weir . (a) A small dam in a stream, designed to raise the water level or to divert its flow through a desired channel; e.g., a "leaping weir". (b) A notch in a levee, dam, embankment, or other barrier across or bordering a stream, through which the flow of water is regulated; e.g., a "wasteweir". Well-designed weirs usually have known dimensions and cross-section areas as a function of height above the base. Graduations of height allow rapid determination of hydraulic parameters, such as wetted perimeter, needed to calculate discharge.