That's true--one way the dead fish serve the cycle is that during spring thaw the nutrients from sunken, rotted carcasses may get scraped off creek bottoms along with silt etc. by ice chunks and then sluiced over nearby low lands. I found it to be most impressive and frightening to watch over a few days, usually late in March in the creeks around here. Once humans build a dam to control flooding, this nutrient replacement system vanishes completely.