Though most outdoor performances would benefit from being indoors, the new choral/dance work Turbine unquestionably belonged where it was Sunday evening, at the Fairmount Water Works overlooking the Schuylkill - even though Saturday's downpour had cost it one of two planned performances.

Early on, one puzzled over the amorphousness of this site-specific collaboration by choreographer Leah Stein, composer Byron au Yong, and the Mendelssohn Club choir. It seemed to be poetic murmuring, with short congenial melodies suggesting a lamentation recalled from a distant past.

Such was the introduction that began in a grove of trees at the north end of the Water Works park, a 19th-century architectural wonder that was built to deliver clean water to Philadelphia. As the chorus led the audience to the overlook area by the river - in a series of migrations with sung Philadelphia street names and quotations from famous travelers who viewed the site over the years - everything came together in a manner both abstract and direct.

The elements intermingled in ways that mirrored the river's crosscurrents. With ripples outlined by foam and bubbles, the water generally runs eastward, but with the illusion of gentle movement every which way. A turbine effect? Similar to the river, the piece had little fortissimo. Few things stood out from the organic whole for very long.

The choreography included a series of arm movements by the choristers, executed with surprising conviction, while members of the Leah Stein Dance Company looped around with often-circular movements that alternately suggested pleading and sign language from a faraway culture. Musical elements - coordinated by outgoing Mendelssohn Club director Alan Harler with help from assistant conductors - were gently insinuating. Often, you didn't know how it was working on you until you were part of the musical currents, along with the performers.

Visually, Stein had a great sense of foreground and background: Near the beginning, the dancers peered out at the river, their backs to the audience, making you wonder what they were looking at. Then you saw that the orange graffiti on a wall across the river was the exact color as many of the dancers' costumes, some of which resembled those of Asian monks. During a section performed near the parking area, a dozen or so dancers migrated to the cliff overlooking the site, in a highly effective framing device.

The pièce de résistance, though, came when soprano Jennifer Beattie suddenly appeared on the river itself, standing on a motorized pontoon boat, looking like a priestess in a full-length red gown, and singing - through a megaphone - one of Yong's graceful melodies, which was imitated and echoed by the chorus overlooking her.

Part of Turbine's many pleasures was that close proximity to the performers. They intermingled with listeners like a flash mob that kept coalescing and dispersing. And choristers who can seem faceless on a traditional stage turned into an intriguing collection of individual faces and voices. Particularly compelling solos were sung by Shahara Benson. With sometimes-impaired sight lines, height was a plus, one reason Jungwoong Kim stood out among dancers.

The audience took some time to settle into the piece, which didn't telegraph early what you were supposed to get from it. Usually in music, you at least have clear-cut signals from the mixture of major and minor keys. Yong's music trafficked in the middle of all that. But it's hard to imagine that anyone walked away from Turbine unchanged, and in ways that might only become fully clear later. Maybe much later.