Anderson da Silva Pantaleão was at the snack bar on his property last Friday when clay-colored water began filling the streets of the southern Brazilian city of Porto Alegre. Soon, he ran into his downstairs shop. At nine at night, the water reached up to his waist.
“Then the fear starts to hit us,” he said. “You’re just trying not to drown.”
He ran to a neighbor’s house on the second floor, taking shelter for the next three nights, rationing water, cheese and sausages with two other people. The members of the group slept in shifts, fearing that another flood of water could take them by surprise in the middle of the night.
On Monday the water began to flood the second floor and they thought the worst. Then a military boat arrived and rescued Mr. Pantaleão. A day later, despite heavy rain, Pantaleão was trying to return in a rescue boat to look for friends who were still missing or stranded.
“I can’t leave them there,” he said. “The water is running out, the food is running out.”
Brazil is dealing with one of the worst floods in its recent history. Torrential rains have inundated the southern state of Rio Grande do Sul, home to 11 million people, since late April, causing severe flooding that has submerged entire cities, blocked roads, breached a major dam and closed the international airport until June.
At least 100 people have died and another 128 have been reported missing. The floods, which have spread through most of the 497 municipalities of Rio Grande do Sul, have forced almost 164,000 people to leave their homes.
In the state capital, Porto Alegre, a city of 1.3 million people located on the banks of the Guaiba River, streets were submerged in murky water and the airport was closed by the deluge, with flights canceled until the end of the month.
The river rose to more than 16 feet this week, surpassing previous high levels seen during a major flood in 1941 that paralyzed the city for weeks.
Flooding blocked access roads into the city and hampered deliveries of essential goods. Supermarkets were running out of bottled water Tuesday and some residents reported walking up to three miles in search of drinking water.
Many of those stranded waited on rooftops for help. Some took desperate measures to flee: When the shelter where her family was staying flooded, Ana Paula de Abreu, 40, swam to a rescue boat while clutching her 11-year-old son under one arm. Two residents of a Porto Alegre neighborhood used an inflatable mattress to remove at least 15 people from their flooded homes.
Search teams, including authorities and volunteers, scoured the flooded areas and rescued residents by boat and air. With nowhere to land, some helicopters have used winches to lift people stranded by floodwaters.
Bárbara Fernandes, 42, a lawyer from Porto Alegre, spent hours Monday on the scorching roof of her apartment building, waving a red rag and her crutches toward the sky. A rescue helicopter finally spotted her late in the afternoon.
“You just don’t know when they’re going to come looking for you,” said Fernandes, who is recovering from ankle surgery and was unable to flee his building before the floodwaters rose.
Nearly 67,000 people lived in shelters across the state, while others took shelter in the homes of family or friends. Some people who did not have access to either option slept in their cars or on the streets in areas that were still dry.
“It seems like we are experiencing the end of the world,” said Beatriz Belmontt Abel, 46, a nursing technician who was volunteering at a shelter in the city of Canoas, across the river from Porto Alegre. “I never imagined I would see this happen.”
In another shelter set up in a gym in Porto Alegre, volunteers distributed food and clothing. There were rows of mattresses on the floor and cardboard boxes served as shelves. Those who had been rescued were busy sweeping the floor and making their temporary beds.
President Luiz Inácio Lula da Silva, who visited the region last week, pledged federal funds to aid rescue efforts. State authorities have also announced aid to pay for search equipment, health services and housing for those whose homes were destroyed or damaged by the floods.
Even as rescues continued, authorities worried that the crisis could worsen because another wave of severe weather was expected in the coming days. With a cold front sweeping through the region, forecasters have predicted heavy rain, hail, thunderstorms and winds gusting over 60 miles per hour.
State Governor Eduardo Leite said authorities were evacuating people from regions vulnerable to more turbulent weather. Some residents have refused to leave their homes for fear of looting. Others have tried to return to their neighborhoods, hoping that the water levels will go down.
“It’s not time to go home,” Leite told reporters Tuesday.
The flood is the fourth climate-related crisis to hit Brazil’s southern region in less than a year. In September, 37 people died in Rio Grande do Sul due to torrential rains and strong winds caused by a cyclone.
Climate experts say the region is recovering from the effects of El Niño, the cyclical weather phenomenon that can bring heavy rain to Brazil’s southern regions and cause droughts in the Amazon rainforest.
But the effects of El Niño have been exacerbated by a combination of climate change, deforestation and disorderly urbanization, according to Mercedes Bustamante, an ecologist and professor at the University of Brasilia.
“We really are looking at a recipe for disaster,” said Dr. Bustamante, who has written several reports for the Intergovernmental Panel on Climate Change, a body of experts convened by the United Nations.
For more than a decade, scientists have been warning policymakers that global warming would lead to increased rainfall in this region.
According to Dr. Bustamante, as deforestation progresses in the Amazon and other parts of Brazil, precipitation patterns are changing and leading to more erratic rainfall patterns. As a result, rainfall is sometimes unevenly distributed, drenching smaller areas or arriving in torrential downpours for shorter periods.
Severe weather has also become deadlier in recent decades, as urban populations have grown and cities like Porto Alegre have expanded into forested areas that once acted as buffers against flooding and landslides, he added.
The latest floods caught Brazil “off guard,” Dr. Bustamante noted, highlighting the need to make cities more resilient to climate change and develop response strategies that better protect residents from extreme weather events, which are sure to be more frequent.
“It is a tragedy that, unfortunately, has been going on for some time,” he said. “We hope this serves as a call to action.”
Manuela Andreoni contributed reporting from New York.