Eyewitness Brandon O'Malley says the incident happened around 1:30 p.m.
A 2006 Lincoln Town Car just in front of O'Malley's car crashed through a guardrail as it headed south along the 8500 block of Krewstown Road.
The car plunged some 50 feet into the creek and flipped over.
Two women, both in their 70's, were inside.
The driver of the Lincoln was dead at the scene.
"We have no further information on her and no ID at this point, she was pronounced dead by fire department paramedics," Philadelphia Police Chief Inspector Scott Small said. O'Malley was able to pull the surviving passenger out of the vehicle.
"We ran down there. The lady in the passenger seat, actually, still had her seatbelt on and was face down in the water when we grabbed her out of the car. Immediately, another gentleman that wasn't in the water started giving her CPR," O'Malley said.
The 71-year-old passenger was taken to Aria Hospial Torresdale Division.
"She is in extremely critical condition, fighting for her life at the hospital," Small said.
Witnesses say the car was moving very slowly on the winding road at the time of the incident.
. There is no word yet on how the driver lost control of the vehicle.