Well, the last weekend of April was AWESOME last year. Of course, streams were low and hatches were very early. In most years, it's blown out as often as it's fishable in late April/early May.
But as far as hatches:
Waning: Grannoms
Peak: Hendricksons
Could be starting: March Browns
Those are the big ones. Other possibilities include green caddis, quill gordon's, blue quills, various midges, etc. And there's always some stonefly nymphs around. If things are warm between now and then, it wouldn't be impossible to see sulphers starting up. But it's early for that.
It's big water. There's big fish if you hit it right. Some of the better pocket water is deep and swift and hard to fish. The pools can fish alright, or be tough. Fish are spoiled and turn on and off like a switch was thrown, you gotta figure out their schedule and meet them on it. And they can rise which makes it obvious, but often the feed underneath even when flies are on the surface, so it's hard to see when that switch was thrown. Be willing to walk. Pools are LOOONGGGGG, and moving up or down just a pool or two can be a bit of a hike.
Be careful about wading, there are lots of places where it's tough, and a big stream with heavy current doesn't help matters. It can also be difficult to cross, learn the crossing areas.