Curt-
I don't know If the large floods in and of themselves would be enough to lower insect populations very much or over long periods but the siltation certainly would. Although over a longer period of time you might end up with a relatively large population of different species that favor silt...
The increase in silt and other small particulates contributes to less species being in areas previously being rocky or gravelly. It also lessens chances for redds in that fry will not get enough oxygen. Insects will not have rocks or gravel to hide in, or to feed off of (algae, diatoms, etc.)...
There are reddish variations of Rhithrogena nymphs. They're fairly common in this area (Northern California). Here are some pictures of Rhithrogena nymphs taken at the Russian River. In this area about 1 in 20 nymphs have the reddish coloration.
Th