersheds on both sides of the mountain range. Our melt estimation is in good agreement with previous studies. Second, we explore the fate of glacial meltwater, and in particular the changes occurring in aquifers, by comparing our melt estimates with other data sets such as geodetic gravity field time series and hydrometric data. While glacial mass change modeling estimates a relatively high mass loss for 2002-2015, geodetic observations show that groundwater storage has increased during the same period. Decreasing glacial mass is compensated by increasing groundwater mass in the total mass change derived from geodetic observation, suggesting water transfers from melting glaciers to aquifers. Field measurements support the hypothesis of a significant groundwater storage increase, but not enough field data are available to precisely and independently quantify this rise.