Oleksiy Kolesnik waded ashore and stood, trembling, on dry land for the first time in hours, rescued on Wednesday morning after spending the predawn sitting on top of a cabinet in his flooded living room.
“The water came really quickly,” said Mr. Kolesnik, who was so weak he had to be helped out of a rubber boat by two rescue workers. “It happened so fast.”
Fetid, coffee-colored floodwaters, with plastic bags and bits of straw swirling in the eddies, lapped at streets in Kherson, a regional capital in southern Ukraine, where rescuers had evacuated a neighborhood cut off by inundated streets. Exhausted residents spilled out of the rubber boats, carrying at most a purse or a backpack, and sometimes a cat or a dog.
The scene, overlooking a flooded square, was just one small snapshot of the vast devastation caused by the destruction on Tuesday of the Kakhovka dam, swelling a more-than-50-mile stretch of the Dnipro River until it swallowed docks, farms, gas stations, cars, factories and houses.
It would be a calamity in calm times, but it hit a region ravaged and largely depopulated by war, where the river forms the front line and providing basic services and communication was already a struggle.
Carrying chemical pollution, dislodged land mines and assorted debris — a refrigerator here, a red armchair there — the Dnipro reached its tainted fingers into drinking water supplies, drowned crops and chased thousands of people from their ruined homes downstream. Upstream, it drastically lowered the reservoir that many Ukrainian farmers need to irrigate their fields and that the Zaporizhzhia Nuclear Power Plant uses to cool its radioactive fuel.
“We were getting used to the shelling, but I’ve never seen a situation like this,” said Larisa Kharchenko, a retired nurse in Kherson who thought she might sit out the flood on Tuesday, when water was knee-deep in her yard but not yet in her home. By Wednesday, it was spilling through her door; in some areas, it reached the roofs of houses.
“It just keeps coming,” she said.
“Somebody needs to arrest Putin,” she added, referring to President Vladimir V. Putin of Russia, who ordered the unprovoked invasion of Ukraine in February 2022. Ukrainian officials charge that Russian forces, which held the dam, blew it up to hinder a Ukrainian offensive, though little evidence about what happened has emerged so far.
On the Russian-controlled river bank, residents of the town of Oleshky pleaded for help on an online chat group, searching for missing loved ones and seeking assistance as floodwaters rose. Some wrote that they were gathering in the tallest buildings in their neighborhoods. Local officials — both Ukrainians who fled last year and those installed by the Russian occupation — said almost all of the town was flooded.
“The water is coming! Help! I am begging you!” one person wrote. “Three people on the roof, one of them elderly.” Another wrote that three adults and a 15-year-old boy were on a roof — and that the boy was panicking.
Another post said three children were stranded in a house. “The second floor is already flooding,” it said. “Asking for help from anyone who cares!”
Kateryna Kovtun posted on the forum, searching for her grandparents in Oleshky, and learned late Tuesday that they had been rescued from a rooftop and taken to a nearby village. “What is next, I don’t know,” she said.
Oleshky was one of 35 towns affected on the Russian-held side of the river, said Vladimir Saldo, the Kremlin-installed regional administrator.
The city of Kherson, a hub of Ukraine’s agriculture industry, lies on the western, Ukrainian-controlled bank of the Dnipro. Last year, it fell to the invading forces, most residents fled, and it was occupied for months. The Russians retreated in November but have continued to bombard the already-battered city and the surrounding region from across the river.
Many neighborhoods, on bluffs above the river, were untouched by the flood, but low-lying areas were a panorama of water and floating debris. Rescuers ventured out in boats to pull stranded, frightened people from roofs or upper floors of homes, with the occasional boom of artillery in the background.
The entire Ostriv neighborhood, one of the areas most vulnerable to Russian shelling, was evacuated.
Alla Snegor, 55, a biology teacher, stepped out of a boat and looked back at the flooded city streets.
“Think of what is in this flood,” she said. “Pesticides, chemicals, oil, dead animals and fish, and also it washed away graveyards.”
Land mines the armies had planted have washed free, some blowing up and others tumbling with the current to new sites, the United Nations warned.
Serhiy Litovsky, 60, an electrician, said he was most worried about the long struggle ahead for southern Ukraine, one of the world’s richest agricultural zones and reliant on irrigation, mostly from the quickly draining reservoir.
“Without water, nobody will live here,” he said. “The legacy of this will last dozens of years.”
The scale of the disruption was hard to fathom, he said: “Without war, this would be a major catastrophe. But this came along with the war.”
Some people displaced by the flood were shuttled by train to Mykolaiv, a Black Sea port city less than 40 miles to northwest of Kherson. Mykolaiv was already strained by its role as a transit hub or temporary home for many people fleeing the fighting. The Mykolaiv region held about 190,000 internally displaced Ukrainians before the dam broke, according to the United Nations’ humanitarian affairs office.
“The flooding,” the office said, “will likely worsen an already fragile humanitarian situation.” Thousands of children were among those fleeing, it added.
Many difficulties lie ahead for southern Ukraine, including finding long-term housing for thousands of people. Towns and cities — including Kryvyi Rih, an iron ore mining and steel smelting hub — were deprived of drinking water, which had been drawn from the reservoir.
The Zaporizhzhia nuclear plant was the south’s major source of electricity before the war; now in Russian hands, it has been damaged by shelling and is not supplying power to the grid. It has enough cooling water for now, but its future remains deeply in doubt.
“This is a catastrophe for the whole south,” said Roman Kostenko, the chairman of the defense and intelligence committee in Ukraine’s Parliament. But on Wednesday, the task was saving people, he said, adding, “Later, we will deal with the legacy.”
Mr. Kostenko, who is also a colonel in the Ukrainian Army, was on Wednesday coordinating the efforts of soldiers who had flown drones to harry Russian forces with dropped hand grenades. Now they were flying bottled water and food to people stranded on rooftops.
Staggering to shore from rubber boats after a night and day spent on roofs in flooded areas, several people said they had been visited by drones as they waited.
“I was sitting on the roof of my house when a drone came by and dropped a bottle of sparkling water,” said Henadiy Rotar, 59. “In 10 minutes, another drone came by and dropped a can of meat.” With his location pinpointed by the drone, a rescue boat soon showed up. “I thought I would spend another night on the roof,” he said.
Kateryna Krupych, 40, and her son, Maksim 12, and daughter, Maria, 4, all came ashore exhausted and barefoot. They had been stranded on a roof on an island near the Russian-controlled eastern bank.
On Wednesday, a Ukrainian special forces unit of the domestic intelligence agency, in coordination with drone operators, began rescues in this area across the roiling, swollen river.
Ms. Krupych said drones had dropped water for the family before its rescue. When the three came ashore, a soldier carrying Maria, a crowd circled around and offered candies to the children.
“Another day and that would have been it,” said Maksim of the family’s time trapped without food and water on the rooftop.
Elena Nechai, a lawyer, said the workshop of her husband’s company, which specializes in repairing construction cranes, was flooded. “All the equipment is under water,” she said.
Building the company was “his whole life,” she said. Ms. Nechai was waiting at the launching point for boats as her husband paddled out to rescue a watchman who was stranded at the site.
The couple had hazard insurance, she said, but early in the war, the insurance company had taken pains to point out a clause in the contract clarifying that it did not cover acts of war.
It would be hard now, she said, to argue that the flood was anything other than an act of war.
Paul Sonne contributed reporting from Berlin, and John Yoon from Seoul.