Mayor Robert Nunes said the wooden Whittenton Pond Dam upstream from the working-class city of Taunton on the Mill River had developed more failures about 2 a.m.
However, gates in the dam were opened to release water from Lake Sabbatia and relieve pressure on the structure, and the reservoir's level had fallen several inches by late morning, officials said at a briefing.
"As the water level decreases, it will be taking a load off the structure and that's what everyone is looking for," said Matthew Bellisle, a private engineer who inspected the dam.
"The condition of the dam at Whittenton Mills has not deteriorated significantly over the past several hours," Nunes said.
City officials said Monday that the 12-foot-high dam was buckling, and later a timber column washed away and officials warned that the entire structure could fail. Early Tuesday, more wooden timbers failed, allowing additional water to leak through and under the dam.
"I've got my fingers crossed that this thing is able to hold," Gov. Mitt Romney said Tuesday morning. "Water's going under the dam. It's going through some areas that are weakened and there's every prospect that it will give way and we'll have a very significant water event.
"On the other hand, a few of us can hope that it hangs together and it ties together as long as possible and that the water is able to leak out in a relatively controlled manner," he said.
Taunton, a city of nearly 50,000 people, has received 11.5 inches of rain this month, including more than 7 inches from Friday through Sunday, according to the National Weather Service.
Nearly 2,000 people were evacuated from homes near the river on Monday.
Taunton, a former 19th-century manufacturing hub about 40 miles south of Boston, lies at the confluence of the Mill and Taunton rivers. The working-class city was last flooded in March 1968 when the same dam was breached.
Karen Anderson of CBS station WBZ-TV reports it was a door-to-door evacuation. Throughout Monday afternoon and evening, police and firefighters knocked on doors to urge residents to stay with relatives and friends or at a makeshift shelter in the field house at Taunton High School.
"I don't want to leave, but my husband and my children are making me leave," said Vicki Rose. "This is my home, I don't want to leave it."
"I'm a nervous wreck, it's never happened to us before," said another resident.
Lisa Campbell, who lives near the river, said she and her children planned to stay at her sister's house on the other side of the city.
"It's better to be safe than sorry," she said. "You saw how many people had to be rescued from New Orleans when they didn't leave."