A weir /ˈwɪər/ is a barrier across a river designed to alter the flow characteristics. In most cases weirs take the form of a horizontal barrier across the width of a river that pools water behind it whilst still allowing it to flow steadily over the top. Weirs are commonly used to to prevent flooding, measure discharge and help render rivers navigable. In some places the crest of an overflow spillway on a large dam may also be called a weir.