It happened at 30 Richman Plaza in the Morris Heights section of the Bronx.
Witnesses said the River Park Tenants Association was holding an event for local families, when Ebrima Wally climbed over a guard rail along the water line.
Emergency responders arrived and pulled the boy from the river.
They say by then he was unconscious and unresponsive, after having been underwater for about 30 minutes.
The boy was taken to Bronx Lebanon Hospital, where he was pronounced dead.
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