I stand corrected. I've only fished the upper Bow in the summer and forgot all about the hot springs. If I'm in Banff in the winter, I'm skiing. Maybe now though I could do a half and half. I still think down stream of Calgary is your best bet. You can stop by any of Calgary's fly shops and get a Bow River map that shows runs and access points.
Here's a nice Bow River rainbow I landed in the winter downstream of Calgary.