Water was rushing over the spillway of the Guajataca Dam on Saturday, having eroded part of it and the land around it. In addition to cracking the dam, the hurricane brought so much water that patches of concrete in the area that normally contains the overflow had collapsed, said Miguel Abrams, the emergency management director of Quebradillas, a nearby city that was also told to evacuate.
“There’s normally a street there,” Gabriel Soto said, referring to a submerged road alongside the dam. He took pictures of the water before going to check on relatives who live nearby.
The Guajataca Dam is 120 feet high and nearly 1,000 feet long, built in 1929 by the Puerto Rico Electric Power Authority. It lies across the Guajataca River, forming a reservoir that can hold about 11 billion gallons of water that is used for drinking, irrigation and power generation.
“This is a serious situation because that’s our drinking water,” Mr. Abrams said. “Seven or eight cities depend on that lake to drink.”
The dam is in the middle of hilly rural neighborhood with curvy roads and steep ridges overlooking the water. A few miles away a maroon sedan that had been caught mid-mudslide hung precariously above the road.
The swelling waterway was such a spectacle that people gathered at the washed-out road and along the ridge to watch. The backyards of many houses abutted the ridge, but the residents there said they felt safe because their houses sat a few dozen yards above the water.
Some of the homes looked stately. Others were submerged.
Water levels have risen significantly, which added to the danger. Another storm system, Lee, was expected to douse Central Puerto Rico in the coming days, which could worsen an already precarious situation.
“Since the ground is already saturated, we don’t know how nature will work,” Mr. Abrams said. “This is a lot of water.”
But he stressed that the ridge that surrounded the dam was 300 to 500 feet high, which protects the vast majority of the homes nearby. Only the neighborhoods in lower areas were in danger of flooding.
Mr. Abrams said only about six households were evacuated from Quebradillas, on the other side of the river. A 2015 study showed that if the dam ever broke, it would be Isabela that flooded.
Even as Juan Morales, the public safety director for Isabela, managed a mandatory evacuation of low-lying areas, he sought to calm tensions.
“These have been preventive evacuations,” he said. “The idea that the dam is collapsing is totally false. There has not been an evacuation of 70,000 people, also totally false. Please calm people. There isn’t a breach, and 70,000 people aren’t going to die.”
Mr. Morales was referring to the number of residents that Gov. Ricardo Rosselló said could be affected by a break in the dam. After the National Weather Service of San Juan announced the breach on Friday afternoon and issued a flash flood warning for Isabela and Quebradillas, the governor, citing the figure, ordered the municipalities to evacuate.
With communications down, Mr. Rosselló went to Isabela himself to get the mayor to evacuate three neighborhoods.
“What is the risk?” the governor said at a news conference Saturday, when he again urged area residents to leave. “The risk is life. Faced with doubt, we want to identify, be able to evacuate and make sure that people are safe.”
Experts from the United States Army Corps of Engineers, the Federal Emergency Management Agency and the National Guard were reviewing the damage and expected to give a report on the risks. But some of the locals are not waiting for the engineering report.
“I packed a winter bag just in case, that’s my plan C,” Luz Rodríguez Pérez, 63, said.
Ms. Rodriguez recalled that the day after Maria’s torrential rains, she heard crushing sounds from the dam, which is about a half mile from her house. “I started loading up the car,” she said. “It was such a loud noise. Rocks were falling.”
She left on Thursday, a day after the hurricane, because of the terrifying sound, but returned the next day. A few hours later, civil defense authorities were driving around her neighborhood with loudspeakers, announcing a mandatory evacuation.
“I have lived in that house for 40 years and have never seen such a thing,” she said. “We assume the dam is strong enough, that it was designed for this kind of thing. It was even reinforced about 35 years ago. But now? I won’t go back.”
Her neighbor Deogracia Román González agreed.
“I’m too scared,” she said from the local elementary school where both women were sheltered. “The water is right up to the street.”
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