The US is warning its citizens in Iraq to make plans to get out of the country - quickly if need be - in the case of a catastrophic dam collapse that could flood the lands downstream around the Tigris River and kill more than a million people.

The Mosul Dan in the country's north is Iraq's largest hydroelectric facility, and it is in bad shape.  Questionably constructed in 1984 on a water-soluble gypsum base, the dam had been undergoing extensive repair work.  Engineers need to drill holes in the gypsum and fill them with a cement grout mixture six days a week in order to correct the structural flaws.

But that was disrupted in 2014, when Islamic State briefly took over the strategically important dam.  IS held it for only eleven days, setting off fears the terrorists release a wall of water that would flood Mosul and Baghdad, killings hundreds of thousands of people.  That didn't happen, but many of the seasoned workers refused to return and regular maintenance did not resume.

"We have no specific information that indicates when a breach might occur," said the statement from the US Embassy in Baghdad, "But out of an abundance of caution, we would like to underscore that prompt evacuation offers the most effective tool to save lives of the hundreds of thousands of people."

The US fears that residents of Mosul would have only minutes to evacuate to at least 6.5 kilometers away from the banks of the Tigris should the Mosul Dam give out.  A fact sheet (.pdf link) identifies 500,000 to to 1.4 million people at risk of death living as far as 480 kilometers downstream from the dam.

Iraqi Prime Minister Haider al-Abadi downplayed the likelihood of such a scenario as "extremely small".