The two boys aged 11 and 18 set off on a canoe trip down the Ardeche river with their father on Sunday afternoon.
According to reports in the French press, the boys got into a trouble when they reached the barrage at Ruoms, which creates a three metre waterfall on the river (see video below).
The waterfall is avoidable, however, with canoes and kayaks able to pass to the side of it.
Initial reports suggest their canoe flipped at this point and the two boys, who were both wearing life jackets, were thrown into the water and swept away by the fast flowing river.
The father was able to bypass the dam and take the route designed for canoes.
He raised the alarm and around a dozen firefighters and a team of divers were sent to the scene to search for the two boys.
A helicopter was also dispatched to help the search teams from the air.
The body of the 11-year-old was found 4km down stream, but the body of the 18-year-old was not located.
The search was due to begin again on Monday.
The family had been on holiday in the region staying in a cottage on the banks of the river.
The Ardeche is a popular tourist destination in summer and one of the area's big draws is the river, which offers great opportunities for kayaking and canoeing.
The river regularly floods after heavy rains which was taken at the Ruoms dam, where Sunday's tragedy occurred.