The teen, Sebastian Sanon, 16, was swept away as he tried to cool off in the water, a friend told WCAU-TV. The two boys and two other friends had been riding their bicycles along the creek.
"He just went to step in a little bit and it took him out," Robert Brown said. "At first he was trying to swim to the side and I tried to tell him to swim diagonal to fight the current."
Rescue workers searched the creek for hours without finding the missing teen or his body. The search was to start again Wednesday morning.
The 22-mile creek begins in the Philadelphia suburbs before crossing northeast Philadelphia and emptying into the Delaware River.
Heavy rain early Tuesday raised the level of the creek by 6 feet, the U.S. Geological Survey reported. The rain caused flooding problems around the Philadelphia region.
In July, Brandon Boyle, 13, drowned in Pennypack Creek, also after heavy rain. In late July, two 18-year-olds, Dan Tadrzynski and Perry Krewson, were swept away by the current in Neshaminy Creek, which empties into the Delaware in Bucks County a few miles upriver from the Pennyback.