Don't put the fly rod up for winter! Fish activity does dwindle a lot during winter but they of course do not Hibernate. Those cold streams with snow on the banks and yes occasional ice and slush floating on top do make it look impossible to net a fish from. However, winter fishing can be quite productive if you understand how fish activity changes, know where to look, and of course have the right flies. You may even find a hatch in the right conditions!
Fish activity changes a lot during winter. Their food supply has decreased, they are worried about predators and shelter more than feeding in the winter, and they won't expend a lot of energy. Explosive takes will be fewer than during the warmer months. Fish won't be everywhere like they were when it was warmer. They will congregate in pools with a slow moving current and deeper water. They will be deep and they won't be in stagnant water because they need a current to bring their food supply to them. They want to conserve as much energy as possible and look for areas with low turbulent. They won't chase food like would in August and wait for food to come to them. Steamer fishing will take more skill than in summer.
Flies are smaller in the winter, don't be afraid of midges in the 22 to 24 range. Big fish will eat tiny flies! TrueFly Supply's Guides do a fantastic job putting together our winter selections. Watch for hatches on the warmer afternoons and look for rising fish in the middle of those slower currents. Springs flowing into rivers are sometimes warmer and fish find them. Look for hatches there as well.
So to sum it up, fish are lazy, stay deep, and the bugs are smaller! Look for warmer afternoons, slower currents, and use small flies. Don't get discouraged by the snow and put your fly rod up. Keep it ready and those fly boxes organized.