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.
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.
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.
City Bureau of Sanitation officials were also responding to the scene to help with the pipe system.