The best correlation is with the bedrock geology. I've dug into this quite a lot.
Agree, but a handheld gps unit or your everyday topo map doesn't tell you what bedrock is underneath. It does give you your elevation, and a map showing the elevation of the streambeds around you.
Reading the study in more detail now. This was all done in the Brokenstraw, French Creek, and Oil drainages, so all west of the Allegheny River in a region where the geology and typical stream gradient is much more favorable to brown trout. They don't name the streams. They are attributing it to stocked trout having access, but I am betting it's more likely to be wild brown trout populations. These are streams that "want" to be dominated by brown trout, the type seeded 100 years ago, not from modern trucks, so if those fish had or have access, they flip to brown trout streams pretty easily. Hence a barrier, most often a long standing dam, is almost required to stay a brookie stream. There just aren't very many brookie streams in those drainages, and yeah, the few I know of are above impoundments. That they found so many, when there aren't that many, gets me scratching my head a bit, they gotta be studying some really tiny little side trickles. Like take 1 brookie stream above an impoundment, and use it and 10 hillside springs that feed it, call them tribs, and say there's 11 brookie streams with a barrier.
But areas like that is where "allopatric" is probably more likely, IMO. I think, statewide, barriers have an extreme correlation with true allopatry (0% browns). Extreme.
From a conservation standpoint, I certainly see value in protecting existing brook trout streams "that want to be brown trout streams". But honestly, long term, I might see less value in them than in all the streams that are 99% brookies despite the presence of browns. I'm sure that statement will be taken out of context, as if I don't care about the ones that are in the most danger, and that's not true. There's value in all of them and we should try to protect the ones in the most danger. It's more a statement to my faith in long term success. Which one will still have brookies in another 100 years? If a stream is just itching to switch to browns, it won't take much to do it. Some kid throwing a couple over the barrier, a flood, etc. Yes, we gotta fight for them, absolutely, but I fear we're fighting a losing battle. The ones that are all isolated in a brown trout watershed, with geology and water chemistry that make them want to be brown trout streams, no genetic exchange with other tribs. That's where it's 4th and long.
But the question was asked if, because browns have found their way to most brookie streams, if we should punt, and there's a sense that the conservation community tends to ignore them in favor of "allopatric". Heck no! The streams in a system where there are numerous brook trout tribs, and most of them have a token population of browns but remain brook trout streams anyway. Those are your gold. Where brook trout are going head to head against browns and winning, streams that WANT to be brook trout streams. And this is not uncommon at all, there are 1000's of them, many of them very healthy. They're the ones where if you protect it, keep siltation and land use issues and such from ruining things, they are the ones that are most likely to still be brook trout streams 100 years from now.