The balancing of incoming and outgoing water allows the Canal du Midi to operate as it does.
Each time a lock operates, large quantities of water are either required to fill it or dump from it into the lower level pound.
Being able to provide this water source was one of the most important problems to be solved by Pierre Paul Riquet, its creator.
Since the drought of 1989–1990, water can also be made available from the Lac de Montbel.
Leaving through the weir, the water simply flows over the side of the canal via a stone or concrete ledge.