Authorities gather near the banks of Apple river in Somerset, Wisconsin, after five people were stabbed while tubing. AP

A Minnesota teenager died and four other people were critically wounded after being stabbed while "tubing" down a river in the US state of Wisconsin, authorities said.

The victims and the alleged assailant, a man aged 52, were all tubing — a recreational activity in which people ride inflated rubber structures on water — down Apple river when the attack occurred, St Croix Sheriff Scott Knudson said.

A motive was not immediately clear and investigators are working to determine if the victims and the alleged attacker knew each other. They were tubing with two different groups that included about 20 people.

Police said the attack occurred on Saturday afternoon.

“We don’t know yet who was connected to who, who knew each other or what precipitated it,” Mr Knudson said.

The river, near the town of Somerset Wisconsin, is accessible by road but the knife attack happened in a more remote area.

Deputies waded about 100 metres through waist-deep waters to reach the victims, the New York Times reported.

The alleged attacker was arrested about an hour and a half later at the exit point of the tubing area downstream.

“Thank goodness a witness had taken a photo of him,” Mr Knudson told the Minneapolis Star Tribune. “Another witness located him at the exit of the tubing area, where he was taken into custody.”

A boy, 17, from Minnesota died. Two of the other victims were flown to a hospital in St Paul, Minnesota, while the other two were taken there by ambulance.

All four surviving victims — a woman and three men all believed to be in their 20s — were in critical condition.

The names of the victims and the alleged assailant were not immediately released but St. Croix County jail records showed that a man aged 52 was being held without bond on suspicion of first-degree homicide, four counts of aggravated battery and four counts of mayhem.

Mr Knudson said he expects charges to be filed early this week.

The Associated Press contributed to this report