Witnesses say the vehicle, which appears to have been traveling from a nearby Costco store, failed to make a turn on the ramp and instead drove straight up a curb, through a fence and into the pond around 2:40 p.m.
The car became submerged in the pond, which CHP Officer DJ Sarabia said serves as a holding tank for the Santa Clara Valley Water District.
Bystanders and emergency responders tried to reach the driver, who was trapped inside, but were unsuccessful.
Sarabia said the woman, whose identity has not yet been released, was removed from the vehicle by divers around 3:15 p.m. and taken to Santa Clara Valley Medical Center, where she was declared dead.
Although initial reports suggested there might be more than one person in the car, Sarabia said divers found only one victim.
According to Sarabia, investigators have not yet determined whether the woman was speeding or what caused the accident.
The ramp and surrounding area has been cleared and is open to traffic.
(ABC7 News contributed to this story)