While potentially deleterious to the earliest life stages, flood
events are essential to the development and maintenance of healthy
stream systems. Floods change the stream structure by altering
the active channel, creating new side channels, and recruiting
and transporting large woody debris. Flood events bring new sediment
to replenish stream banks, and fresh seeds and propagules to
colonize open soils. High flows carry coarse sediment and deposit
gravel in downstream reaches while flushing fine sediment from
spawning gravel. While a severe flood year may result in a low
survival rate for the present year cohort of salmonids, the resulting
habitat changes can create improved habitat for future generations.