is a barrier across the width of a river that changes the height of the river level. They are used to control the flow of water for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. Pictured below is the Imberfal 
industrial weir in Konstancin-Jeziorna, which used to accumulate water for the needs of the local paper factory.