The streams of the Smokies are naturally more acidic than most Western, Northeastern and Upper Midwestern streams. This does not have as much to do with acid rain as does the parent rock formations the streams flow over. This really becomes evident when a road cut is made through an acid bearing shale formation like the one at Newfound Gap and the Cherohala Skyway. Water trickling over these fresh road cuts significantly drops the pH of nearby streams for years. This is evident in upper Walker Camp prong and McNabb and Hemlock Branches in the CNF. The one outlier stream in the Smokies is Abrams and it flows over and through more of a limestone base.
It is a combination of low pH, low mineral content and low buffering capacity that keep the streams of the Smokies relatively infertile. They are aesthetically pleasing and do hold some gorgeous wild fish, but will never be in the same league as the famous Western and Northern streams unless the fish receive supplemental feedings. There will always be a relatively few really large brown trout in just about any water they inhabit, but they are the exception.