Depends on water chemistry and water temperature plus how much the water level changes for electrical generation. Varying water levels with heavy flushing followed by low water periods are bad for aquatic organisms.
Hatches implies the aquatic phase of an organism. You may have mainly aquatic only organisms that DO NOT HATCH.
I would add cress bugs to Dave's list of organisms. Note that of the organisms Dave mention, only midges actually "hatch".
I would collect samples with a kick net which is window screening tacked to two broom handles. Kick and shuffle the river bottom above the net in several areas to see what is available. It is more accurate than rock turning.
Regards,
Silver
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy