Abstract Seasonality strongly influences hydrological and chemical transport in permafrost‐underlain mountain catchments. In spring, snowmelt delivers large volumes of water, but frozen ground limits infiltration, causing shallow flow pathways to quickly route water to streams. As thaw progresses, storage capacity increases, flow paths deepen, and previously frozen water becomes mobile. Changing storage capacity and activation of deeper flow paths can alter the degree of storage turnover and transit time distributions of outgoing fluxes. Here we characterize the storage and release of water in two headwater catchments underlain by continuous permafrost located in Tombstone Territorial Park in Yukon, Canada. Our objectives were to: (a) evaluate the rate, timing, and magnitude of all hydrological fluxes, (b) utilize Bayesian mixing analysis to partition runoff into rain and snow contributions, and (c) apply the StorAge Selection (SAS) framework to characterize water age dynamics in both catchments. Results show ∼400 mm of precipitation entered the basins, ∼45% as snow, which melted over 4 weeks. Evapotranspiration (ET) was roughly equal to discharge, increasing throughout the summer. Mixing results suggest nearly all (>90%) of runoff during freshet was snow water in both catchments, indicating limited mixing with old water. In contrast, most of the rain left the basins as ET. The water balance and SAS framework highlight significant contributions from melting ground ice post‐freshet. Additionally, high flows resulted in a more uniform SAS function, indicating greater mixing of storage. ET was comprised of mainly young water, likely due to the high field capacity of organic soils and the shallow rooting of tundra vegetation.
Key Points Rapid transmission of snow water to the stream in permafrost catchments due to limited mixing with immobile (frozen) storage during freshet Ground ice is a significant source of water for runoff post freshet Increasingly uniform SAS function for discharge during higher flows, indicating mixing in storage during wet conditions