As I see it, in my head: The insect life starts in the sediment at the bottom of a river which could be mud, gravel or sand. When the river rises slowly, the moving water does not affect the sediment that much, but, if the river rises quickly, as in a flash flood, it will wash away a lot (not all) of the bottom sediment and move this sediment and insects living in it, further down the river. The insect life will come back as soon as the river recedes to it's normal level. Now a tailwater system, below a generating dam, will be affected the same way plus the changes in water temperature, when generating, just adds to the problem. Just my thoughts and nothing more....