Aquatic insects washed downstream by flooding falls into the category of CATASTROPHIC DRIFT. The opposite of flooding is drought and it can also cause catastrophic drift, as can pollution. CATASTROPHIC DRIFT is one form of insect drift with the other two being Constant Drift and Behavioral Drift. Behavioral and Constant Drift allows the insects to colonize the entire river.

Downstream drift is why insects fly upstream to mate and lay their eggs. This upstream egg laying behavior both corrects for downstream drift and distributes the insect eggs so they eventually inhabit the entire stream. Downstream drift and upstream egg laying are adaptive behaviors and the upstream egg laying re-established the insects upstream after flooding.

A fly fisher can take advantage of behavior drift to increase the chances of nymphing success. Behavior drift peaks after dusk and before dawn, at dark so the insects are less likely to be seen. Again this is an adaptation that insures that the fewest drifting insects will be seen and taken by feeding fish. But this is when the fish will be looking for drifting insects so your chances of success are higher when insects are drifting.

See:

http://books.google.com/books?id=UUt... drift&f=false

http://books.google.com/books?id=wiT... drift&f=false