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.
Many impoundments have irregular shapes that create coves and peninsulas.
Depending on whether the dam has the ability to regulate the amount of water passing through the dam, the water level in the impoundment may or may not be controlled. Most small dams in Wisconsin are run of the river and the level of the impoundment is determined by the amount of water in the river. When a free-flowing river meets an impoundment behind a dam, the river s flow slows considerably. The river uses the energy of its flow to carry sediment in the water, so as the flow nearly stops in the impoundment, coarse silt and sand and gravel settle on the bottom of the impoundment. Finer silt and clay suspended in the water are carried out into the impoundment and past the dam.