Salmonflies, most likely. That’s what I’d call the bug in @SquatchinSince86 ’s pic above, anyway.
Issaquah Creek is a good sized creek. Water quality impacts aside, I’m less surprised to see stoneflies there than at my house. The only moving water near me is really small and doesn’t have the substrate I typically associate with stoneflies. But, it must be the source of these bugs. There just aren’t any other options. Prehistoric survivors, they are.