These are structural barriers built to obstruct or control the flow of water in rivers and streams. They are designed to serve two broad functions. The first is the storage of water to compensate for fluctuations in river flows or in demand for water and energy. The second is the increase of hydraulic head, or the difference in height between water levels in the lake created upstream of the dam and the downstream river. By creating additional storage and head, dams are able to generate electricity, supply water for agricultural, industrial, and household needs, control the impact of floodwaters, and enhance river navigation. They can be simultaneously operated in a manner that augments downstream water quality, that enhances fish and wildlife habitat, and that provides for a variety of recreational activities such as fishing, boating, and swimming.