Officers would not release the man's name or comment on why he went over the falls. "It's still under investigation," Niagara Parks Police Inspector Paul Fortier said. He said the man is from Michigan. Police said there appeared to be no evidence of foul play.
Suicides are not uncommon at Niagara Falls, although police are reluctant to give numbers.
Niagara Falls historian Paul Gromosiak said he believes the man must have been attempting suicide, given the way he went over and the thin odds of survival.
If it was a stunt, the man could be fined $10,000.
Witnesses described seeing the man float by Monday in the swift Niagara River, go head-first over the churning 180-foot waterfall and then pull himself out of the water onto rocks below.
"[He was] very calm. He wasn't screaming, he wasn't flailing or anything. He was just calm and floating along feet first and went right over the falls," Brenda McMullen said Tuesday on CBS News' The Early Show.
Water rushes over the falls at a rate of 150,000 gallons per second.
"I saw him disappear over the edge of the falls," McMullen's husband Terry said. The Columbus, Ohio, tourists snapped photographs afterward, showing the man dressed in street clothes, apparently lying on the shoreline at the base of the Canadian Horseshoe Falls.
Her husband Terry said it was amazing the man survived.
"He crawled up on rock, which was amazing to me, 'cause I figure he was gone," he told CBS affiliate WIVB Buffalo. "You hear about people practicing for years and doing barrels and stuff. This guy's not wearing nothing but a parka and a hat."
His wife "started crying, because she figured the guy was dead," he added, but then the crowd started cheering.
"Everybody was going crazy," he told Early Show co-anchor Julie Chen. "This is not something you see every day, so it was a pretty amazing sight."
"The guy just basically jumped in the Falls," said witness Diedre Love, of Largo, Md., who was at the Falls with her husband to celebrate their first wedding anniversary. "I saw him go over. He didn't yell or anything."
Since 1901, 15 daredevils have taken the plunge in barrels or other devices, including a kayak and a water jet-powered personal watercraft. Ten survived, said Gromosiak, who has written books on the subject.
The only other person known to have survived a plunge over the Canadian falls without a barrel or other apparatus was a 7-year-old boy wearing a life preserver who was thrown into the water in a 1960 boating accident.
No one has ever survived a trip over the narrower and rockier American falls.
Video shown by the Buffalo television station showed officers walking from the scene with a shirtless man in handcuffs and a blanket covering his face.
Lynda Satelmajer, of Brampton, Ontario, said she and her family watched the man as he prepared to get in the water and then watched him go over the Falls, all in smiles.
"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."
About a dozen daredevils have taken the plunge in barrels or other protective chambers since 1901. About half have survived.
Rescuers descended the gorge in a tourist elevator to an observation deck and reached him from there.
He was taken to Greater Niagara General Hospital for treatment, said police. He was reported Tuesday in stable condition.