A dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a reservoir, lake or impoundment. Dams are classified according to their structure, intended purpose or height. On the basis of structure and material used, dams are classified as: . Timber dams . Embankment dams or masonry dams. According to their height, dams can be categorised as: . Large dams or major dams . Low dams . Medium height dams . High dams