I'd suggest the Upper Colorado.....namely from Pumphouse campground to State Bridge. The water and air temps shouldn't be cold per Colorado standards. August and September are usually warm and nice. The snow hits more toward October. Great camp sites along the river with fire rings, pit toilets, and picnic tables, so you don't need to haul all those things with you. Perfect for an overnight canoe trip.
If you get out here soon enough, you could stiil hit the Ark at around 600csf. It's running around 1000 right now.