Modeling changes in summer temperature of the Fraser River during the next century

SummaryThe Fraser River basin in British Columbia has significant environmental, economic and cultural importance. Healthy river conditions through sufficient flows and optimal temperatures are of paramount importance for the survival of Pacific salmon, which migrate upriver toward the headwaters to spawn near the end of their lives. Trends have been detected which indicate that the annual flow and summer temperature have been increasing since the middle of the last century. In this study we examine the observed trend in summer temperature of the Fraser River and compare it with temperatures calculated as part of a global climate model (GCM) simulation in which atmospheric greenhouse gases are increasing. We then use the GCM to consider how these trends might continue through the present century. Both the observations and model indicate that during the last half of the 20th century, the summer temperature near the river mouth has been increasing at a rate of approximately 0.12 °C per decade in August. In this study we use an online method in which river temperatures are calculated directly as part of a GCM simulation and project how summer temperature near the mouth of the Fraser River might change by the end of the present century. The results indicate that between 2000 and 2100 river temperatures will increase in all summer months with a maximum increase of 0.14 °C per decade in August. This result is consistent with an offline modeling study by [Morrison, J., Quick, M.C., Goreman, M.G.G. 2002. Climate change in the Fraser River watershed: flow and temperature projections. Journal of Hydrology, 263, 230–244] in which they used output from two GCMS to drive a hydrologic model and predict future changes in river temperature and supports their contention that the timing and magnitude of the increase could be crucial for salmon migration. Future work can extend this analysis to other river systems in an effort to project the potential effects of climate change on the behavior of the world’s large river basins, as well as identify the potential biological effects that may accompany these changes.