Rachel Buchanan, from Oxford, was left with a "frightening" drive through floodwaters that had hit a site near a river in the neighbouring Ardeche region.
"We woke up in about a foot of water".
A top Gard official, Thierry Dousset, told BFM-TV about 750 people were evacuated from five campsites.
In the Gard region, where strong rains poured down all day on Thursday, rivers quickly overflowed their banks and 119 children were rescued from their campsite at Saint-Julien-de-Peyrolas, the French interior minister said in a statement.More news: Kim Kardashian Wants to Change Chicago West's Name
A search was underway for him.
Videos posted on social media showed flood water rushing through populated areas. We just have what we're wearing, ' Rita Mauersberger, a visitor from Germany who was among the campers taking shelter in a local hall in Saint-Julien-de-Peyrolas, told France Info radio.
Hundreds of firefighters and police took part in the rescue operations, using helicopters to help spot affected campsites and carry out some people.
The public prosecutor for the region confirmed to AFP that the president and vice-president of the association that brought the children from Leverkusen to the village of Saint-Julien-de-Peyrolas were being held on suspicion of causing unintentional injury and endangering the lives of others.More news: How to watch the PGA Championship on Eleven Sports
The weather in the southeast of France has improved this morning but authorities are urging people to stay vigilant as the flooding are receding.
Roads remained close throughout these Gard, Ardeche and Drome, but will be reopening as the water level gets lower.
Heavy rain meant tents were destroyed, while about 17,000 homes were without power.More news: Kawhi Leonard thanks Spurs in first statement since trade to Raptors