The problem is as old as asphalt and as hard to fix as tapping a tight town budget.
There is so much pavement that when it rains, water that in years past would have soaked into the ground now shoots off downhill toward the nearest stream.
The Wissahickon Creek is emblematic of the problem: Its first trickles emanate from the Montgomery Mall parking lot.
In the last decade, engineers, regulators, and developers have embraced the idea of turning parking lots, fields, and roofs into giant sponges that hold rainwater long enough to let it percolate into the ground.
Philadelphia has been touted as being emblematic of the solution: Its $2 billion plan to foster a plethora of smaller swales, gardens, green roofs, and more - instead of a giant underground tank that simply holds the water until it can be treated - is considered a national model. The city already has 10 acres of green roofs. But the whole plan is going to take 25 years to implement.
Part of the region's problem stems from the fact that so much of it was built before regulations for storm water existed.
"In the storm-water world, the 1980s was the beginning of the first shot at storm-water management," said Chad Pindar, supervisor of watershed planning and compliance for the Delaware River Basin Commission, an interstate agency. "Anything built prior to the '80s, you can write off."
Gradually, as the problems became harder to ignore, new techniques emerged and regulators responded.
"By the early 2000s, you started to see better management," Pindar said.
But it was aimed mostly at new construction. "The big hurdle is what do you do with Darby Borough? It's already built out," Pindar said.
