Flows can fluctuate in May. High but steady or decreasing flows are fishable, but once you get above 6500 cfs or so (Madras gage), it's hard to find good spots to fish. When flows are high, you should look for the same TYPE of water that you usually fish, but it may be in a different place. And there won't be as many places to fish as there are at lower flows.
Mahogany duns might last until mid-month. The blue-winged olives are on the way out, too, but you can encounter important hatches of them for the first part of the month. Between these two insects, you might have hatching bugs from noon to 3:00 p.m.
The big stories for the month are the stonefly hatches. The salmonflies and golden stoneflies will dominate the river until mid-June. Salmonflies are predominant in the upper portions of the river, while golden stoneflies are more prevalent in the lower miles. The two species meet in the Maupin area, where you can find plenty of both.
Of these two big insects, salmonflies are the first to become active. As the water warms up, nymphs begin to crawl about. In the process they get knocked loose and drift in the current, where trout pick them off. About the third week of May, the salmonflies will crawl to shore and hatch on land. Adults are often blown or stumble onto the water and are sucked down by waiting trout.
The hatch starts in the river's lower reaches because the water has had more time to warm up. Then it progresses upriver until it arrives in the Warm Springs area. Many years, the hatch near Warm Springs begins a week or more after the first salmonflies appear in Maupin.
Stonefly nymphs such as Kaufmanns Stoneflies, Rubber Legs, etc. will do well all month. Your imitation needs to be on the bottom. Dead drift it through rocky areas, along current seams, and through slower, deeper water just below riffles. Use tight line or trout indicator tactics. Avoid spawning area; stay off gravelly flats, because that's were the spawners are.
Team your stonefly nymph with a smaller wet fly, such as a beadhead Prince (size 12-14), Zug Bug (size 14), Green Rock Worm (size 12-14), Pheasant Tail (size 16-18), or Sparkle Larva (size 12-14).
Golden stoneflies have a life history similar to that for salmonflies, but their hatch cycle starts a couple of weeks later. The very bottom part of the Deschutes, near the mouth, has golden stoneflies that usually start to hatch mid-May because the water is warmest down there (there aren't many salmonflies in that part of the river). The lower river isn't great stonefly habitat, but you'll find fewer people fishing for trout.
The closer to you get to Warm Springs, the fewer golden stones you'll see. The closer you get to the mouth, the fewer salmonflies you'll see. Trout usually prefer the goldens to the salmonflies when both are available.
I've tried a number of salmonfly patterns, and in my opinion it's hard to beat a Clarks Stonefly. The Clarks Stonefly works well on the soft bankwater, lands gently on the water, casts more accurately than other stonefly patterns, and--due to its light weight--lets you use a finer tippet. It's also a really easy fly to tie. For some other good patterns, see Graduate School Stoneflies.
When casting salmonfly patterns, remember that most of the action is near the bank. Drift your fly through quiet water that is near overhanging vegetation; or water that is two-four feet deep and flowing over boulders; or pocket water just behind boulders; or current seams where fast water meets slow; or drop-offs below riffles; or riffly water near deeper water.
In the hatch's late stages, you'll probably do best with low-riding patterns. Take a look at the Titanic Stone for a sunken-adult pattern. This can be very effective at the end of the salmonfly/golden stonefly cycle.
Green caddis and spotted caddis hatches will also be going on, so don't just get stuck in a stonefly frame of mind. See the Rivers in General section for caddis tactics and patterns.
Near the end of May the pale morning duns may be hatching. At the beginning of May you may see some remnant March browns.
All month the Deschutes will be full of salmon and steelhead smolts. They are idiots and will take any surface or near-surface fly you toss at them. The risk of injury to these fish is great. Don't target them, and don't fish a dry fly, Soft Hackle, or emerger in backeddies unless you know you are casting to mature trout, not smolts. If you're hooking 5-9 inch fish, they're probably smolts; move on.
For more on May tactics and flies, see the Rivers in General forecast