THE PHILADELPHIA Water Department needed to build a 4 million-gallon underground basin in Manayunk to capture stormwater and prevent sewers from overflowing into the Schuylkill.

The city's Parks & Recreation Department and the Manayunk Development Corporation needed to replace the crumbling Venice Island playground with a performing-arts center to serve as a neighborhood cultural magnet.

In the course of a few jaw-dropping minutes at a community meeting, a marriage was made between strange bedfellows.

Tomorrow, 10 years after the wedding, PWD's lake-size underground storage basin and Parks & Rec's Venice Island Performing Arts & Recreation Center will make their public debut.

During a preview tour, Leo Dignam, Parks & Rec's deputy commissioner for programs, stood among flat granite boulders in the children's spray garden and laughed when the sprinklers suddenly cut on and showered him.

"This place is amazing," he said. "Even though it took 10 years, it's worth the wait."

The $46 million development, nestled between the Manayunk Canal and the Schuylkill, includes a $4.5 million, 250-seat, state-of-the-art theater, built above the river's 100-year flood level, that will house drama, dance, concerts and corporate conferences.

Gigantic "Venice Island" lettering practically shouts a welcome across the canal to Manayunk mainlanders.

Alongside the theater is an outdoor amphitheater with lush grassy tiers. Absorbent rain gardens are everywhere.

The PWD pump house is built over the 4 million-gallon underground basin - longer than a football field and 25 feet deep.

"We'd been looking into having a theater here since the '80s," said Kay Sykora, the Manayunk Development Corporation's founding executive.

Michael Rose, managing partner of the Manayunk Brewing Co., said: "But even if we could have built a theater, we didn't think we could sustain it. We didn't want a white elephant."

Sykora said: "When we started having meetings about Venice Island, the Water Department was talking about an underground tank and a path around the tank.

"And then one day, Joanne Dahme, from the Water Department, showed up and said, 'How about if we build a theater?' "

Sykora's eyes turned dreamy, and she said: "It was like I was hearing a disembodied voice saying, 'You want a theater? Can we build one here?' I went, 'Yes!' "

Federally mandated by the Clean Water Act to build the underground basin, PWD paid for the theater, too.

Parks & Recreation, which serves 5,000 children in its performing-arts programs, will pay for maintenance, eliminating Rose's "white elephant" worries.

"This is urban life at its best, right here," Rose said, beaming.

Jane Lipton, executive director of the Manayunk Development Corporation, agreed.

"We have the only canal and towpath in the city," she said. "We have vistas of the river. And for people who say, 'You can't park in Manayunk,' I want to stand in the middle of our new, well-lit, 180-space Venice Island parking lot and shout, 'Yes, you can!' "

Gazing up at the gleaming, new performing-arts center, Lipton said: "Every day, 70,000 people can see this from the highway. We could have entertainment here every night. This will establish Manayunk as a destination. You have to be willing to dream."