OK, here's some info from research done in New Zealand that helps explain why didymo is more of a problem in tailwaters and large western rivers than in the freestone mountain streams in this area. (Warning - Large PDF file > 1mb) http://www.biosecurity.govt.nz/files...nts-sep-05.pdf
According to the table on page 16, didymo fares best in water ranging in temp between 6 - 20 degrees Celsius (42 - 68 degrees Farenheit). Score one for tailwaters. Freestones generally have wider variation.
Didymo also needs high exposure to sunlight (not shaded). Score another point for the tailwaters vs the mountains.
The steadier, more regular flows of lake-fed and dam-controlled rivers suit didymo better than the variable flows of mountain streams. It also appears that periodic high flows (flooding) help scour the streambed and discourage didymo growth.
And last, but probably not least, the pH levels of the Smokies streams are generally too low to suit didymo. The streams in the Smokies range from 4.8 - 7.4 in pH, with the average at 6.3. According to the above referenced report, didymo does better with more alkaline water averaging around 7.7 pH.
What that all boils down to, at least to me, is that the tailwaters provide an ideal growing environment for the nasty bugger that is rock snot, but the majority of freestone streams in the mountains really aren't that threatened. At least not by didymo. We'll save the discussion of acidity, etc. for another day.
The other report I thought I had seen can be found here: http://www.biosecurity.govt.nz/files...ort-jun-07.pdf
(Warning: another 1mb PDF file). According to what I gathered from a quick skimming, while didymo did lead to an overall increase in invertebrate drift biomass, the average size of the individual bugs was somewhat smaller. Also, there was insufficient data to conclusively prove anything one way or the other and more study will be required (of course).
Anyway, those reports should provide anyone who really wants to numb their brains several hours of light reading before bedtime. Enjoy.