Kirk Jones, 40, of Canton, Mich., is the first person known to have gone over Niagara Falls without safety devices and lived. He could be fined $10,000.
Brian Merrett, chairman of Niagara Parks Commission, called the stunt "stupid."
"Our people went down in the gorge and got him," Merrett said. "That's why we don't condone this. It puts all of our people: the fire department, the paramedics everyone at risk to do the rescues. That's why were are so adamant about stunting."
Stunned tourists described seeing Jones float by on his back Monday in the swift Niagara River, go headfirst over the churning 180-foot waterfall and then pull himself out of the water onto the rocks below.
"He just looked calm. He just was gliding by so fast. I was in shock really that I saw a person go by," Brenda McMullen told WIVB-TV in Buffalo.
Jones was not seriously injured and remained hospitalized in Niagara Falls in stable condition.
Police said they were ruling out the possibility it was a suicide attempt.
"We're investigating it as an intentional act," Niagara Parks Police Inspector Paul Fortier said.
He said psychological tests were being conducted at the hospital.
Fortier said police have a videotape of the act that they believe was made by someone who accompanied Jones. That person has not been charged.
Water rushes over the falls at a rate of 150,000 gallons per second.
Only one other person is known to have survived a plunge over the Canadian Horseshoe Falls without a barrel or other contraption - a 7-year-old boy who was wearing a life preserver when he was thrown into the water in a 1960 boating accident.
No one has ever survived a trip - with or without safety devices - over the narrower and rockier American falls.
Since 1901, 15 daredevils have taken the plunge in barrels or other devices, including a kayak and a personal watercraft. Ten survived, said Niagara Falls historian Paul Gromosiak, who has written books on the subject.
Suicides are not uncommon at Niagara Falls, although police are reluctant to give numbers.
Lynda Satelmajer, of Brampton, Ontario, said she and her family watched the man as he entered the river and then went over the falls.
"He seemed a bit edgy, kind of jumping around," she said. "He walked over to where we were standing and he jumped and slid down on his backside and went over the brink.
"It was really freaky, actually. He was smiling."
Copyright 2003 Associated Press. All rights reserved.