...will the relatively sudden rise of the water after a rainstorm affect the emergence of the bugs?
If the nymphs are mature and ready to emerge, a rise in water level after a rainstorm probably won't have any effect on whether they emerge, but it may affect where and (perhaps) how they emerge. Locations of emergence sites can change as water levels change. For some species, methods of emergence might also be affected. For example, changes in water level might have some influence on the percentage of Isonychia
nymphs that crawl out on rocks to emerge vs. those that emerge in the water. Related water temperature changes might also alter the daily timing of the hatch.
i've heard of hurricanes wiping out certain hatches because of the untimeliness of the flood.
Catastrophic flooding can certainly damage populations in the short term. Mostly, what I have noticed is a tendency for bugs (and fish) to get flushed out of the constricted areas of a stream into the places where the water can spread out. Some are certainly lost in the process, but repopulation usually occurs in the affected areas unless the flood has also eliminated suitable habitat.
The Brodheads had a nice hatch of green drakes (E. guttulata
) prior to the catastrophic flooding caused by Hurricane Diane in 1955. Some attribute the loss of the hatch to widespread spraying of DDT in the wake of the flooding, but other hatches returned in due course. I suspect that the wholesale flushing of silt from the system eliminated most of the nymphs, but the more significant long-term effect was probably due to the disappearance of their former habitat.