Ukraine on Tuesday accused Russian forces of blowing up a major dam and hydroelectric power station in a part of southern Ukraine that Moscow controls, sending water gushing from the breached facility and threatening what officials called an “ecological disaster” due to massive flooding.

Officials from both sides in the war ordered hundreds of thousands of residents downriver to evacuate.

The fallout could have broad consequences: Flooding homes, streets and businesses downstream; depleting water levels upstream that help cool Europe’s largest nuclear power plant; and draining supplies of drinking water to the south in Crimea, which Russia controversially annexed in 2014.

It is also expected to have a devastating impact on the ecology of the region and will sweep mines from the banks of the Dnipro River into villages and farmland downstream.

Ukraine’s president, Volodymyr Zelenskyy, declared the destruction of the Nova Kakhovka dam as an act of terrorism and the “largest man-made environmental disaster in Europe in decades.”

Andriy Yermak, head of Zelenskyy’s office, also labelled the alleged act “ecocide.”

Ukraine’s nuclear operator Energoatom said in a Telegram statement that the collapse of the dam “could have negative consequences” for the six-reactor Zaporizhzhia Nuclear Power Plant, which has been held by Russian forces since the early days of the war.

But the operator also wrote that for now the plant has enough cooling water in its own reservoirs and that the situation is “controllable.”

The UN’s International Atomic Energy Agency wrote on Twitter that its experts were closely monitoring the situation at the plant, and there was “no immediate nuclear safety risk” at the facility.

The IAEA’s chief Rafael Grossi said the plant should have enough water to cool its reactors for “some months” from a pond located above the reservoir of the Nova Kakhovka dam.

“There are a number of alternative sources of water. A main one is the large cooling pond next to the site that by design is kept above the height of the reservoir,” he said in a statement.

Rosatom, Russia’s state nuclear corporation, which has declared itself the Zaporizhzhia plant’s operator, also said the dam breach didn’t pose a threat to the plant.

“At the moment there are no threats to the safety of Zaporizhzhia nuclear power plant. Five units are in “cold shutdown” state, 1 in “hot shutdown” state,” said Yury Chernichuk, the Rosatom appointed director of the plant. “The water level in the cooling pond has not changed and is 16.67 meters.”

Dmitry Gorchakov, a nuclear expert with Bellona, agreed with these assessments.

“All participants — Ukraine, Russian the IAEA —concur that the situation is not critical for the Zaporizhzhia plant and does not pose a direct risk to it,” he  said. “I have not seen such unity for a long time, and that could indicate that this is the true state of affairs.”

The Zaporizhzhya nuclear power plant. Credit: Ralf1969

Other parts of Ukraine that are downstream of the dam break will not be so lucky.

Aerial footage showed the dam missing a broad mid-section with the reservoir behind, which had been filled to record levels, pouring over it and roaring downstream. Towns along its path were inundated and complete houses could be seen floating away in the waters, while countless pets and wild animals scrambled to survive.

The collapse will also rob Ukraine of long-term capacity for generating hydroelectric power and the loss of upstream reservoirs threatens water supplies to Kherson and Zaporizhzhia regions and Crimea.

The dam, a Soviet power project, was completed in 1956 and was 30 meters high, holding back a vast reservoir of 18 cubic kilometers of water — about the size of Salt Lake in the US state of Utah.  It sits about 32 kilometers upstream from Ukrainian-held Kherson.

The Ukrainian Interior Ministry called for residents of 10 villages on the Dnipro’s right bank and parts of the city of Kherson downriver to gather essential documents and pets, turn off appliances, and leave, while cautioning against possible disinformation.

Around 16,000 people on the west bank of Kherson region are in a “critical zone,” Oleksandr Prokudin, the Ukraine-appointed head of the Kherson region military administration, said.

Ukraine controls five of the six dams along the Dnipro, which runs from its northern border with Belarus down to the Black Sea and is crucial for the entire country’s drinking water and power supply.

Zelenskyy warned last November that Russia was plotting to blow up the structure and that doing so would cause “a large-scale disaster” affecting people living downstream.

Blowing up a dam can be considered a war crime, the Geneva conventions say, as it “may cause the release of dangerous forces and consequent severe losses among the civilian population.”

Bellona, in its recent report on the state of the Zaporizhzhia nuclear plant, conducted a detailed analysis of a possible failure of the Nova Kakhovka dam and what threats that might pose to the plant.

Gorchakov, who authored the report, concluded that such an event would not pose an immediate threat to the facility. But he added today that continued monitoring of the cooling capacity of the plant is critical.

“And all this will certainly complicate the future launch of the plant into operation after it is liberated,” he added.  “For its full-fledged operation at capacity, a normal water level in the reservoir is still needed.”