When the two puppies arrived at a makeshift shelter in the southern Brazilian city of Porto Alegre, their rail-thin legs buckled from exhaustion. They had treaded water for hours, fighting to survive as floods submerged the city, turning streets into rivers.

“We tried to get them to walk, but they couldn’t,” said Dr. Daniel Guimarães Gerardi, a veterinarian volunteering at the shelter. “Your heart aches at times like these, for these poor animals suffering.”

Two days after being rescued, the 6-month-old mutts — one tiger-striped, the other jet black — mostly dozed on donated blankets amid chew toys, still drained from their ordeal. When awake, they wobbled around the shelter on unsteady legs, tails wagging and ears pinned back tightly.

They wore no name tags, and since they were found on May 21 nobody had come looking for them. “We hope that, if they have caregivers, they will be found,” Dr. Guimarães said. If not, he added, the aim will be to find them a good, safe home.

More than a month after catastrophic floods battered southern Brazil, its worst disaster in recent history, the region is still reeling. The floods submerged entire towns, destroyed bridges, shuttered an international airport and displaced nearly 600,000 people across the state of Rio Grande do Sul. At least 169 people were killed, and 56 are still missing.