Anderson da Silva Pantaleão was at the snack bar he owns last Friday when clay-colored water began filling the streets in the southern Brazilian city of Porto Alegre. Soon, it was rushing into his ground-floor shop. By 9 p.m., the water was up to his waist.
“Then the fear starts to hit,” he said. “You’re just trying not to drown.”
He dashed up to a neighbor’s home on the second floor, taking refuge for the next three nights, rationing water, cheese and sausage with two others. Members of the group slept in shifts, fearing another rush of water could take them by surprise in the dead of night.
On Monday, water began flooding the second floor, and they thought the worst. Then, a military boat arrived and rescued Mr. Pantaleão. A day later, despite heavy rains, Mr. Pantaleão was trying to go back on a rescue boat to search for friends who were still missing.
“I can’t leave them there,” he said. “The water is running out, the food is running out.”
Brazil is grappling with one of its worst floods in recent history. Torrential rains have drenched the southern state of Rio Grande do Sul, home to 11 million people, since late April and have triggered severe flooding that has submerged entire towns, blocked roads, broken a major dam and shut down the international airport until June.
At least 100 people have been killed and 128 others have been reported missing. The floods, which have stretched across most of Rio Grande do Sul’s 497 municipalities, have forced nearly 164,000 people from their homes.
In the state capital, Porto Alegre, a city of 1.3 million perched on the banks of the Guaiba River, streets were submerged in murky water and the airport was shuttered by the deluge, with flights canceled through the end of the month.
The river rose to over 16 feet this week, exceeding the previous high levels seen during a major flood in 1941 that paralyzed the city for weeks.
The flooding has blocked roads into the city and hampered deliveries of basic goods. Supermarkets were running out of bottled water on Tuesday, and some residents reported walking up to three miles in search of clean drinking water.
Many of those stranded awaited help on rooftops. Some took desperate measures to flee: When the shelter her family was staying in flooded, Ana Paula de Abreu, 40, swam to a rescue boat while grasping her 11-year-old son under one arm. Two residents of one Porto Alegre neighborhood used an inflatable mattress to pull at least 15 people out of their inundated homes.
Search crews, which include the authorities and volunteers, were scouring flooded areas and rescuing residents by boat and air. With nowhere to land, some helicopters have used winches to pull up people stranded by the flooding.
Barbara Fernandes, 42, a lawyer in Porto Alegre, spent hours on the scorching roof of her apartment building on Monday, waving a red rag and her crutches toward the sky. A rescue helicopter finally spotted her in the late afternoon.
“You just don’t know when they’ll come for you,” said Ms. Fernandes, who is recovering from surgery on her ankle and could not flee her building before the waters rose.
Nearly 67,000 people were living in shelters across the state, while others have taken refuge in the homes of family or friends. Some people who had access to neither option were sleeping in their cars or on the streets in areas that were still dry.
“It seems like we’re living through 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 meals and clothes. Rows of mattresses lay on the floor, and cardboard boxes served as shelves. Those who had been rescued busied themselves sweeping the floor and making their temporary beds.
President Luiz Inácio Lula da Silva, who visited the region last week, pledged federal funds to help the rescue efforts. The state authorities have also announced aid to pay for search crews, health services and housing for those whose homes were destroyed or damaged by floodwaters.
Even as rescues continued, the authorities worried that the crisis could worsen because another wave of severe weather was expected in coming days. With a cold front buffeting the region, meteorologists have forecast heavy rains, hail, thunderstorms and winds over 60 miles per hour.
The states’s governor, Eduardo Leite, said the authorities were evacuating people from regions vulnerable to more turbulent weather. Some residents have refused to abandon their homes, fearing looting. Others have tried to return to their neighborhoods, hoping water levels will recede.
“It’s not time to go home,” Mr. Leite told reporters on Tuesday.
The flooding is the fourth weather-related crisis to hit Brazil’s southern region in less than a year. In September, 37 people were killed in Rio Grande do Sul by torrential rains and punishing winds caused by a cyclone.
Climate experts say the region is reeling from the effects of El Niño, the cyclical climate phenomenon that can bring heavy rains to Brazil’s southern regions while causing drought in the Amazon rainforest.
But the effects of El Niño have been exacerbated by a mix of climate change, deforestation and haphazard urbanization, according to Mercedes Bustamante, an ecologist and professor at the University of Brasília.
“You’re really 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 well over a decade, scientists have been warning policymakers that global warming would bring increased rains to this region.
As deforestation advances in the Amazon and elsewhere in Brazil, precipitation patterns are shifting and leading to more erratic rain patterns, according to Dr. Bustamante. As a result, rainfall is spread unevenly at times, drenching smaller areas or coming in torrential downpours over shorter periods.
Severe weather has also become more deadly in recent decades, as urban populations have grown and cities like Porto Alegre have pushed into forested areas that once acted as buffers against flooding and landslides, she added.
The latest floods caught Brazil “unprepared,” 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 bound to become more frequent.
“It is a tragedy that, unfortunately, has been coming for some time,” she said. “We hope that this serves as a call to action.”
Manuela Andreoni contributed reporting from New York.