background: when it comes to acid rain effects, we have everything in PA from limestone valley streams where the limestone bedrock buffers away any acid rain effect, to some mountain tribs that flow through low buffering bedrocks and have no trout at all.
bedrock geology might matter to trout populations in small headwaters mountain streams (down in valleys the buffering is often higher.)
if the bedrock geology that a mountain trib has flowed through has very low buffering capacity, there may even be zero trout due to acid rain. this can happen for example with Pottsville bedrock which covers 15% of PA, and exists in both eastern and western PA (there is a large area of Pottsville bedrock in NWPA).
of course habitat, pools, water temp, not drying out, etc. have to be considered... as I can say from first hand fishing experience, a dried out streambed in high buffering bedrock does not fish well
interestingly, lower and higher buffering bedrocks can be right next to each other. in Schuylkill, it could be Pottsville bedrock (lower buffering) next to Mauch Chunk bedrock (higher). in Sullivan, it could be Burgoon bedrock (lower buffering) next to Huntley bedrock (higher). In Centre, it could be Tuscarora bedrock (lower buffering) next to Juniata bedrock (higher).
so you might check whether a mountain trib has flowed through a bedrock area with some buffering above where you fish it.
this map can show you the bedrocks. hit geology tab upper right.
http://www.gis.dcnr.state.pa.us/maps/index.html
actually using bedrock info isn't too bad in NEPA, SEPA, NCPA, and SWPA, because there are fewer bedrock types in the mountain tribs. but in SCPA or around centre county there are more bedrock types and more changes (see map linked above).
here are some bedrocks with my thought on how I would look at a mountain trib section that has mostly flowed through them... this wont begin to cover all the bedrocks out there, but there are some common ones mentioned, particularly in NEPA, NCPA, & SWPA mountain areas:
what to look for:
bedrocks that are a bad sign for trout: Pottsville, Allegeheny, Llewlyn, Burgoon, Tuscarora. Montalto, and other bedrocks mentioning quartz when you click the bedrock color in linked map above. also mentions of coal in the descriptions of any bedrock in map linked above (thanks Pat).
moderate: Pocono, Chemung, Reedsville, Coburn, Clinton, Lock Haven, Hamilton, Bloomberg & Mifflin, Mahantango, Trimmers, Bald Eagle, Spechty Kopf.
nice: Mauch Chunk, Huntley, any of Catskill bedrocks, Shawangunk, Juniata. mentions of dolomite or limestone if you click the bedrock color in linked map above.
btw, if a stream emerges from swamps, it may have natural acid from plant decay so it may need more buffering...
