The Yakima River runs very high during June - August, like super high and swift. This makes for GREAT boat fishing and hopper fishing up against the shore. Late in the summer however, typically around September 1st the river drops fast and the river gets very clear and low. The trout are hungry and condensed to smaller spaces. The fishing can range from epic to dismal. The trout are often selective after the water drops to below 2,000 cfs. The GOOD NEWS is that the wading and DIY rowing gets much easier than it was.
Lighter tippets, smaller flies, and a stealthy approach is critical during this time. Look for hoppers, stoneflies, and big October Caddis to be effective in the evening hours with small baetis nymphs, mayfly nymphs, and other general nymphs in the #16 - #20 range to be effective during the day.
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.