Cold, clean water is among a trout's most critical needs, and once you understand how trout react to the temperature you'll have a better idea of the best times to go to the river and the best types of water to target. Some anglers are fanatical about water temperature. They will carry a thermometer at all times and keep a journal of entries to compare temperature, fish activity, and even insect activity.
Most people have a difficult time conceiving of how a trout lives. Its environment and comfort zone is nothing like ours. The first thing to consider is that trout are cold-blooded, so their body temperature is the same as the water they live in. People are warm-blooded with constant body temperature. This allows us to feel differences in temperature all around us. A doorknob is usually room temperature, we'll say about 70 degrees, and our body temperature is 98.6. The doorknob feels cool when we grasp it because of this difference in temperature. A mother is very aware when her child has a fever. She can feel a 101 temperature quite easily, and that's only a two and a half degree difference.
Temperature is the most critical factor when it comes to a trout's activity level. Just like us, they are less active in extreme cold and heat.
Trout are sensitive to temperature, but not the way we are. Virtually everything in a river is the same temperature including the fish. If they touch a rock on the river bottom, it's the same temperature as the water which is the same as their body temperature. Trout rarely react to the "feel" of the water, but the temperature has a direct effect upon their metabolism. Cold water will feel numbingly cold to our 98.6-degree hand, but a fish's body is the same temperature as the water, so it doesn't feel the cold the way we do. This is a dramatically different way of perceiving the world from the way we do. The cold affects their metabolism, so their body functions are generally slower in the cold than they are in warmer water.
There are only a few instances where a trout will "feel" a difference in temperature such as where a spring enters the river. Depending upon the season this water may be quite a bit warmer or cooler than the flow it enters. As a result, they may very well move into this water.
Trout are cold water fish, so they thrive in waters that remain below 70 degrees throughout the year. They can tolerate water above 70 degrees for a short period, but it's hard on them and detrimental to their health.
Trout are relatively slow moving when water temperatures are down in the 30's. This makes sense when you consider water freezes at 32 degrees, so they are only a few degrees from being a block of ice.
Trout are most active when water temperatures are in the 50's through the low 60's. There will be some local variation to this as some rivers rarely see temperatures in the 50's, but on average this is an ideal water temperature range.
Cold weather doesn't necessarily mean you can't catch trout on a fly. Understanding how temperature effects trout behavior will go a long way toward knowing where fish will be and how to target them.
When water is in the low 40's down through the 30's a trout's metabolism is extremely slow. During this time, trout burn calories slowly and have no reason to expend energy. This doesn't mean that they will not eat, but they aren't putting much effort into feeding if food isn't present. They will linger in slower waters where they don't need to expend as much effort swimming, but may very well eat any nymph that tumbles to them in the current. Some rivers may see good hatches of midges or other aquatic insects in cold water and trout will absolutely take advantage. This allows them to get more out of their food. Fewer calories go to the primary metabolic function, and more go toward growth.
As temperatures warm through the 40's into the 50's a trout's metabolism starts to kick into gear, and they require more food to survive. Fortunately, this is synchronized with natural cycles as aquatic insects also become active in this temperature range. Trout will move into swifter water to feed. During this time, A trout's body requires more energy, but conditions are also ideal for them to find food. Fish will start to spend more time in riffles and position themselves at the base of a plunge. These habitats require them to expend more energy, but they are also in a position to eat more food in these areas.
A great way to think of trout is to compare them to Wall Street investors. During an economic downturn, many investors keep their money tight and will typically keep their money in very safe investments like bonds and CD's that make a very modest return on investment. These investments don't make much money, but they rarely lose money. On the other hand, investors are likely to take on riskier investments when the economy is active, growing in leaps and bounds. There is no guarantee the stake in a new business venture will pay off, but chances are good they will, and the pay off is far higher than simple interest. Investors are similar to trout as they are more careful with their resources when things are slow, but get out and do much more when opportunities abound.
The savvy angler will match his tactics to the current behavior of the fish, and this is really at the core of success. Places, where you found actively rising fish on a pleasant afternoon in May, will probably have few active fish in January. Once you understand these differences, you should be able to identify good water at any time of year wherever you go.