The Aude River reached levels not seen in more than 100 years, the French agency responsible for monitoring, Vigicrues, said.
One person was also injured when a house collapsed in Cuxac, a local official, Alain Thirion, said on television. Firefighters responded to more than 250 calls overnight, as 160-180mm (6-7in) of rain fell in Carcassonne, he added.
In the villages north of Carcassonne, roads were cut off, schools closed and people were asked to stay at home. There was up to two metres (6ft) of water in the streets of Conques.
(CNN)Six people have died after flash flooding hit the Aude region of southern France, local officials said Monday.