The 2021 heatwave results in simultaneous but different hydrological responses over Canada west of 100°W.

• New high air temperature records were established over a large domain over a few days. • Changes to streamflow coincided with the air temperature extremes. • The hydrologic regime determined the type of response to the heatwave. • Hydrological responses were conditioned by the existing snowpack. The 2021 Western North America heatwave resulted in record high air temperatures over a large area of Canada west of 100°. The heatwave persisted from mid-June into July, depending upon the threshold used to define the heatwave. The heatwave was a weather event that was short lived but had a widespread transient impact on hydrology in this single year. These impacts were more evident in nival systems because of the prominent role of snowmelt. In normal years, the timing of hydrological processes, particularly snowmelt, is a function of latitude and elevation; in 2021 the heatwave resulted in simultaneous high rates of snowmelt across rivers where a snowpack existed at the time of the event, and strong diurnal melt signal was a diagnostic. In 2021, rivers throughout Canada west of 100°W responded strongly to the heatwave, but the response depended on three factors: location relative to the heat dome, the hydrologic regime, and the amount of snow present at the time. The melt signal was strongest in those basins that had not reached their annual peak often having discharges in the highest 5 % of historical observations for that day of the year. Discharges rapidly declined in basins already in recession indicating the rapid depletion of the snowpack. In basins with little or no remaining snow, streamflow often declined during the same period often to levels in the lowest 5 % of historical observations for that day of the year. [ABSTRACT FROM AUTHOR]