From the usually idyllic Schuylkill to the winding Neshaminy in Bucks County, the region's waterways - normally considered an asset - turned into a threatening menace, forcing evacuations and rescues across the suburbs.
Rising floodwaters swelled the Brandywine River in Chester County and the Perkiomen and Wissahickon Creeks in Montgomery County to near-record levels hours after the storm had cleared. Their waters - as much as eight feet above flood stage in some places - cascaded into riverfront properties and washed out roads, stranding motorists who thought the worst had passed.
"I thought I could go through," said Mike Sands, 55, minutes after Tinicum police plucked him from his Mercury Mountaineer, which was stranded in waters from the Long Hook Creek near Philadelphia International Airport. He had dodged a National Guard truck set up to block traffic, but police let him go with "a stupidity warning," he said.
Elsewhere, inland flooding kept emergency responders busy throughout the day with hundreds of swift-water rescues in, among other places, Darby, Fort Washington, Horsham, Chadds Ford, and Whitemarsh.
The rising Schuylkill forced evacuations of the waterfront Riverwalk and Londonbury apartment complexes in Conshohocken, and tourists with hastily packed suitcases fled the borough's nearby Marriott Hotel. Water also spilled onto Norristown's Lafayette Street, where river levels rose to just under 20 feet - seven feet above flood stage, according to the National Weather Service.
Along the Delaware River, roads were closed, power was out, and residents had evacuated or were leaving homes in Lower Makefield, Yardley, and New Hope in Bucks County. The worst wasn't expected until Monday afternoon, when officials predicted the river would crest at nearly 31 feet.
In Hatboro, it was the rescuers who found themselves in need of rescuing after three firefighters fell into churning floodwaters early Sunday while trying to fish out a flooded car near Upper Moreland. The men were trapped for nearly two hours until another crew pulled them out unharmed, Enterprise Fire Company Chief Keith Gordon said.
Elsewhere, Alison Greenwood and Travis Jackson thought they had prepared their flood-prone house along the Wissahickon Creek in Whitemarsh, installing three sump pumps, a retention basement, and walls along the back patio and the front of their house. It wasn't enough.
Rushing water filled their basement, rose three feet up the first-floor walls, and sent them - like many of their neighbors - fleeing to their attic to await rescue from National Guard troops roaming the area.
"This is higher than anyone's ever seen it around here," Jackson said while surveying the damage hours later.
But with their power out and homes largely unscathed, some approached the roaring waters with a sense of wonder. By midday, many Philadelphians were venturing out to see for themselves what Irene had wrought.
The Schuylkill was the main attraction in Manayunk, where its brown waters roared through downtown, swirling and seething around bridge abutments and flooding blocks of Main Street.
Riverbanks took on an almost street-party atmosphere, with passersby snapping photos and reporting on conditions via cellphone. Philadelphia police arrested two men who tried to brave the blocked-off Main Street in an inflatable raft.
But count lifelong resident Leo Maziarz, 46, among the unimpressed.
Although water crept across a parking lot near the Green Street Bridge - climbing up a billboard advertising "Manayunk's Ultimate Waterfront Apartments" - he insisted he'd seen far worse. "With the really good ones, the canal and the river meet," he said.
For residents without power near the flood-prone Brandywine Creek in Chester and Delaware Counties, scoping out the extent of the damage also became a popular Sunday-afternoon activity. Cars, trucks, pedestrians, and bicyclists congregated near bridges in Chadds Ford and West Chester.
Steve Conary, chairman of the Pocopson Township supervisors, said he had been driving around to assess the storm's impact when he ended up as part of an almost festive convoy of spectators marching along the Route 52 bridge over the Brandywine, each member taking a peek at the submerged buildings and the Brandywine picnic park below.
"This area always floods, but I don't think I've ever seen the water this high," Conary said.
The National Weather Service forecast Sunday afternoon that most Philadelphia-area waterways had reached their peak levels and that floodwaters should recede throughout the week. However, it remained unclear what effect runoff from Irene's trek up the East Coast would have as rains poured back downriver.
Meteorologists predicted the Delaware River near Easton could continue to rise into Monday afternoon, reaching a height of more than 30 feet - nine feet above flood stage. Yet, as of Sunday night, the river there remained below flood stage.
Yardley code enforcer Wes Foraker stood along the river its banks watching for what still might come.
"We escaped from the wind," he said. "The water kind of snuck up on us."
Inquirer staff writers Bill Reed, David O'Reilly, Mari A. Schaefer, Kathleen Brady Shea, Kathy Boccella, and Joelle Farrell contributed to this article.
Contact staff writer Jeremy Roebuck at 267-564-5218, firstname.lastname@example.org, or @inqmontco on Twitter. Read his blog, "MontCo Memo," at www.philly.com/montcomemo