Rachel Buchanan, of Oxford, was one of thousands of holidaymakers who had headed for camp sites in Gard, Ardeche and Drome when heavy rain destroyed tents as the heatwave broke with a vengeance.
She woke up to discover her camper van was one foot deep in water and had to race to escape waters gushing over hillsides into the site.
Ms Buchanan was left with a “frightening” drive through floodwaters that had hit a site near a river in the neighbouring Ardeche region.
She told the BBC: “The level and speed of the river today was extraordinary.
“We woke up in about a foot of water.
“Driving in our camper van was very frightening as the road by the river was completely under water and waterfalls had appeared from nowhere crashing down the gorge.”
A 70-year-old German man who was helping to supervise children at a summer camp at Saint-Julien-de-Peyrolas in Gard is feared to have been swept away by floodwaters, together with with his van.
Nearly 120 children were evacuated from the site as emergency teams helped around 750 people to safety across the Gard region.
District official Thierry Dousset said it was not certain whether the 70-year-old had been in his van at the time.
Four German children had been hospitalised for hypothermia in Bagnols-Sur-Ceze, a town on the Ceze River, Mr Dousset added.
More than 400 firefighters and police officers, many from other regions, helped with the evacuations, using helicopters to spot camp sites and rescue people.
Many roads in the area were left cut off, with the authorities urged members of the public to be prepared for more disruption.
The French weather service has issued orange and yellow alerts for parts of south-east France that will remain in place until 4pm on Friday.
UK Met Office meteorologist Marco Petagna said nearly 3 inches of rain fell over the area in just 24 hours.
He added: “It’s located in the area of hot weather that has been affecting much of Europe just recently, although there is a cold front moving south.
“Where you have got cold air trying to come in, at the boundary the hot air rises quite quickly and creates these rain clouds.
“There are still storms around, although they are starting to ease a little bit.”
He said parts parts of the Mediterranean could still see wet weather, with the Balearic islands and Sardinia at risk of heavy rain.
It’s all something of a contrast to recent weeks, which have seen much of the continent parched in the face of soaring temperatures.
EDF France was even forced to shut down four nuclear reactors in the country because the rivers from which they draw water to cool them were becoming too hot.
Meanwhile firefighters have been battling wildfires in Spain, Portugal and even within the Arctic circle in Sweden.