The global hydrologic cycle is a complex physical system connecting ocean, land and atmosphere, and rivers are the conduit which connect all of these media. There have been several studies on the impact of climate change on river flow, but relatively few long term studies focusing on the impact of climate change on river temperature. This dissertation examines the potential impact of climate change on river temperatures for mid and high latitude Northern Hemisphere locations. Present trends in temperature of the Fraser and Lena Rivers are extended by using a global climate model to project how river temperatures in the basins might change by the year 2100. During the second half of the 20th century, observations indicate that river temperatures in the Fraser River are increasing and extreme temperatures are more frequent. This can negatively affect the reproductive fitness of Pacific salmon during their upstream migration to their spawning beds. The model projects that the observed warming trends will continue to 2100 and that the frequency of extremes, particularly temperatures above 18°C, will increase the risk to the salmon population. During mid summer, the model projects that the frequency of days with temperatures above 18°C will increase from 3.8 days per month now to 21 days per month by 2100. For the Lena River, the model projects that river temperatures will increase during the summer by 2.0°C--3.0°C by 2100, with the largest increase (approximately 4.5°C) at the mouth of the basin in late September. There are also changes in the timing of the peak summer river discharge which occurs earlier in the spring. This study is a useful starting point in understanding future water resource requirements and overall ecological fitness in the Fraser and Lena basins.