I don't know what you are reffering to as spring and summer fish, but I will just offer what I know since I do alot of research on steelhead behavior.
First of all summer run fish do not spawn in the summer months, they spawn in early to mid winter, usually December-February. When skamania were planted years ago, the fish stock consisted of three fish, two females over 32" and one male over 36" that would be the broad stock of the entire species here in the GL region. One female was an early runner(june-mid july), the other a later runner(later july - september). These fish enter the rivers not to spawn, but to "holdover", feed and mainly because gentics were altered to create this fishery. Therfore saying this you are correct about skamania entering the rivers during a spate, cooler water flows, etc. but just as easily they will slip out to the lake when conditions are less than ideal(ex: water temp, condtions, rising or falling baro, etc). I have learned this from expirence from GL surf flyfishing, where I walk the beaches looking for mouths that can be full or absent of fish depending on many factors.
If you were speaking about the spring run, you may be correct on smaller tribs, but usually on larger river systems you can expect fish to stage longer since the river would usually be longer. Even if we did have rain for a straight week or two weeks, you would still probably have fish staging when conditions are back to ideal.