I am sitting here watching it rain outside and daydreaming about trout streams. I know that tomorrow a couple rivers will look like chocolate milk and be difficult to fish. However, I know of a few higher gradient streams which will stay fairly clear and if this were late spring or summer I might be excited. If you just thought to yourself "why", its all in the drift.
(Caution Potentially Boring Reading )
The drift of stream insects and other invertebrates refers to their downstream transport in stream currents. While stream invertebrates are very well adapted by whatever means to maintaining their position in running waters, it is to be expected that on occasion an individual will lose its hold and drift downstream. On other occasions the nymph may be free swimming and just get caught up in a faster current. Another example would be a spider, ant, or grasshopper being washed into the stream by a heavy rain storm These are simple examples of why insects may be in the drift. There are actually four Major types of drift. Besides boring you with a long writen out explanation of each type, I figured I would just make another easy to read chart.
Catastrophic Drift : Occurs only when floods wash insects into the water or stir them up from there substrate. Or, droughts, or chemical spills occur and force insects to escape and area. This would also include terrestrial insects getting washed into rivers during a rain storm.
Behavioral Drift: A daily activity used by many aquatic insects and small fish to find new food sources, uncrowded habitat, or avoid predators.
Emergence Drift: Occurs when a mature nymph or pupae leave the stream bottom and drift in the current to the surface for adult emergence.
Surface Drift: Occurs only when adult insects emerge on the surface or when they return to lay eggs.
(There is one occasion that is a combination of emergence and surface drift. It is when the female of some species of caddis must swim back to the bottom of the river to lay her eggs).
With that being said, some species are much more likely to to be in the drift than others. An example of this would be a stone-cased caddis larvae which is not very mobile and unlikely to be caught in the current. On the opposite end a mayfly species like a blue-winged olive that swims quite well often end up in the drift. Here is a really cool chart that was made by some biologist after studying a stream in Oregon. If you look at the two most common species they both live just about everywhere in the US. It really makes you consider what you should tie on during periods of peak drift.
Insect % of total collected in the drift
Mayfly (Baetis) 18%
Mayfly (Centroptilum) 3%Mayfly (Paraleptophlebia) 2%
Stonefly (Sweltsa) 2%
Stonefly (Calineuria Californica) 1%
Diptera (Chironomidae Larvae) 29%
Diptera (Chironomidae Pupae) 6%
Diptera (Chironomidae Adults) 6%
Water Mites 7%
Although some behavioral drift occurs all of the time there are actually periods of peak drift. The first period starts roughly and hour before and continues until an hour after sunrise. The second peak is roughly from an hour before and extends to an hour after sun set. The third peak is in the middle of the night from approximately midnight to two a.m. These periods of drift have been found to be fairly consistent throughout the year with only slight differences from day to day caused by weather and temperature. This could be the scientific reasoning why most anglers like to fish early mornings and just before dusk.
There is also peak times of emergence drift. In fall, winter, and early spring, is is usually mid day from about eleven a.m. to three p.m. During the summer however most emergence drifts happen in early morning and evening with a few species emerging at night.
I hope that if you read through this I didn't make it too boring. There is a lot more info on this subject but I don't want get crazy on a blog post. I hope that you found this information useful and apply it next time you are on the stream.
See you on the trail,