A commonly used concept for floodplain planning and zoning is the 100-year floodplain. This is the area that would be inundated by the largest flood that hydrologists estimate has a 1 in 100 probability of occurring in any given year. This is based on previous years' flow records. It could occur in two consecutive years. Sediment is important to determining the morphology of river systems.
Rivers naturally evolve and change their shapes by eroding, transporting, and depositing sediment.
These factors influence the types of wildlife habitat available in the river system. When a dam impounds a river system, the area that is flooded behind the dam no longer has a river channel morphology. Instead, this part of the river takes on the morphology of an impoundment or pond. The water in the impoundment is relatively still, and a shoreline may have grassy banks or sandy beaches. The shape of the valley behind the dam and the height of the dam determine how much land is flooded and the shape of the impoundment.