Here in Charlotte, NC we have been snowed in with 4 to 5 inches of snow followed by freezing rain. This is a good time to sit at the computer and try to answer a popular question about winter fishing. We often hear the question: “Which is the better fly to use in winter, dry fly or nymph?” Perhaps the question would be better phrased: "when does the water get to cold to catch trout on the dry fly".
That decision can not be made solely on water temperature as there is no magic range for the temperature. However, with some exceptions the general answer is around 50 degrees. Some trout have been caught in the park on a dry adult caddis pattern at the slightly lower temperatures of 42 to 44 degrees. This was after the Little Black Caddisflies began hatching following a day or two of 50 degree water.
Generally speaking, the trout will respond on the surface very well when the water reaches 50 degrees for a day or two. Dry fly fishing can be excellent at this temperature provided hatches are occurring. The trout will usually eat the emerging pupae much more readily but in the example above, they continued to hit the dry flies with the temperature as low as 42 degrees.
However, we generally will start out fishing a nymph midge pattern for example if the water temperature is below 50 degrees. Trout will eat the nymphs readily when the temperature of the water is between 45 and 50 degrees and they can be caught very well in this temperature range provided other things are favorable.