I'm curious about this: "spring-fed mountain stream"
What do these springs look like? And are the springs right up at the source of the stream, so way up on the mountain?
And are they large, i.e. with a lot of water flowing?
Ultimately, most of PA's mountain freestoners are also spring fed. The springs are small (compared to limestone springs, anyway), they come right out of the ground, and can be anywhere from near the ridge above to right down by the streamside. If you follow any draw up from a stream you'll find a spring at the head of it. Yes, the headwaters of any freestoner are littered with these, most have literally hundreds, and more dump in throughout the length of the stream.
The spring water isn't deep springs, generally it's runoff that flows under the loose rock and topsoil, but on top of the bedrock. So you get a much larger number of smaller springs, with outputs that are more constant than true runoff but still vary with only the last few weeks of rainfall. Whereas with a limestoner, the spring water penetrates the bedrock, leading to much fewer, but larger springs, that generally vary in output based on months of rainfall rather than weeks. Because of the soluble bedrock, limestone springs also have a lot more minerals dissolved in them than freestone springs.
Freestone springs are plenty deep enough to emerge at a nearly constant temperature year round, though, just like limestoners. It's just that because they are numerous and small, the streams they feed grow very gradually. If you get far enough downstream, you are a long ways from the majority of the source springs, and thus the stream is warmer. Since most freestoners in an area grow at a relatively similar pace, any area will generally have a "maximum" size, beyond which gets too warm to support trout year round. That maximum size decreases, obviously, with thermal pollution or being exposed to sunlight.
Throughout the northern tier of PA, if such a spring is near a road, it's commonplace to see a pipe sticking out for people to get drinking water from. My cabin has a nice spring. This is in Forest County, technically in the Tubbs Run drainage. It's on a flat on top of a mountain, and originally was just a little swampy area that rose up out of the ground, which was drained by a ditch along the road, which eventually becomes a small stream and goes into the headwaters of Tubbs. We dug a hole and lined it with rock, then put an overflow on it to the same ditch. Drained the swampy area and left us with a nice covered 3' x 3' x 3' pool, which my family uses to store minnow buckets and the like. We occasionally use it for emergency water too. We have found trout in it. Very small, pale ones. Presumably they got there underground. The water is crystal clear and about 53 degrees 365 days per year and has been tested to be drinking water quality.
Another family member built a small pond by damming such a spring. They stock trout in it, and yes, they live year round with no temperature issues.
To state that freestoners are fed purely by runoff is incorrect. Unless you count small, top of the bedrock, but still underground streams to be "runoff".
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