Governor Ricardo Rosselló said the extent of the damage to the dam is not fully known and urged people living in the municipalities of Isabela and Quebradillas to flee.
“Close to 70,000 is the estimate of people that could be affected in the case of a collapse,” he said.
“We don’t know the details. It’s time to get people out.”
At least 13 people were killed when Hurricane Maria barrelled through Puerto Rico – virtually flattening the entire island and leaving a path of destruction in its wake.
The onslaught of powerful winds and torrential rain from the deadly hurricane has caused structural damage to the dam, near Lake Guajataca in northwest Puerto Rico.
Apocalyptic footage shows massive torrents of water cascading through the landscape – plummeling everything in its way.
Flash flooding caused by the dam failure has created a “life threatening situation”, according to The National Weather Service (NWS).
The agency issued an alert telling people to move to higher ground after the dam started showing signs of failure last night.
it read: “This is an extremely dangerous and life-threatening situation. Do not attempt to travel unless you are fleeing an area subject to flooding or under an evacuation order.”
Local resident Adwin Caban is concerned for the safety of his family after his entire town was evacuated.
Writing on Twitter, he said: “The dam near my hometown just collapsed… my town evacuated… still no word from my family.”
The dam was built in 1929 and has a capacity of 11 billion gallons of water in 1999, according to US Geological Survey.
It is used for drinking water and irrigation in Puerto Rico.
Abner Gomez, executive director of Puerto Rico’s emergency management agency, said there is “no way to fix the dam” due to rising waters from Hurricane Maria.
He warned that “thousands of people could die” if the dam tops over or fails structurally.
The emergency situation comes just 48 hours after Hurricane Maria – the most powerful storm in more than 80 years – slammed into Puerto Rico’s southeastern coast.
It came barely a week after monster category 5 Hurricane Irma ripped through the Carribean before making landfall in Flordia.
Maria as “one of the greatest natural disasters” in recent US history, on a par with Hurricanes Katrina and Sandy, Gomez said.
The destruction in some parts of the island “looked more like a tornado than a hurricane,” he added.
The storm is now a category 3 hurricane, and is moving away from the Turks and Caicos Islands.
“On the forecast track, Maria should move away from the Bahamas into the open waters of the western Atlantic today,” the National Hurricane Center said.
The hurricane claimed the lives of 30 people in the Caribbean, including at least 15 people in the small island Dominica.
US President Donald Trump said the island was “totally obliterated” by the storm and pledged to visit and offer assistance.