Rising stream and river temperatures pose problems for salmon. Salmon favor cool water - the maximum temperature most can tolerate is in the range of 68 to 75 degrees fahrenheit. Recent climate models predict that stream temperatures will rise in many locations over the 21st century. As stream temperatures increase, up to 50% of existing salmon habitat in the United States will become too warm. As the fish become increasingly restricted to colder streams, their populations will be more fragmented and more isolated, since the populations will no longer be able to intermingle in the warmer water downstream.
Climate change  also threatens to accelerate the spread of salmon diseases. Whirling disease, which is found in mountain streams, damages the brains and spinal cords of young fish. A marine disease called ichthyophoniasis, or "ick" for short, infects the heart and other vital organs. Warmer waters seem to increase both the generation of these organisms, and the damage they do when they infect a host.
Damming of rivers also poses a threat to salmon, as they swim upriver to get to spawning areas, and are unable to proceed further when they reach a dam.