The break sent 10 million to 15 million gallons of water gushing through the streets, beginning around 8 p.m. Sunday, before the main was shut off about 1:30 a.m. Monday.
The flooding destabilized a sidewalk, which hit a gas pipe, but Philadelphia Gas Works sealed that leak about 10:30 a.m. Monday.
Mayor Nutter, who toured the mud-caked streets Monday afternoon, said PGW had not restored gas at 27 homes for safety reasons.
Residents had running water again Monday, and there were no reports of structural damage, Nutter said. About 40 homes were evacuated Sunday and the residents taken to a shelter or to hotels.
On Monday, all were allowed to return, except those who lived in four homes close to the work area.
Crews from the Water and Streets Departments will clean the streets and storm drains over the next few days. The water flushed from the water-main break was sent to a water treatment plant to be recycled.
McCarty said residents should not be concerned with the state of Philadelphia's water infrastructure, which she said was "in good condition."
There are typically about 221 breaks per thousand miles of water piping in the Philadelphia area per year, compared to the national average of 270 breaks per thousand miles, and these usually occur in much smaller pipes, McCarty said.
McCarty said it was too early to tell the extent of the damage. The Water Department will cover costs up to a total of $500,000, McCarty said. Residents with homeowners and car insurance are expected to be covered.
Peco Energy Co. spokesman Ben Armstrong said the company would continue rerouting electric service Tuesday so Water Department employees could work safely. Peco customers will not be affected, he said.
Several cars around 21st and Bainbridge were towed to make way for work crews. Residents looking for their cars are directed to call 215-683-9773.
The city's aging pipes are victims of constant pressure that makes them vulnerable, engineering experts said. Water courses through the pipes while vehicles pound on them from above.
"In any type of an old city like this, there are going to be a lot of problems," said Robert Traver, a professor in Villanova University's department of civil and environmental engineering.
He said many factors could have caused the break.
"It could be the material of the pipe. It could be the bedding underneath or soil supporting it," he said. "It could be the cow jumping over the moon. There are a lot of different possibilities."
The Water Department said it did not know the age of the cast-iron pipe that broke.
In Philadelphia, the average pipe is 78 years old, said Anh P. Nguyen, a civil engineer with URS Corp., an engineering firm.
Nguyen did not share McCarty's positive view of the city's water system. The American Society of Civil Engineers gives Pennsylvania's drinking-water infrastructure a grade of D+ because the state will need $15.5 billion over the next 20 years to replace aging facilities and comply with safe-drinking-water regulations, he said.
"It is generally understood in the engineering world that water infrastructure in the Northeast, including older cities such as Philadelphia, is in a deteriorated state and in need of repair," Nguyen said.
Bill Miller, an associate professor of civil and environmental engineering at Temple University, said Philadelphia's system was among the oldest because the city was one of the first to install a modern water system.
"Water distribution in Philadelphia started back in the late 1800s," he said. "This was one of the first cities to produce filtered water for consumption, so there have been pipes in the ground since that time."
Victor Agiv stood Monday morning outside La.Va, the cafe he owns at 2100 South St. and watched crews moving dirt. Agiv said he hoped the city would let him reopen Tuesday. He has to fill out paperwork to get reimbursed for damage to his property, but his primary feeling was relief.
"It was a big mess," he said, "but nobody got hurt, so it's good."
Late Sunday, neighbors near the water-main break hurriedly packed what they could as the water rose.
Fire Department employees, concerned that the water might have swept up large objects such as manhole covers that could hurt people, carried some residents out by boat, including Daniel Cohen, 35, and his wife.
"The whole street was like a river, with about three feet of water," Cohen said as he returned home Monday morning.
Hilary and Andrew Nicholls and sons Toby, 9, and Silas, 10, visiting from suburban London, were forced out of their apartment near 21st and South Streets.
"We had to pack quite frantically," said Silas, who grabbed only a change of clothes, pajamas, a toothbrush, and toothpaste.
Toby asked his parents if they could take the ironing board along to surf in the water. That request was denied.
Homeowners or residents who have experienced damage and not had contact with the Water Department are advised to call 215-685-6300.
Contact staff writer Miriam Hill at 215-854-5520, email@example.com or @miriamhill on Twitter.
Staff writer Peter Mucha contributed to this article.