If summer flow regimes do indeed trend towards overall decrease in water then salmonids are in deep trouble. I would not be surprised if we see a repeat of the incident that occured on the Klamath. With lower waters rivers will be warmer and the fish of all life stages will be competing for decreased available oxygen. Upstream migrations are likely to be impeded as channels dry up and riffles turn into trickles. If riparian areas are not kept intact, water temperatures can be expected to go higher. Flooding and sedimentation could increase as a result.
The matter of global warming and its effects on salmon are not completely difficult to predict assuming that there is a warming trend on a scale of decades and that this is not just a blip in the overall temperature. Receding glaciers are real evidence that our streams are being supplied with less water during warmer periods of the year. With less snowpack all around the world, the albedo will decrease as the glaciers continue to shrink. What this means is that less incoming solar radiation will be reflected back into the atmosphere. If the solar radiation that does reach earth becomes converted into infrared radiation, the likelyhood of an increase in global temperature is highly probable. However, if there is less water stored on mountains that brings up the question: where? There are two potential locations for that water to be stored, either in the ocean or in the atmosphere as water vapor or clouds. If the case is such that overall cloud cover in the Pacific Northwest goes up, then there exists the possibility that one, more solar radiation could be reflected back into space or two, whatever solar radiation that does get converted into infrared by the earth and reflected away will simply be reflected back at the earth by the clouds. The second situation could increase the temperature for the region overall, but may also keep it at some relative constant. Clouds could be considered the wild card in determining the outcome of the effects of global warming on salmon due to their highly ephemeral and variable nature.