Modeling future Fraser River temperature with a global climate model

Hydrologic conditions in the Fraser River basin, located in British Columbia, Canada, can have a significant impact on the health and annual survival rates of Pacific salmon. In the final stages of their lifecycle, Pacific salmon migrate upstream in the Fraser River (100 to 1100km) to their spawning beds. Adverse hydrologic conditions, including high flow rates and elevated water temperatures, can prevent the salmon from successfully completing their single lifetime opportunity to reproduce, jeopardizing the future generation s of salmon. We have obtained historical river temperature and discharge for the Fraser River, and we use a global climate model to project changes in both to the end of the 21st century. During the 20th century, increasing trends in water temperature have been observed. We start by discussing the increasing trends in monthly river temperatures, and extend these trends to 2100 through the use of a global climate model. We then work with higher resolution temporal river temperature data to do the same. The monthly analysis is expanded through an examination of observed daily trends, and comparing these to the results of a daily model simulation. The model's daily variability is similar to the observed during the summer for the present climate. The model projections for the last two decades of the 21st century indicate that Fraser River temperature will increase by about 2°C. Of particular importance is the number of days per year when temperatures are above the critical biological threshold for salmon migration of 18°C. For the present climate (20th century), there are 11.2 days per year when river temperatures exceed 18°C. The model projects that this will increase 56.5 days per year by the end of the 21st century (2081-2100). The model also projects temperatures to exceed 19°C an average of 30.3 days per year, and temperatures above 20°C will occur 8.25 days per year during the last two decades of the 21st century. These future conditions may pose a serious threat to the health of the species of Pacific salmon which are susceptible to subtle changes in the basin's hydrologic characteristics. This study is a useful starting point in understanding future water resource requirements and overall ecological fitness in the Fraser Basin. The methodology described herein can be employed in the analysis of other large river basins that may be impacted by future climate change.