Rescuers found Jesse Hernandez on Monday when they lifted a manhole cover to drop a floating camera into the complex of pipes,
CNN affiliate KCBS reported.
He was safe and returned to his family after being decontaminated and evaluated at a hospital.
The harrowing incident began during an Easter gathering of 20 family members at Griffith Park in Los Angeles. The boy was playing in an old shack in the late afternoon when boards covering a pipe opening gave way.
He plunged about 25 feet into the sewer network.
"I was playing, and I didn't see that it was a little piece of wood, and I stepped on it, and I just fell down -- the current took me," Hernandez told KCBS.
"I was thinking, like, I was going to die."
"I stopped myself because the little tunnel started getting smaller, so I just stood up fast," he said. "There was this big circle thing, and I just went up. Like, I just stood up there for the last 13 hours."
The Los Angeles Fire Department said about 2,400 feet of pipe had been inspected when a manhole cover in a highway median was opened so a camera could be dropped into the pipes.
That's where Jesse was found, about two-thirds of a mile from where he fell, KCBS said.
'Big burly guys so filled with joy'
The network of pipes runs along the Los Angeles River and crosses under freeways. The environment is "extremely dangerous and toxic," Los Angeles Fire Department spokesman Erik Scott said. The fluid in sewage pipes varies in depth and moves at about 15 mph.
Rescuers and municipal officials used floating cameras from third-party vendors to look through the system of pipes, Scott said. Such cameras are regularly used to inspect pipes for repair.
"The vendor cameras are able to both float at the top of the pipe and crawl along the bottom. The number of pipes providing different travel paths required a systematic approach to ensure each was searched," said Margaret Stewart, a fire department spokeswoman.
The rescuers were elated, Scott said.
"I've never seen a command post of big burly guys so filled with joy," he said.
Hernandez recalled his cell phone conversation with his family to KCBS: "Mom, I'm alive. Come pick me up."
The mayor's crisis response team thanked the Hernandez family for its support, patience and optimism.
The fire department noted the "excellent teamwork" among agencies -- which include fire and its urban search and rescue, police, highway patrol, city recreation and parks, park rangers, the water and power department, and experts from the sanitation, engineering and contract administration bureaus.