Bitter winds and frigid winter temps have driven most anglers off of area rivers. There are always some "hardy" anglers who bundle up, rig up, and stand in ice-cold flowing water for a few hours before coming to their senses and realizing that any fish holding in the pools might be unable to strike due to being frozen to the bottom. Regardless, sub-zero winter weather can keep your favorite pools free of angler pressure, but you must exercise caution when you wade and fish in these conditions.
Extreme cold and water do not mix, for a variety of reasons. First here is the chance of frostbite or freezing of the skin. Exposed skin can be subject to frostbite, and the chances are increased when the skin is wet (from line stripping, landing a fish, etc). Wool or fleece gloves (without additional insulation) insulate even when wet, while synthetic insulations such as Thinsulate are useless once saturated. Your hands are most likely to become wet during regular fly fishing activities - pay attention to how they feel when you are out in the elements. Lucky enough to have hands resistant to cold/wet conditions, I rarely wear any gloves if I can get away with it. However, I always have a pair in my vest just in case.
Wading safety goes without safety here, and there's an added glitch that can come with winter fishing. Shelf ice forms along the shorelines of rivers in the depths of winter, and can be a dangerous obstacle to anglers wading or walking the banks. River flows are slower along the banks and shallow areas, and this slack water can freeze in place if conditions are right. As the ice at the water's edge builds up, it can get thick enough to prevent an angler from getting out of the water, particularly along deeper areas. This can be extremely dangerous for someone who has taken a fall, gotten soaking wet and needs to exit the river immediately.
Shelf ice can also tempt a bank angler with easy access to the water's edge. As it's nearly impossible to determine whether or not shelf ice is thick enough to be safe, bank anglers are better off fishing from the exposed bank, rather than a shelf of ice extending over moving water. Slick, unsure footing next to a strong current is a recipe for a cold swim or something worse. Winter weather can completely freeze the surface of a river, making it possible to cross over the ice from side to side. As the water below is still moving, this is a situation to avoid at all costs.
Whether you're wading during the wintertime, or walking the banks of your favorite river, the bitter weather needs to be respected. Between the chances of frostbite, and the possibility of encountering shelf ice along the river, hazardous conditions can be everywhere - be careful when you are enjoying the great outdoors. Tight lines!