Reports identified the body as Salvatore Montagna, although police would not comment.
The FBI once called him the acting boss of the Bonanno crime family – prompting one of New York’s tabloids to call him the “Bambino Boss” because of his rise to power in his mid-30s.
Nicknamed “Sal The Iron Worker”, he owned and operated a successful steel business in the United States.
Montagna’s death is the latest in a series of Mafia-related killings and disappearances over the last two years. He was considered a contender to take over the decimated Rizzuto family.
A provincial police spokesman said that a private citizen called after seeing a body along the shores of the L’Assomption River. He also reported hearing gunshots.
A post mortem is being held to find the cause of death.
Montagna was born in Montreal but raised in Sicily. He moved to the US at 15 but never obtained citizenship.
The married father of three was deported to Canada from the US in 2009 because of a conviction for refusing to testify before a grand jury on illegal gambling.
He pleaded guilty to the minor charge, but it made him ineligible to stay in the US. Montagna had no criminal record in Canada and re-entered without trouble.
His arrival in Montreal occurred only months before members of the Rizzuto family began being killed.
There has been speculation that Montagna was part of the new Mafia leadership in Montreal and was trying to reorganise the group.