It happened Sunday afternoon at about 2 p.m. when the child's mother and a friend attempted to take the kids across the Kings River in Fresno County to a rock.
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Deputies were called to the bottom of Pine Flat Dam to search for the 8-year-old girl and 4-year-old boy who went downstream.
The Fresno County Sheriff's Department helicopter, Swift Water Rescue team and other agencies joined forces in the search.
An hour later, deputies found the 8-year-old girl underwater.
The Kings River remains closed to the public in Fresno County.
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Rescue crews wrapped up their search efforts at sundown and started looking for the boy again Monday morning.
"This is a very sad day for all of us, and it's a very sad day for the family," says Lt. Brandon Pursell. "We need people to listen to us, pay attention."
Officials say if parents allow their children in the water, they could face child neglect and child endangerment charges.