Courtesy of "The Perfect Fly"
https://perfectflystore.com/fly-fishing-cold-water-for-trout/
When the weather and water turns cold, the first question we usually get from anglers is when they can they use dry flies and when should they use sub-surface flies such as nymphs. The first warning I could give is that making this decision solely based on water temperature could be a big mistake. There are times when you may want to fish a nymph when the water temperature is 65 degrees F. and higher as well as times when it is 40 degrees F. There is no magic range for either the dry fly or one designed for subsurface presentations.
A better question may have been “when does the water get too cold to catch trout on the dry fly”. With some exceptions I won’t get into, the general answer is around 45 degrees F. I have caught trout on dry flies at slightly lower temperatures ranges from 40 to 44 degrees F. This sometimes happens after an insect begins hatching following a day or two of 50 degrees or warmer water. If there is a lot of food on the surface the trout will usually eat it. By the way, when this happens, trout will usually eat the emerging pupae and nymphs much more readily.
During the spring months, trout will usually respond on the surface very well when the water reaches 50 degrees F. for a day or two. Dry fly fishing can even be excellent at this temperature provided hatches are occurring. Angie and I generally would start out fishing a nymph if the water temperature was below 50 degrees and not expected to rise to that temperature. Trout will eat the nymphs and pupae readily when the water is between the 45 to 50 degrees F range. You can catch them very well in this range provided other conditions are favorable. If you find the fish fairly concentrated, you can sometimes catch a lot of them when it is between 40 and 45 degrees F.
The next subject I will address pertains to where to find trout in cold water. More specifically on the bottom, mid-range, or surface. There’s no good answer to this question. The trout may be at either of those water levels depending on many factors. The water temperature is not one of them.
When trout are inactive, they tend to stay on or near the bottom. This also gives them some protection from overhead predators. You may also find them in very shallow water and of course, suspended between the bottom and surface. The water temperature is not the factor that controls this. Remember, the fish are perfectly comfortable anywhere in the stream, as far as the temperature is concerned. The main thing to look for is slow to still water. Trout want tend to hold for long periods of time in fast, very cold water because they will expend more energy than they can acquire food to replenish.
Now the problem with this is that you may be looking at a fast run, for example, with the surface water moving very fast, when down near the bottom, between and behind rocks, or within holes in the bottom, the water may be moving very slowly. In some situations, it may move very little. You cannot always go by what you see on the surface. For another example, water near the banks may be swift. However, there is usually some pockets along the bank where it is moving slowly. In eddies, water can even be practically still. Just keep in mind, moving water can be deceptive. Trout do not need depth from a temperature standpoint. It would be a rare situation in a trout stream for the water to get warmer deeper. Trout streams are generally not that deep. Even if they were, keep in mind, trout do not move to warmer water for comfort. They will feed more in warmer water but they won’t move strictly for temperature purposes.
Another cold-water question we often get, is very easy to answer – do you need to fish the sunny areas of the water in the winter months”? The answer to that unless you can find some still water in the stream you are fishing, and there is usually little of that, is no. The sun has the effect of helping to warm the entire stream, even if it shines in only a few places. Since the water is moving, it makes little to no difference in the water temperature, even in the area the sun is shining on. It may help you keep warm, and it may even help you think you will do better but that is about it. It doesn’t mean the water is warmer there than it is downstream.
As ridiculous as it is, I sometimes believe that just the fact that anglers sometimes feel cold when they are fishing during very cold weather, allows their mind to play games on them. Solutions like “Fish the sunny spots line of thinking that the trout are cold and looking for warm water in the same sense we warm blooded creatures get cold can be very deceptive. “Fishing deep and on or near the bottom is a much better line of thinking. Line of thinking such as “you can’t catch trout in cold water” and other mistaken beliefs about fly fishing cold water, compare well to some of the best “Old Wives Tales”.
