In western Canada, internal natural variability is the dominant source of uncertainty for the climate model projection of precipitation and related variables. Tree-ring records capture this natural variability and also enable testing of the capacity of climate models to simulate significant modes of variability. We compare millennial tree-ring reconstructions of the weekly flows of the Athabasca and North Saskatchewan Rivers to weekly flows from a hydrological model forced with climatology from various runs of CMIP6 Earth System Models. Relative to the proxy streamflow time series, the climate / hydrological models have a high-frequency bias and underestimate decadal scale hydroclimatic variability. Inter-annual to decadal variability represent more of challenge for water resource management than incremental slow-onset changes in water yield resulting from the regional impacts of global climate change.