The summer Stonefly hatch is beginning and large dark colored stonefly nymphs can be very effective in the swift water. The trout do get smart however! They only get fooled a couple of times on these big bugs before they get cautious. Fish early in the morning at dawn, or late into the evening with dry flies like Chubby Chernobyls, Fatboys, Gypsy Kings, and other large dry flies.
The Caddis hatches are slowing down, but will be prevalent in the evenings at dusk. If targeting the Caddis hatch, be sure to be in a great spot at dusk and look for feeding fish.
The Mayfly hatches are pretty much done until fall and mid day fishing will be primarily nymphing now that the PMD hatch is pretty much wrapped up for the year. There are few big yellow mayflies hatching in the evenings, so if you see this bug and trout demonstrating a gentle rise you can almost count on the fact they are eating a mayfly. Use a Rainy's Everything Dry for that situation.
Strategies for Fishing the Yakima Canyon
Wade fishing can be challenging when the river is over 2,500 cfs so boating can be an advantage in higher flows. If you are wading, Euro style nymphing behind large boulders can be very effective. Dry dropper setups work well as the hatches begin mid morning on warmer days, on colder days a stonefly nymph/dropper combo is a great prospecting setup. As fish begin to feed on emerging caddis use a light rod with Caddis or PMD emerger patterns tied on a 5X tippet. Whenever possible try to target a single rising fish, and don't flock shoot.