Oregon Steelhead Fly Fishing comments about the delay of metalheads into the Deschutes R.: “I know that ODFW has received a lot of complaints about the water temperatures and they are currently working with PGE and Oregon DEQ to try to resolve the problem.”
“My guess to why the fishing has been so slow is that the water temps in the Deschutes river are really warm. We were getting morning temps between 63 and 67 with evening temps between 67 and 70. The water certainly got warmer towards the mouth. The Columbia is still running at about 64 degrees, thus there is no incentive for fish to move into the Deschutes when they can stay nice and cool in the Columbia. All we can hope for is the Columbia warms up and the Deschutes cools down so the fish start seeking refuge in the Deschutes.” OSFF Blog
SwittersB & Exploring 
Hello AJ….A very complicated issue. To say water temps are influencing the fish to stay in the cooler Columbia R. is only part of the picture. Most probably true and often true in many river systems in Oregon and world wide. The why’s are a mix of natural occurrences, dams and water flows, farming/irrigation, stream side and in stream habitat, timber management, soil conservation. I don’t pretend to know everything about it. You didn’t say whether you fish primarily for steelhead, trout, salmon, bass etc. Water temps influence fish year round. When waters do get over heated, you are wise to fish well oxygenated riffles etc where fish often hold until the cool down at night or perhaps, in some regions, don’t even fish at all as the fish will be too stressed. I would research more on the subject re habitat etc. Cooler temps are coming and September/October will be less crowded and hopefully more alive.
http://www.denverpost.com/sports/ci_15883655 Example of Temps Elsewhere
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