Water over 70°F is generally lethal for trout

Trout, and their kin salmon, are coldwater fish.  The US EPA reports 78°F as the lethal threshold for juvenile trout and salmon. Adults are even more sensitive to water temperatures, dying when waters sustain temperatures of 70°F.[[Appendix B in EPA Region 10 Guidance for Pacific Northwest State and Tribal Temperature Water Quality Standards (April 2003).]]

These thermal limits are general benchmarks, as each fish species and life-stage has different tolerances.  An EPA report by Dale McCullough of Columbia River Inter-Tribal Fish Commission discusses temperature thresholds for fish throughout the trout and salmon family.[[McCullough, Dale A., Shelley Spalding, Debra Sturdevant, and Mark Hicks. “Summary of Technical Literature Examining the Physiological Effects of Temperature,” (Abstract) May 2001]]

Since temperatures are rising in general in the American West, and the relatively cold water from melting snow diminishes earlier in the summer, stream water temperatures are likely to increase, presenting widespread threats to trout habitat. (Trout that live in tailwater fisheries may be somewhat protected, however).