The 40-ish man, whose name has not been released, suffered life-threatening injuries, according to the Niagara Parks Police Service.
But other officials later said he was expected to survive his injuries, which included a collapsed lung, a flail chest and numerous gashes.
Authorities had one simple explanation for why the man survived, before he was pulled to safety during an almost two-hour rescue involving multiple emergency agencies on the Canadian side.
"He was very lucky," Niagara Falls Fire Dept. Platoon Chief Dan Orescanin said. "One thing that helped get him to shore, he just happened to come down the river into an eddy, and that enabled him to get out. If he had been in the main current, he wouldn't have survived."
The man's injuries showed the evidence of the rocks he struck on the way down, as he suffered gashes to his head and shoulder, a collapsed lung and chest and rib injuries.
"By the time we got to him, he was hypothermic," Orescanin said. "He was shaking like a leaf."
Authorities said the man willingly jumped into the water above the falls, but they did not say whether they believe it was a suicide attempt or a stunt.
But the man's surviving both the plunge and the powerful river currents defied the odds.
"Oh, No. 4," historian and author Paul Gromosiak, of the Town of Niagara, said when told about the news. "This would be the fourth person to go over the falls unprotected."
The three previous survivors were a 30-year-old western Ontario man who attempted suicide in 2009; U.S. resident Kirk Jones, who was accused of staging a stunt in 2003; and 7-year-old Roger Woodward, who survived an accidental 1960 plunge wearing only a life vest, after his boat capsized in the upper Niagara River.
Gromosiak claimed that Niagara Falls attracts about one or two suicide attempts per week, including people who are rescued in the water above the falls.
"There are odds that you will survive, but they're so miniscule that it's impossible to comprehend," he said. "Out of the thousands who have come here to commit suicide over the years, we have two who did survive."
While authorities still haven't pinpointed a motive, if the plunge indeed was a suicide attempt, no charges are expected to be filed. Authorities previously have said that an attempted suicide does not violate Ontario law.
The incident began when witnesses told the Niagara Parks Police Service that they saw a man climb over a retaining wall about 6 to 10 meters above the falls and then deliberately jump into the water.
Seconds later, he surfaced in the lower Niagara River basin adjacent to the Journey Behind the Falls observation platform, police reported.
A Niagara Parks Police officer located the man along the rocky Ontario shoreline after he collapsed in water up to his waist, that agency stated in a news release.
Emergency workers from the Niagara Falls Fire Department, the Niagara Regional Police Service and Niagara Emergency Medical Services all participated in the rescue effort.
Seven firefighters rappelled themselves down onto the shoreline, where the man was pulled to safety in a basket by an aerial fire truck.
He then was transported to a waiting air ambulance, before being taken to a Hamilton-area hospital, where no further word was available about his condition.
"Everything went well," Orescanin said of the rescue. "The guys were great. All our training came in handy."
The Niagara Parks Police Service is continuing its investigation into the incident.
