Madousou would have been a 4th grader this year. Her family says she loved Hannah Montana.
Lt. John Walker says the current of the river was too swift for the girl to swim when she went in around 8:00 Tuesday night.
Madousou walked a trail with eight friends in a wooded area along 51st Street in Southwest Philadelphia.
The children were playing in the water but Madousou, who could not swim, was carried away.
"She's about four foot in height, about 100 pounds, a thin build. So, to go into the river with the currents the way they were last night, the weather conditions, she was pulled out pretty quickly," said Lt. Walker.
Her parents say they did not know Madousou and her sisters had come to the river to play, saying they usually went to a city pool. However that was not an option, since the city pools have recently been drained and closed.
Police said late Tuesday that the effort had turned from a rescue mission into a recovery mission, meaning the search is for a dead body.
Investigators used underwater cameras and sonar equipment during the search Wednesday.
Her mother, Mawa Trawally, a native of the African nation of Liberia, was at the water's edge while the search continued.
"All she wants right now is to see the body of her child," said an interpreter.
Residents say this is the second time a child has drowned in this part of the river in recent years, adding the water is rougher than it looks on the surface.
"On top it doesn't look rough, but when you get under the currents, it'll take you under like a tornado," said Mike Hero of Southwest Philadelphia.
"Kids should not be in this area, they should not be along the Delaware or the Schuylkill River. If they go in this water, 90-percent of the time they don't come out alive," said Lt. Anthony Napoli of the Philadelphia Police Marine Unit.
Police are investigating this as an accident. Lt. Walker said police have spoken to the children who were with the girl at the time of the accident, saying they are very traumatized by what happened.
The Associated Press contributed to this report