Fly selection for Steelhead and Salmon is less important than it is for inland trout fishing. Since we are not matching the hatch, you don't really need to be as concerned with what fly you are using. The typical trib fly box holds a set of flies that we’ll try and put into a few different general groups, egg patterns, naturals, streamers and attractor flies. It still makes sense to go by the old adage "Bright day-bright fly, dark day-dark fly", but don't get locked into it. Try out various patterns and sizes. After missing a strike, I like to change to a smaller version of the same fly. If that doesn't bring a strike, I go to a fly that's completely opposite of the original. A tiny polar shrimp would replace a huge egg sucking leach.
Egg patterns are good choices, the fish are spawning in the rivers and they do eat eggs of there own kind and others. An egg pattern can be as simple as a chunk of yarn tied to a hook or as fancy as multiple egg patterns with an egg veil. The fish will often hit egg patterns very lightly, thus the strikes are harder to detect. When I fish with a tandem set of flies, one of them is almost always an egg pattern.
Naturals, hex nymphs, stone fly nymphs, hare's ears, are all good choices. These patterns represent items that the fish see in the rivers and will feed on from time to time.
Streamers, egg sucking leach is an old favorite, any streamer that looks like a bait fish from the Great lakes is also a good bet. Streamers will often receive more violent strikes.
Attractor flies will have some kind of movement in the water or are bright flies designed to get the fish’s attention or tick off. When the fish are spawning they can be territorial, and will hit any thing that comes into their territory. Purple Peril, Popsicle, Spey flies and Bunny Leeches are all good choices