The Haute Perche Canal (French: Canal de Haute Perche [kanal də ot pɛʁʃ]), despite its title, is a natural river, the Rivière de Haute-Perche, that has been slightly canalised to improve navigation between its mouth in the Bay of Bourgneuf at the port town of Pornic and the upstream settlements of Le Clion-sur-Mer [fr], Chauvé and Arthon-en-Retz.
The mouth of the river is a ria (or Breton: aber), eroded along a fault line in the Precambrian mica-schist rocks of the Armorican Massif which define the north coast of the Bay of Bourgneuf.
Formed mainly before the Pliocene era, it was inundated by the rise in sea level due to the Flandrian transgression and has gradually silted up since.
[25] The sluice of the former lock at the causeway bridge in the port of Pornic is the main control point for the water level in the canal.
Another sluice with three vertical check valves is located 1.25 km upstream in Boismain, near Pornic's wastewater treatment plant at Les Salettes.
[27] Human influence on the river began in the 13th century during the time of Pornic Abbey [fr], with the construction of the causeway in 1225 which linked the two banks and formed a dam equipped with four tide mills.
[2] Finally, a decree from the prefect dating from 1933 considerably reduced the use of the Pornic lock: access authorisations were subsequently very restricted.
The German army blew up the lock and let the ria and marshes flood as far as Arthon in an act of passive defence of the Atlantic Wall.
[4][2] At the end of the 18th and beginning of the 19th centuries, Alexandre de Brie-Serrant [fr], last lord of Retz, made proposals to extend the Haute Perche canal in order to create a link with the Loire.
This proposed improved navigation to avoid the dangerous sandbanks between Nantes and the Loire estuary and offered a more direct route to the Bay of Biscay.
The Haute Perche canal was also proposed as a drainage route for a project to drain the Grand-Lieu lake in 1806, but was excluded because the course was more than double that of the Acheneau.