Abstract
Quantification of climate change impacts on the thermal regimes of rivers in British Columbia (BC) is crucial given their importance to aquatic ecosystems. Using the Air2Stream model, we investigate the impact of both air temperature and streamflow changes on river water temperatures from 1950 to 2015 across BC’s 234,000 km2 Fraser River Basin (FRB). Model results show the FRB’s summer water temperatures rose by nearly 1.0 °C during 1950–2015 with 0.47 °C spread across 17 river sites. For most of these sites, such increases in average summer water temperature have doubled the number of days exceeding 20 °C, the water temperature that, if exceeded, potentially increases the physiological stress of salmon during migration. Furthermore, river sites, especially those in the upper and middle FRB, show significant associations between Pacific Ocean teleconnections and regional water temperatures. A multivariate linear regression analysis reveals that air temperature primarily controls simulated water temperatures in the FRB by capturing ~80% of its explained variance with secondary impacts through river discharge. Given such increases in river water temperature, salmon returning to spawn in the Fraser River and its tributaries are facing continued and increasing physical challenges now and potentially into the future.
Highlights
The Fraser River Basin (FRB), western Canada’s largest Pacific River watershed, spans one-quarter of British Columbia (BC) from its headwaters in the Rocky Mountains to its outlet to the Pacific Ocean at Vancouver[1] (Fig. 1)
Higher root mean square error (RMSE) scores are mainly due to higher day-to-day variability in the observed data, which remains challenging for the Air2Stream model to simulate accurately
This study formulates an important baseline for the historical changes in the simulated water temperatures across the FRB using the Air2Stream model forced with observational datasets
Summary
The FRB, western Canada’s largest Pacific River watershed, spans one-quarter of BC from its headwaters in the Rocky Mountains to its outlet to the Pacific Ocean at Vancouver[1] (Fig. 1). Climate change impacts have been observed across the FRB where observed air temperatures have risen 1.4 °C (1949–2006)[8] and up to 2.3 °C (1970–2009) in parts of the Nechako sub-watershed[9,10]. In response to such warming in the latter half of the 20th century and early 21st century, river flows throughout the FRB show increased variability due to changes in precipitation patterns and phase[8], glacier retreat, along with climatic phenomena such as El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO)[11]. The lack of comprehensive water temperature data limits our understanding of long-term river thermal changes, their predictability and the potential impact on species such as salmon. The Air2Stream model was previously used in several river basins globally[23,24,25,26] including the Mackenzie and Yukon River basins in Canada with an ability to perform better than statistical regression
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