Reports identified the body as Salvatore Montagna, although police wouldn't immediately confirm or deny the identity.
Nicknamed "Sal The Iron Worker," he owned and operated a successful steel business in the U.S.
The FBI once called him the acting boss of the Bonanno crime family — prompting one of New York's tabloids to dub him the "Bambino Boss" because of his rise to power in his mid-30s.
His death is the latest in a series of Mafia-related killings and disappearances over the last two years. Montagna was considered a contender to take over the decimated Rizzuto family.
Montagna was born in Montreal but raised in Sicily and, although he moved to the United States at 15, he never obtained U.S. citizenship.
The married father of three was deported to Canada from the United States in 2009 because of a conviction for refusing to testify before a grand jury on illegal gambling.
He pled guilty to the minor charge, but it made him ineligible to stay in the U.S. Montagna had no criminal record in Canada and re-entered this country without any trouble.
His arrival in Montreal occurred just months before members of the Rizzuto family began being killed.
The FBI had called Montagna the acting boss of the Bonanno crime family, an allegation his lawyer publicly denied.
The Bonanno crime family is one of the five largest Mafia families in New York — one of the notorious criminal gangs that formed the original Commission, along with Al Capone and Lucky Luciano. The Bonannos also had close ties to the Montreal Mafia.
There had been speculation that Montagna had been part of the new Mafia leadership in Montreal and was trying to reorganize the leaderless group.
His death comes just two months after another man with Mafia ties, Raynald Desjardins, narrowly escaped death in a shooting in Laval, a suburb north of Montreal.
Desjardins had close ties to Vito Rizzuto, the reputed head of the Montreal Mafia who is currently imprisoned in the United States.
A rash of killings and disappearances in late 2009 and early 2010 decimated the operation and have robbed him of many of his closest family members.
Rizzuto's father and son were gunned down, as were other friends, while his brother-in-law simply vanished.
Montagna became the latest name on the list.
A provincial police spokesman said Thursday that a private citizen called just after 10 a.m., having spotted a body along the shores of the L'Assomption River.
The same person also reported to local police that he heard gunshots, but Sgt. Benoit Richard said he couldn't confirm just how the victim died.
"When (police) arrived, they saw a man lying near the river, they took him out of the water and started doing CPR with the help of the emergency personnel," Richard said.
"The man was transported to the nearest hospital where he was pronounced dead."
Richard said police will await the results of an autopsy, scheduled for Friday, to determine the cause of death.
CORRECTION: An earlier headline on this story incorrectly spelled the Bonanno surname.
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