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Variability and trends in runoff in the rivers of British Columbia's Coast and Insular Mountains

This study examines the 1914–2015 runoff trends and variability for 136 rivers draining British Columbia's Coast and Insular Mountains. Rivers are partitioned into eastward and westward flowing rivers based on flow direction from the Coast Mountains. Thus, eastward and westward runoff trends and influence of topography on runoff are explored. Our findings indicate that rivers flowing eastward to the Nechako and Chilcotin plateaus contribute the lowest annual runoff compared to westward rivers where runoff is high. Low interannual runoff variability is evident in westward rivers and their alpine watersheds, whereas eastward rivers exhibit high interannual runoff variability. On Vancouver Island, some of the rivers with the highest annual runoff exhibit high interannual variability. A significant (p < .05) negative correlation exists between mean annual runoff (Rm) and latitude, gauged area, mean elevation, and its corresponding coefficient of variation. However, a significant positive correlation was found between the glacierized area of mountainous regions and Rm. The mean coefficient of variation in annual runoff is significantly negatively correlated with latitude and glacierized area, but significantly positively correlated with longitude. Annual and seasonal runoff trend analyses of each river were performed for an early (1936–2015), a middle (1966–2015), and a late (1986–2015) period using the Mann–Kendall test. Trend analyses revealed a shift towards more positive detectable (signal‐to‐noise ratio > 1) trends in annual and seasonal runoff from the middle to the late period across the study domain. Most positive detectable seasonal runoff trends in the middle period occur in spring in glacierized westward rivers located >1,200 m, whereas in the late period, they all occur in fall and are regionally coherent around Vancouver Island and south coastal BC. Rivers draining eastward exhibit more positive trends over 1986–2015 compared to westward rivers. This study provides crucial information on the hydrology of mountain watersheds across British Columbia's coast in response to Pacific Decadal Oscillation phase changes, the elevational amplification of regional climate change, and their influences on precipitation and glacier retreat.