The incident occurred in the 16200 block of San Fernando Mission Boulevard at about 3:17 p.m. The victim, identified as Philo Ragni, was a senior at John F. Kennedy High School, L.A. Unified School District officials confirm.
Friends and family laid flowers at a makeshift memorial set up in front of the scene.
A witness who did want to be identified said "everybody was screaming 'Oh my God!'"
Police say the LADWP vehicle was heading northbound when a group of kids riding bicycles suddenly crossed the street westbound.
"It just looks like it's a traffic accident. It's an unfortunate traffic accident," said LAPD Lt. Manuel Romero.
Vera Rabadi says her son was riding alongside Ragni when he was killed.
"I'm still in shock. It could be my son down there," she said. "These kids are BMX, they don't have no helmets, no brakes and it scares the hell out of me!"
The LADWP says it is working with the LAPD on the investigation.
"LADWP extends heartfelt sympathies to the family and loved ones of the deceased," the agency said in a statement.
There are no indications that the LADWP driver was under the influence of drugs or alcohol at the time of the incident. Romero said it is unlikely that the investigation will lead to charges.
The LAPD says an MTA bus that happened to be driving by at the time of the crash likely caught the incident on surveillance cameras. The investigation is ongoing.