I'm thinking of a spot I know in SWWI - fairly well known. Small creek dumps into a big pool that I would guess was caused by the bridge that crosses the river at this point.

You can see dozens (hundreds?) of fish from the bridge just downstream of the bridge in the pool.

I'm betting this is a similar situation?

I imagine there's food coming down from the running water but the pool has nearly no discernible current. It's like a pond. And trout aren't territorial in stillwater. They'll congregate.

I've had good success on dries casting down stream from upstream of the bridge. The fish will come up under the bridge in the shade.

At this time of year, try a beetle or hopper. Cast it out and let it sit. If nothing comes up after a bit, try to pop it a little like fishing for gills on a lake.