Dams, weirs and barrages all are types of head works used to increase the head of water on upstream side. There is confusion between these terminologies which are explained in this article.
A weir is a diversion headwork constructed across a river to raise the water level on the upstream side. Like Barrage its main aim to divert water as it does not have a storage reservoir. The water is raised up to the required height and the water then flows over the weir. In a weir the water overflows the weir, In Barrages water flows through adjustable gates. Weirs have traditionally been used to create mill ponds. They are also used to prevent flooding, measure discharge, and help render a river navigable.
A Barrage is a diversion headwork. it’s main aim is to divert the flow of river it does not have a storage reservoir on its upstream side. The water is elevated only to few feet. The main difference from the wear is that it has adjustable gates installed on top of it. Water level can be maintained using these gates.
A Dam on the other hand is a storage headwork so the main aim of dam is to create a storage reservoir on the upstream of the dam. Dams are mostly multipurpose projects used for Irrigation, hydroelectric power, Flood control, recreation etc.