A. The following is from a column I published in Trout magazine a few years ago and summarizes much of what we know about drift and when it happens.

When fishing wet flies, nymphs, emergers, or any other sub-surface flies, anglers are usually imitating the immature form of aquatic insects. Collectively, these are the nymphs and larvae of the various aquatic insects and other true aquatic organisms that normally are attached to the substratum but also occur suspended in the water column. Biologists refer to this suspended community of organisms as "drift."

Scientists recognize three reasons or sources of these drifting organisms: normal drift that results from dislodgment of organisms by the current or from organisms actively entering the water column, behavioral drift that results from actions such as predator avoidance or competition for space, and catastrophic drift that occurs as a result of dislodgment by floods. Is drift purposeful? Most drifting insects probably don't enter the drift of their own volition - after all, they have no guarantee of finding a better site downstream and actually become more susceptible to predation by trout while suspended. But it does serve a purpose as a mechanism for recolonizing denuded areas resulting from such things as flood scouring, emergence, or human activities such as dredging, or rechannelization. It also allows organisms to compensate for overcrowding and competition for available food resources.

If you suspended a fine-meshed net in the current for a given time, say 20 minutes each hour over a 24 hour period, would you find the same numbers in each sample? Generally, the answer is "no." Intuition and some popular writing would predict that drift would increase as the waters warmed during the day, resulting in increased insect activity and enhanced opportunity to be dislodged. There are some isolated case for some species drifting more during the day, but the majority of studies of this phenomena show that drift is mainly a nighttime occurrence. In fact, one study not only showed increased numbers of drifting organisms during a cloudy night, it also showed that a brief emergence of the moon decreased the numbers of drifting insects. Why at night? During darkness, it is less likely that insects will be seen by predators, and increased activity and movement of the insects at this time enhances the likelihood of their being dislodged into the current. This increased activity during darkness has been documented by visual observance of aquatic insects using infra-red lighting.