I think the early post is correct. The Chewuch River closes on Aug. 15th, but is currently open up to the 8-Mile Bridge (Closed Waters beyond that). The water is fairly low, but you should be able to find a few pockets here and there that hold 8-10 inch rainbows.
The Methow may be your better bet as there is tons of access along HWY 153 and there is more water.
Please be sure to keep any caught fish in the water as they are released...Both the water and air temps are warm, so there is a potential for high mortality. Also, be sure to check the regs....they get very specific on where and when you can legally fish the streams and rivers.
I have no info on Pearygrin, sorry.