A weir is categorized as spillover devices used to measure or control water flows. A weir is helpful in creating an impenetrable barricade around a river for the purposes of elevating the water level against the current. The weir pushes the water upt to a specific height and the water proceeds to flow throughout the weir. Eventually, the water begins to overflow the weir, however in a dam the water overflows in a more controlled manner, specifically through a zone called the spillway.