The initial report came in just before 4:30 p.m. of a 13-year-old boy who fell into a drainage pipe that feeds into the river. Firefighters have been searching the water near the 5200 block of W. Zoo Drive but have seen only water and no person.
The drainage ditch is about 13 feet wide and two feet deep, with swift moving water.
More than 100 firefighters are involved in the search, which was still going as of 10 p.m. and expected to continue through the night if necessary.
Officials said the boy and some friends had climbed a chain-link fence and were playing in an abandoned maintenance shed near the river when a wooden plank gave way, opening a hole that led into a drainage pipe and the boy fell in. The water was moving fast, about 6-10 mph, officials estimated.
Officials say the effort involves a detailed search along each possible path of travel inside a lengthy closed system of sewage pipes.
Technical gear used in the search includes a camera mounted to a flotation device - similar to a boogie board - tethered to a rope and extended 300 feet down a pipe.
The boy was identified as Jesse Hernandez. After searching more than four hours, firefighters had still not found him and were asking the public for help.
More than 20 members of the Hernandez family were picnicking in the Griffith Park area at the time as part of an annual Easter tradition.
Authorities and the family released a photo of Jesse in the hopes that he may turn up somewhere outside of the pipe system and someone will spot him.
City Bureau of Sanitation officials also responded to the scene to help with the pipe system.