It amazes me that we still have salmon and trout in some of the places on the east side. It's a testimony to their ability to survive. But ultimately they will be gone if we don't do something. High stream temperatures are sometimes lethal for fish. Warm water handicaps their ability to maintain themselves.
Temperature regulates the rates of process in biology.
The east side has open country interspersed with forest, more sunny days and relatively hot stream and river temperatures in basins such as the John Day and Grande Ronde. They probably did. But there was generally more plant cover along steams and their channels were often narrower and more sinuous. There were no roads or ditches blocking the flow of cooler water underneath flood plains into the river. There were probably more cool places for fish to hide.