On the subject of fish over wintering in streams, in the last place we lived we had what is termed as a "Riparian" area behind our house, which had a heavy flowing stream that ran through it and right past our house when we moved in. Over the years the stream slowly dried up until the water no longer ran past our house, but it continued to run about 100 yards upstream and just disappeared into the ground. When it was flowing strong there was a decent population of brook trout in there and local kids use to go fishing and catch them. On day, after the stream had stopped flowing past our house, my boys and I were exploring the flowing water upstream and noticed an area in the stream where there was a sort of under cut cave in the stream that contain about 1000 gallons of water. While we checking it out we noticed some trout in there. They were essentially trapped as the water flowed down like a waterfall into the cave area, and disappeared down into the ground only a few yards down stream. For the next few years we would regularly visit the trout and see how they were doing. Fortunately the water flow seemed steady up to this cave area and didn't dry up, but we noticed the fish were getting thinner, clearly not getting enough food. So on subsequent trips we always brought fish food and worms with us to feed the trout and actually were able to observe them gain weight. When we first observed them they hid from us the best they could near the back of the cave, but after regularly feeding them they seemed to anticipate getting fed and swam excitedly around the pond out in the open.
This area was deep in the riparian undergrowth and obviously somewhat protected from the cold because the pond area in the cave never froze over even in the coldest part of the winter.
We eventually moved from that place, so I don't know their fate, but it was a nice experience to share with my young sons.