I seem to recall reading somewhere that either the wider variation in water temps or the comparatively lower amount of direct sunlight on mountain streams vs tailwaters is believed to account for why didymo is not generally found in the mountains. I'll have to see if I can find where I read that and post a link.
Also, FWIW, I think I recently ran across a report that said recent research in New Zealand(?) showed improved populations of scud and other small aquatic critters in the didymo infested rivers that might actually lead to improved trout food sources in those rivers. Therefore didymo might not be the harbinger of doom that folks have been worried that it would be. Certainly something that may deserve more research.
Fly fishing - it's cheaper than a bass boat!