The Canal Du Midi is unique because many of its locks are still in their original conditions, retaining their characteristic oval shaped basins.
The Freycinet gauge (French: gabarit Freycinet) is a standard governing the dimensions of the locks of some canals, put in place as a result of a law passed during the tenure of Charles de Freycinet as minister of public works of France, dating from 5 August 1879. The law required the size of lock chambers to be increased to a length of 39 metres (128 ft), a width of 5.2 metres (17 ft) and a minimum water depth of 2.2 metres (7 ft 3 in), thus allowing 300 to 350 tonne barges to pass through.
Consequently, boats and barges, such as the péniche, built to the Freycinet gauge could not exceed 38.5 metres (126 ft) in length, 5.05 metres (16.6 ft) in breadth and a draught of 1.8 metres (5 ft 11 in). Bridges and other structures built across the canals are required to provide 3.7 metres (12 ft) of clearance.
In the late nineteenth and early twentieth centuries many French canals were modernised to conform to the Freycinet standard. Even the Canal Du Midi was worked on. Luckily, the Canal Du Midi was the least important canal at the time, and work was postponed till the late 1970's. Money and labor was instead used to update other canals elsewhere in France.
The Canal de la Garonne was updated first, from Bordeaux to Toulouse. Once this was finished, attention was turned to the Canal Du Midi.
From 1977 to 1979, the following sections of the Canal Du Midi were upgraded to the Freycinet gauge:
- Toulouse to Baziege (28 km)
- Beziers to Etang de Thau (32 km)
- Junction Salleles d'Aude to Port La Nouvelle (31 km)
After this work was completed, all attention turned to the famous Nine Locks of Beziers. It was decided that all efforts should be focused on building a water slope here, before continuing with enlarging the remaining locks on the Canal Du Midi. Work started here in March 1982.
The water slope in Beziers never was a success, and only served for a few days. Shortly after the transport of merchandise became unpopular and the Canal Du Midi was no longer required to have its locks enlarged. Thus, in a way, the malfunctioning water slope in Beziers saved the Canal Du Midi locks from being completely destroyed. (Source: http://archives.assemblee-nationale.fr/7/qst/7-qst-1983-10-10.pdf )