PITTSBURGH – The flash flooding that killed four people and forced others to swim to safety or climb onto car roofs was a freak incident caused by heavy rainfall that overwhelmed the sewer system just as rush-hour traffic clogged low-lying city streets, officials said Saturday.

A mother and her two daughters died in Friday’s flooding after becoming trapped in their vehicle and rising water pinned it to a tree. Another woman’s body was washed into the Allegheny River, where she was found Saturday morning.

Back-to-back storms pounded the city with 3 to 4 inches of rain. The water drained rapidly onto Washington Boulevard, a main street near the Allegheny River on the city’s east side, with a force too great for a pair of sewer pipes 9 feet in diameter. The torrent blew off 60-pound manhole covers.

“We had geysers here,” said Raymond DeMichiei, deputy director of the city Office of Emergency Management.

“There’s only so much any drainage system can handle,” said Jim Struzzi, a spokesman for the Pennsylvania Department of Transportation. PennDOT maintains the roadway but the city is responsible for the pipes underneath.

The area’s topography presents near-perfect conditions for flooding.

Hills line both sides of the section of Washington Boulevard that flooded and the boulevard itself slopes down as it stretches north toward the Allegheny River. When heavy rains hit, water rushes in from three directions.

The low-lying area was apparently once a creek bed, and “the watershed is huge for this,” DeMichiei said.

Flooding is not uncommon in the area, but this was “a little bit more spectacular than usual,” he said.

Rescue crews used inflatable boats to reach drivers marooned by floodwaters as deep as 9 feet, while others swam to safety on their own.

Rhodearland “Bob” Bailey of Penn Hills, who is about 80, was rescued from the roof of his car.

He said he heard a woman yelling for help but couldn’t see anything because the water was coming down so fast, he told the Pittsburgh Tribune-Review.

“I’ve never seen nothing like this in my life,” he said. “Lord have mercy.”

Authorities identified the mother and children who died as Kimberly Griffith, 45, of Plum, and her daughters, Brenna, 12, and Mikaela, 8.

The body of the other victim, a 70-year-old woman who had been reported missing Friday, was found in the river near the Highland Park Bridge. She was not immediately identified.

In 1998, a thunderstorm accompanied by a tornado that struck Mount Washington touched off worse flooding in the area, though it did not result in deaths.

DeMichiei said emergency officials will discuss steps to avoid future tragedies from flooding at the site.