A stunned college student captured the moment when a mother laughed as her boyfriend threw her 4-year-old son off a 27-foot-high (8.22m) bridge in Washington state.

The boy survived the incident, but his body can be heard loudly smacking the water. "When he landed, I thought he broke his neck," said witness Brianna Jones, 19.

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"I looked up and was like, 'Whoa, there's a kid up there.' This guy just threw the kid right off the bridge, and he smacked down right on his neck," Kaylub Fawley, the student who took the video, told the Washington Post.

"As soon as that kid hit the water, he went straight down and straight back up," Fawley, 18, added, noting that the boy vanished underwater before his life jacket yanked him back to the surface. "He was just screaming the whole time."

Swimmers contacted police, and officials are deciding whether or not to press charges against the mother.

Young adults do jump from the Devonshire railroad bridge during the summer over a quiet section of the Wynoochee River in the town of Montesano. But they must go straight in; a poor entry or a bad fall can cause serious injuries.

"We've been jumping for so long, we know how to do it without getting hurt," Fawley explained. "But we also know the consequences. You have to be careful."

Jones said the boy nearly struck his head on one of the steel girders as he fell, and barely missed large boulders in the water. The mother, who has not been identified by police, continued to act like it was a joke even after her son hit the water.

"We all started freaking out and yelling at her, 'You can't do that!'" Jones recounted. "She said, 'He's my kid, and it's none of your business.'"

Police and an ambulance arrived soon after and the boy was treated on the scene and released back to his mum. But a spokesman for the Grays Harbor County Sheriff's Office told the Post that the local prosecutor may charge the mother with child endangerment, an offence that is classified a class C felony. The boyfriend is also under investigation.

Fawley posted his video to his Facebook page with a note: "This is what happens when some parents just don't care." It quickly went viral.

"You just don't throw a kid off a bridge," Jones said. "It made me so mad. People like that shouldn't have children."