A major dam on the Dnipro River in southern Ukraine was destroyed early Tuesday, sending torrents of water cascading through the breach, flooding a war zone downstream, putting tens of thousands of residents at risk and raising the possibility of long-lasting environmental and humanitarian disasters.
Ukraine and Russia quickly blamed each other for the calamity. Officials in Kyiv said Moscow’s forces had blown up the Russian-controlled dam in the predawn hours, and President Volodymyr Zelensky of Ukraine pointed the finger at “Russian terrorists.” The Kremlin’s spokesman, Dmitry S. Peskov, denied any Russian involvement and described the destruction as “sabotage.”
It was not immediately clear who or what had caused the destruction of the Kakhovka dam and hydroelectric power plant near the city of Nova Kakhovka. But some top European officials denounced Russia. Engineering and munitions experts said a deliberate explosion inside the dam had most likely caused its collapse. Structural failure or an attack from outside the structure, they said, were possible but less plausible.
The dam’s destruction was a “monumental humanitarian, economic and ecological catastrophe,” and “yet another example of the horrific price of war on people,” said António Guterres, the United Nations’ secretary general.
The dam, in the Kherson region, had held back a body of water the size of the Great Salt Lake in Utah. Ihor Syrota, the head of Ukraine’s main hydropower generating company, Ukrhydroenergo, said in an interview that it had collapsed after an explosion about 2:50 a.m. Tuesday.
“The damage is huge, and the station can’t be repaired,” he said. “The lower part of it has already been washed away.”
Residents in the town of Antonivka, about 40 miles downstream, described watching in horror as floodwaters swept by carrying trees and debris from washed-out houses. The Ukrainian authorities raced to evacuate people by train and bus.
For miles along the flood plain, others waded through water, rescuing pets and belongings, videos and images on social media showed. Some people rode bicycles down streets submerged in muddy water. In Mykolaiv, an emergency train collected those fleeing the rising waters in Kherson.
Daria Shulzik, 38, an office manager, awoke to what sounded like pouring rain — but it was rushing water filled with the detritus of the town. There were “a lot of dirt, branches, parts of buildings, fences, cattails from swamps — everything,” she said.
Ms. Shulzik said the Russian military had created a disaster. “I don’t know why they started this war, and why they carry on,” she said. “Agriculture will suffer, and the Black Sea will suffer because all this is flowing into the sea. Even the fish will suffer now.”
About 16,000 people total remained in the “critical zone” on the Ukrainian-controlled west bank of the river, said Oleksandr Prokudin, the regional military administrator. The National Police of Ukraine said that 23 towns and villages had been flooded so far, and that the water level in the Dnipro had risen by nearly 11 feet in the city of Kherson. By 9 p.m. local time, at least 1,366 people had been evacuated from flooded zones, the police said on the Telegram messaging app.
The destruction occurred a day after American officials said they had detected what could be the beginning of Ukraine’s long-anticipated counteroffensive to repel Russian forces east of the Dnipro in the Donetsk region. Russia’s military said it had beat back several attacks by Kyiv’s troops.
Sergei K. Shoigu, Russia’s defense minister, accused Ukraine of destroying the dam because it wanted to move forces and equipment defending Kherson to other parts of the front to help with its counteroffensive. Ukraine said it was Russia that blew up the dam to prevent Ukrainian troops from crossing the river downstream.
The dam, the southernmost one on the Dnipro, was built between 1950 and 1956 as part of a broader effort to capitalize on the economic power of the river known as the “Great Dnipro.” It was downstream from the Russian-controlled Zaporizhzhia Nuclear Power Plant, which relies on the Kakhovka reservoir to cool its reactors.
Initially, there was concern that if the river level fell far enough, the plant, Europe’s largest civilian nuclear facility, would be unable to draw water, potentially leading to a meltdown. But Ukraine’s state nuclear company, Energoatom, said in a statement that while the destruction “may have negative consequences” for the Zaporizhzhia plant, it had sufficient water for now from a nearby pond for cooling.
“The situation is under control,” the statement said.
Experts were still waiting to understand the full scale of the disaster. Videos verified by The New York Times and images on social media showed water flooding communities downstream. Floodwaters swamped houses, spilled over into farm fields, blocked roads and deluged a zoo in the Russia-controlled city of Nova Kakhovka, the mayor, Volodymyr Kovalenko, said. City Hall and the Palace of Culture there were also inundated.
A satellite image showed the dam was breached in three places. About 200 yards of its central area was destroyed, and a structure at the hydroelectric plant sitting atop the dam was split in two. A drone video initially showed part of the southern end of the dam still intact. A few hours later, that area was underwater.
