The death toll rose from six to 10 on Sunday as investigators continued to parse through the damage and identify the deceased.
Video of the tragedy shows dozens of people on multiple boats at Furnas Lake in the Brazilian state of Minas Gerais.
In a matter of seconds, a massive slab of the rock dislodged and slammed into multiple boats as people nearby screamed.
Minas Gerais Gov. Romeu Zema wrote on Twitter that “heavy rains… caused the loosening of a wall of stones,” leading to the tragedy.
Recent heavy rains in the area forced about 17,000 people from their homes.
Pedro Aihara, a spokesman for the Minas Gerais Fire Department, told the Brazilian newspaper G1 that the wall of the canyon are vulnerable to the elements.
“We have a rock formation that is basically composed of sedimentary rocks, so these are rocks that naturally have a much lower resistance to the action of winds, water, and the natural elements that act on the region,” Aihara told the newspaper.
Furnas Lake was created about six decades ago for the installation of a hydroelectric plant.
Brazilian President Jair Bolsanaro called the incident a “disaster” on Twitter, saying that the Navy is working alongside the Fire Department and other agencies in rescue efforts.
At least 32 people were injured in the incident.
The Associated Press contributed to this report.