Century‐Scale Shifts in Flow Dynamics of a Canadian Rockies Headwater Watershed Under a Changing Climate

ABSTRACT Mountainous regions play a pivotal role in global freshwater supply. Changes in river flow dynamics in upstream reaches can have significant consequences for water availability and management. This paper examines changes in flow seasonality and hydrological extremes in the upstream reach of the Bow River, located in the eastern slopes of the Canadian Rocky Mountains—a region that substantially contributes to downstream river flow. Using a century‐long hydroclimatological dataset (1923–2022), we applied trend analysis, correlation analysis, apportionment entropy, circular statistics and wavelet coherence to explore temporal variations in flow seasonality, the magnitude and timing of high and low flows, and their associations with climate variables. We defined the low‐flow period using the change points detected on the rising and falling limbs of the annual hydrograph. Results show a weakening of flow seasonality (at the monthly scale), characterized by increased flow during the overwinter season and decreased flow during the open‐water season. Despite these seasonal shifts, no significant trend was detected in mean annual flow at the headwater stations. These seasonal changes coincide with rising temperatures, decreasing snowfall, increasing rainfall during the overwinter season and declining snow water equivalent (SWE), accompanied by earlier melt termination and reduced melt duration at the upper stations. The increase in overwinter flow is also consistent with enhanced groundwater contributions, which sustain the river flow during the season. Hydrological extremes also exhibit non‐stationary behaviour: low flows increased consistently across the watershed, while high flows (≥ 90th percentile) declined at lower elevations. High flows, particularly at the upper stations, show a stronger association with snow‐related metrics (i.e., annual maximum SWE and melt intensity) than with antecedent precipitation inputs. The timing of both high and low flows also shifted earlier in parts of the headwater watershed. Wavelet analysis confirms strong coherence between flow and climate variables, especially at an approximate annual cycle, with temperature showing more consistent and pronounced associations with flow than precipitation and large‐scale climate indices. Overall, the findings demonstrate climate‐driven hydrological changes in the headwater watershed of the Bow River over the past century, underscoring the need for adaptive water management strategies under future climate scenarios.