i think the toutle may be an example of the survival mechanism involved in straying in wild fish....could be that wild fish of toutle stock strayed to other rivers and returned to spawn there or recolonized it once some areas of the toutle became good enough to use again...
straying can protect against the loss of an entire stock due to some catastrophic event in the home stream...if there were no straying, areas that lack spawners due to poor conditions or restricted access won't become recolonized if conditions improve...