I recently found some photos from an outing several years ago to Montech, southwest of Montauban. The town is on the plain between the River Tarn and the River Garonne. We went to see a pente d’eau (water slope, for want of a better phrase) that was designed to bypass the existing lock system on the Canal de Garonne.

Until superseded by the railway and then by roads, the canal system in France, as in the UK, was a principal means of transporting goods. Most rivers were usually too fast-flowing, too shallow or simply too unreliable for the purpose. The problem with canals was the natural slopes that had to be negotiated by means of locks. This was time-consuming and caused long traffic jams.

The Canal de Garonne was constructed in the 19th century to connect the Canal du Midi at Toulouse with the Garonne south of Bordeaux. At Montech, five locks had to be constructed to negotiate a slope of 13m30 on the canal. They took just over an hour to get through in order to travel a few kilometres. Until about 1970, the canal was still mainly used for commercial traffic. After that, it became largely the preserve of pleasure boats and tourism.

The canal was upgraded in the 1960s to cope with larger boats. All the other locks on the canal were converted to a length of 39m (the Freycinet gauge – gabarit Freycinet) but the five at Montech retained the old gauge of 30m. At Montech, the engineers decided to experiment with a revolutionary technique to circumvent the lock system.

They created an artificial channel parallel to the existing lock, following the natural lie of the land. Two gigantic engines – effectively diesel locomotives – pull between them a watertight barrier that pushes a triangular wedge of water before it on the ascent and retains it on the descent. A boat or barge floats on the water and passes up or down the slope in 20 minutes, thus saving about 45 minutes on the original journey through lock system. It is reserved for boats of 40-50 metres in length. Smaller pleasure boats still have to pass through the locks.

The system went into service in July 1974. Apparently, it is unique. When we visited, we saw it in action aboard a boat. The guide told us that other places had copied the technique but hadn’t been able to make it work. According to a local magazine, scientists the world over have visited. A delegation of Chinese engineers even came to look at it with a view to using the technique on their own waterways.

As I understand it, la pente d’eau has been out of action for a couple of years to enable repairs and maintenance to take place. But you can still enjoy a cruise on the canal and see the machinery in place. Go here to find out more.

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