Ognon lock has two lock chambers and was built in 1679 by architect Emmanuel de Lestang, who was also entrusted with the construction of Répudre aqueduct. Immediately downstream from the lock, the canal crosses the Ognon river. Like many other rivers in the Mediterranean, the water levels can rise very quickly here in stormy weather. There is even a local proverb: "Avec l’Ognon, gare au bouillon !" (When it comes to the Ognon, watch out for the gushing waters).
When he built the canal, Pierre-Paul Riquet, designed it so that the waters of the Ognon, and many other waterways, would flow into the canal and then leave via an overflow mechanism called a weir.
However, when water levels were very high, the Ognon river was out of control and the waters quickly overwhelmed the weir and flowed downstream into the canal, flooding the Pechlaurier canal pound. To avoid this, a large safety gate was built in 1689 to contain the waters of the river in their natural course.
To be able to intervene quickly, a house was built on-site for a lock warden. This person was in charge of closing the safety gate when water levels were rising. During the big flood of 1999, this house was partially flooded and the safety gate was under water. Since then, the gate has been controlled remotely by the employee on duty at Ognon lock. This person closes the gate in accordance with weather alerts that he receives two or three times a year.
To solve the issues related to high water levels on the Ognon, civil engineering engineer Pradal put forward a proposal for an aqueduct in 1823 to divert the course of the Ognon. This new aqueduct was built in 1827-1828, just downstream from the lock.
Since then, the river has passed under the canal along a new route with better water flow. But when water levels rise considerably, the river is still capable of submerging the aqueduct. This is when the safety gate has to be closed, also known as the 'Demi-Ognon' gate.