More than 70 people were still missing and at least 23,000 had been displaced in the state bordering Uruguay and Argentina, which had nearly half of its 497 cities affected, according to Rio Grande do Sul's civil defence.
In several towns, streets essentially turned into rivers, with roads and bridges destroyed. The storm also triggered landslides and the partial collapse of a dam structure at a small hydroelectric power plant.
A second dam in the city of Bento Goncalves is also at risk of collapsing, authorities said, ordering people who live nearby to evacuate.
A highway is partially submerged by floodwaters in Porto Alegre. (AP PHOTO)
The state is at a geographical meeting point between tropical and polar atmospheres, which has created a weather pattern with periods of intense rains and others of drought.
Local scientists believe the pattern has been intensifying due to climate change.
Heavy rains had already hit Rio Grande do Sul last September, as an extratropical cyclone caused floods that killed more than 50 people.
That came after more than two years of a persistent drought due to the La Nina phenomenon, with only scarce showers.
"It's not just another critical situation; it's probably the most critical case the state has ever recorded," Rio Grande do Sul Governor Eduardo Leite said in a live broadcast on social media.
He added the death toll will likely climb as authorities have not been able to access certain locations.
President Luiz Inacio Lula da Silva travelled to the state on Thursday to visit affected locations and discuss rescue efforts with the governor.
with AP
