Drift

A macro invertebrate behavior pattern keyed to the day/night cycle (i.e., diurnal rhythm). Many species of arthropods participate in drift each night during the summer months. Typically, organisms rise up into the water column at two distinct times during the night; the first at about 1 hour after dark and the second at 4-6 am. They drift downstream until they reach the bottom of the pool in which they reside. As morning approaches, they migrate long the bottom of the river back to their original place of lift off. The purpose of drift is still being investigated.

Links
  NPS