This summit pound (a canal pound is the stretch of water between two locks) is the highest point of the canal. It is at the watershed point between the Mediterranean and Atlantic basins. Today, this summit pound is 5 km long and is 189.43 metres above sea level. It is the section of the Canal du Midi between the Ocean lock and the Mediterranean lock.
In 1674, when navigation was opened between Toulouse and Castelnaudary, the Naurouze basin, at the watershed point between the Mediterranean and Atlantic basins, was used as the summit pound. It was only natural that Pierre-Paul Riquet would name the first lock from Toulouse, the Ocean lock, and the fisrt one towards Narbonne, the Mediterranean lock.
However, in 1688, as recommended by Vauban as part of his plans to modernise the canal, the section via Naurouze basin was abandoned and replaced by a channel dug out on the south side of the basin, and so boats no longer passed through these Ocean and Mediterranean locks. On the Atlantic side, Montferrand lock was renamed the Ocean lock, and on the other side, Médecin lock was renamed the Mediterranean lock.
The locks leading to the basin were no longer needed and so one of them was filled in (the Ocean lock), but the other one (Mediterranean lock) is still used and the supply waters are channeled to the summit pound via this ancient lock chamber.
Commemorative and informative panels now mark the location where the waters separate in the canal pound, known as the summit pound.