August isn't a bad time to fish in the high country, but it is the monsoon season so keep that in mind. This means it will likely rain every afternoon that you are there though the rain duration can be highly variable. I've spent a week at the end of July in Breck each of the last three years and fishing has been variable each of those years. In 2008 we had great weather with little rain but the fishing was horrible due to a heavy snowpack that year and a late spring/summer which pushed runoff into July. In 2009 the weather was horrible all but the first two days we were there. By the end of the week we'd had snow and temps in the mid-50s to low 40s for highs. Fishing was good early in the week but once the rain and low temps set in it was all over. This year we had pretty good weather all week, but the best was again at the beginning of the week. Just as an example I fished the N Fork of the S Platte river on Monday and the flow was 160 CFS, by Friday the flow was 320 CFS due to the rain.
There is a lot of fishing in the area with most folks focusing on the rivers in Summit county. As a result if you are willing to drive 45 minutes to an hour south into Park County you'll generally find open fishing on quality waters there. There are multiple wildlife management areas along the forks of the S Platte which are open for public fishing so access isn't a issue.
If you can't find someone local to fish with I'd recommend Breckenridge Outfitters. I've fished with them in 2009 and this year and had very enjoyable trips each year. Ned Parker is the owner and he runs a first class operation. Their rates are no higher than anyone else in the area so keep them in mind.
Brian
"My father was very sure about certain matters pertaining to the universe. To him, all good things - trout as well as eternal salvation - come by grace and grace comes by art and art does not come easy." Norman Maclean, A River Runs Through It