Fisrtly, having read your posts on invasive species and their control, I doubt that your questions will be silly.
I would think that in most rivers they tend to move, as you suggest, to slower, deeper water, but remember that their appetite falls off as the water cools. That slower water might just be a pocket behind a streambed obstruction, such as a rock ( or a supermarket trolley in my local rivers). Odd ones still get caught in conditions cold enough to give you that snow + barbel photo, but in general winter barbel are like winter carp, slow to feed, and ignored by many. Fish that are not feeding frequently may actually need to find quieter areas of the river to conserve energy, but of course fish can go for long periods without food too. The barbel is an expert at spreading out its fins and just clinging effortlessly to the bottom without further fin waving. Obviously when not too many are being caught it can be harder to identify where they choose to lie, but sometimes a flood will bring both a rise in temperature and a chance to localise the barbel.
Don't let me put you off winter barbel, but it is not the best time to get started on the species.
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