All the Furnace Hills streams are in a down cycle from my experiences. IMO it has to do with natural sedimentation from the red sandstone that makes up their watersheds. It’s pretty, and has some decent buffering qualities, but it breaks down into well, wait for it, red sand.
All of those streams got a good flushing out after the flood of 2011. Holes that were a foot or 18 inches deep tripled in depth. All the red sand got pushed out of the stream channel into huge sand bars on the banks. The fish populations responded, and I had very very good fishing on those streams from about 2013 through maybe 2017 or so. Hammer, Shearers, Segloch, Furnace, Walnut, all of them.
But, slowly and surely the holes have refilled with sand. There’s spots on Segloch where there is literally 18 inches or more of sand in the bottom of holes before you’d hit bedrock. I know this from seeing and fishing those same holes after the floods. I was amazed at how deep they were.
The geology of all those streams is the same (Hammer has limestone in its headwaters, but is mostly sandstone once you get down into the SGL), and the same thing is happening now on all of them. This is their natural geology, and I’m sure they’ve gone through this cycle numerous times before. I still fish them, mostly because they’re the closest reliable wild Trout streams to where I live - maybe 40 minutes or so, but they probably won’t fish all that well until they get scoured out good again. The sand just kills the habitat. I was amazed at how good the habitat became after the 2011 floods. Before, and again now, it’s just meh. And “meh” is about how they’ve fished for me for the last 5 years or so. There’s still fish there, but none of them are fishing great at the moment. It’s not just Hammer, and there’s nothing “wrong” with them per se, it’s just their natural geology and its impact on fish habitat.