Police received a 911 call at about 5:39 p.m. on Wednesday by a passerby about the child in the area of the Upper Pond of Grace Lord Park.
While searching the area, cops discovered a 35-year-old female and an 11-year-old child in the water. The unattended child, age 6, was unharmed. She was standing by the bank of the Rockaway River yelling out for her mother.
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Police discovered footprints leading down to the water and found the bodies on the top of the waterfall that the river flows into.
The identities of the mother and son had not yet been released.
The Morris County Prosecutor's Office was leading the investigation.