Hey Biggie, I just scoured 4 pages of the Complete Angler's Guide to Oregon (by John Shewey). The Middle Fork, has redband trout to 14" with the best time from June - October. And you find the same fish (and sizes) on the South Fork; with best fishing from May - early July.
The book also suggests to leave these South Fork fish alone, during the hottest part of the summer. Water levels fluctuate greatly, which is used for crop irrigation. Therefore, the water temps are dangerously high. Trout stressed by a fight and then released, may not survive. But, the fishing picks up and gets good again in the Fall.
Hope that this helps.