After a hot spell, the flash flooding in the northern part of the Gard region turned the Ceze and L'Ardeche rivers into churning waterways that quickly spilled out of their banks.
She told the BBC: "The level and speed of the river today was extraordinary".
Divers were taking part in the search for the missing man, an AFP journalist at the scene said.
About 750 people in total - including many British holidaymakers - were evacuated in Gard, mainly from campsites, district officials reported.
The group of 136 children and youths from the German city of Leverkusen, were now safe.
A fellow camper told France's BFMTV channel that as the river surged through the site, he could hear children's cries in the trees.
Ten people were taken to hospitals to be treated for minor injuries, including four German children with hypothermia.
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She said she heard a "crack, crack, crack, four in a row" in rapid succession. "That's when they (police) were shot", Gaudet said. CTV said Moore reported that "police escorted people from their homes" after announcing a suspect was in custody.
He was the only person unaccounted for after a rescue operation at three campsites in the area, where a heatwave ended abruptly this week in storms that caused flash flooding.
Hundreds of firefighters backed by four helicopters helped in the evacuation.
The weather in the southeast of France has improved this morning but authorities are urging people to stay vigilant as the flooding are receding.
A spokesman for the Gard Prefectures said: 'It is hoped that the flooding will reach its peak by around 11pm on Thursday'.
The statement added that 119 children from a campsite in the town of Saint-Julien-de-Peyrolas in southern France had been "sheltered" from the floods.
Pictures showed rescuers wading through flooded campsites with damage to tents, caravans and bicycles. Around 17,000 homes in the southwest and northeast were without power.