The man, who is believed to be in his 70s, was helping supervise over 119 German children and youths who were camping in Saint-Julien-de-Peyrolas.
The Gard administration also said four German children were hospitalized on Thursday for hypothermia in Bagnols-sur-Ceze, a town on the Ceze River.
According to the interior ministry, a total of 1,600 people, majority campers, were evacuated as a precaution in Gard and the nearby Ardeche and Drome regions.
BFM-TV reported that the missing person was a 70-year-old German man.
Trapped by a muddy, fast-flowing torrent after the Ardeche river burst its banks he sought shelter inside his caravan, but the vehicle was swept away.
A senior Gard official, Thierry Dousset, told the rolling news channel BFMTV that hundreds of people had been evacuated from five campsites.
Divers were taking part in the search for the missing man, an AFP journalist at the scene said. All in the group, who are from the city of Leverkusen, are all accounted for and safe.
"No one has suitcases".
Hundreds of firefighters backed by four helicopters were involved in the evacuation mission.
Pictures showed rescuers wading through flooded campsites with damage to tents, caravans and bicycles.
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.
Electricity supplies have been cut in some villages, while roads have been gridlocked because of the number of people trying to get to high ground.