I read an article in the past couple of years (tried unsuccessfully to Google it) about how changes in tailwater releases on the Arkansas River in Colorado had a tremendous negative impact on the infamous Mothers' Day caddis hatch. My recollection is that it was thought some early water releases (very cold water) had altered the hatch cycles of the caddis reducing the quality of the hatch. So, it was posited that the timing of the water temperature changes, rather then the flooding/rising water, had reduced the hatches. This made sense to me since the river deals with the spring runoff annually. Having fished the Mothers' Day hatch for years, the quality of the hatches has been reduced considerably since about 2006 or so. I've heard from others that it might be improving.