Emergency crews were heading to southern Ukraine from Kyiv, the head of the state emergency service, Serhiy Kruk, said in a statement. Vehicles designed to be driven through floodwaters, generators, mobile water treatment plants, water trucks and other equipment were also on their way. Volunteers from the Red Cross unloaded aid in Mykolaiv.
Even as water levels rose, Russian troops were still shelling the town on the outskirts of the city of Kherson.
Tatyana Yeroshenko, 32, a teacher and volunteer with an aid group, said by phone that she woke up around 5 a.m. Tuesday to artillery explosions. “I heard a boom, and my windows shook,” Ms. Yeroshenko said.
Then she checked her phone and saw news reports of a major flood.
In this area of southeastern Ukraine, where the Dnipro River separates Russian and Ukrainian troops, the floodwaters flowed into towns where tens of thousands of people had already evacuated after Russia’s full-scale invasion 15 months ago. In Antonivka, about 4,000 residents out of a prewar population of about 13,000 had remained before the flooding, said Ms. Yeroshenko.
In a sign of how widely the dam’s destruction may be felt, the head of Russian-controlled Crimea, Sergei Aksyonov, issued a warning about water levels of the North Crimea Canal, which supplies fresh water to the peninsula from the Dnipro. On Telegram, he said that Crimea had enough water reserves in its reservoirs, but that levels could drop.
John Kirby, a spokesman for the National Security Council, said the United States was monitoring the effects of the destruction. “It’s very clear that the deliberate destruction of civilian infrastructure is not allowed by the laws of war,” he said.
“We know there are casualties,” he said, “including likely many deaths, though these are early reports and we cannot quantify them.”
He gave no further details, and Ukrainian officials did not immediately release any information on casualties.
Dam warfare in Ukraine is not new. In August 1941, during World War II, retreating Soviet troops blew a hole in the Dnipro hydroelectric station, flooding roughly 50 miles downstream and killing thousands. The Germans also dynamited the dam years later as they retreated after repairing the Soviet damage.
Last fall, as Ukraine edged closer to retaking Kherson, officials in Kyiv and Moscow both warned that the other side would try to damage the dam. Appearing via video, Mr. Zelensky told a meeting of European leaders in Brussels that Russia was preparing a “false flag” operation to blow it up and frame Ukraine for the humanitarian and ecological disaster that could ensue.
Military analysts said then said that neither side had anything to gain from destroying the dam, since it would affect both armies.
Some military analysts cautioned against assigning blame with limited information.
“It’s too early to tell whether this is a deliberate act by Russia or the result of negligence and prior damage inflicted to the dam,” said Michael Kofman, the director of Russian studies at CNA, a research institute in Arlington, Va.
“This is a disaster that ultimately benefits nobody,” Mr. Kofman added. “Russia is responsible because it controlled the dam, and it’s actions in Ukraine led to this outcome, one way or another.”
But on Tuesday, Josep Borrell Fontelles, the European Union’s top diplomat, said the disaster represented “a new dimension of Russian atrocities.” He vowed in a post on Twitter that “all commanders, perpetrators and accomplices” would be held accountable for this “violation” of international humanitarian law.
Ukraine’s foreign minister, Dmytro Kuleba, described the dam’s destruction as “ecocide,” adding: “Russia destroyed the Kakhovka dam, inflicting probably Europe’s largest technological disaster in decades and putting thousands of civilians at risk. This is a heinous war crime”
Experts said the flooding was expected to intensify as the waters from the reservoir continue to flow before peaking in a day or two.
The loss of the dam was not expected to severely affect Ukraine’s energy grid, said Alex Riabchyn, Ukraine’s former deputy energy minister, because the hydroelectric plant had not been operating in the power grid since October. But it could cause a severe shortage of drinking water in the Dnipro, Zaporizhzhia and Kherson regions, he said.
The flooding could also expose underground land mines and wash them downstream. The HALO Trust, a British American charity that has been clearing mines planted by Russian troops, said that it was now operating in the flooded areas.
Others expressed concern about the potential for industrial pollution and the threat to nature conservation areas.
“It will have a series of acute and also long-term environment effects,” said Doug Weir, the research and policy director at the British-based Conflict and Environment Observatory. “It’s going to have an enormous legacy.”
Reporting was contributed by Victoria Kim, Eric Schmitt, Paul Sonne, Maria Varenikova, Anna Lukinova, Evelina Riabenko, Farnaz Fassihi, Max Bearak, Matthew Mpoke Bigg, Isabella Kwai and Monika Pronczuk.