August on the Juan is great fishing, it's always pretty good. Fish small stuff and fish drys and you'll be fine, show up early to get a good area. If you are unfamiliar, you'll find fish everywhere, catching a good drift can be tricky in a back eddy though. Ask me when it gets closer and I'll give you the scoop on what they are eating. The river just finished run-off and is fishing good. I'm heading up this weekend. Also Check out sanjuanriverforum.com bunch of great guys always updating info on the river
Jeff