Mark Munro and James Robertson had been drinking before the fatal attack on Russell Robertson at Bainsford Bridge,in Grahams Road, Falkirk in the early hours of May 29 last year.
The victim struck his head as he fell and fractured his skull and his body was retrieved from the Forth and Clyde canal more than an hour later.
A court heard that Russell would have been unconscious when he went into the water.
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A judge told the pair at the High Court in Edinburgh: “Russell Robertson will be missed forever by his family.”
Lady Carmichael said she had received a victim impact statement which very clearly set out the daily grief caused by the 27-year-old’s death.
The judge said: “You both participated in an act the consequences of which were unpredictable and which undoubtedly carried some degree of risk.”
Lady Carmichael said: “No one may ever know why you pushed him over the railings. The whole incident was over within less than a minute.”
The judge said that both Munro (31) and Robertson (27) had been drinking for some hours before the fatal attack, but added that provided “no mitigation or excuse of any kind”.
Munro, Temple Denny Road, Denny and Robertson, of Aldour Court, Pitlochry earlier stood trial accused of the murder of Mr Robertson, formerly of Carradale Avenue, Tamfourhill, which they denied.
Both were found guilty of the lesser offence of culpable homicide by a jury at the High Court in Glasgow.
Lady Carmichael said no weapons were used in the killing and told Munro and Robertson: “Both of you have essentially lived useful, hard-working, productive lives until now.”
All three men had been at the Warehouse nightclub in Falkirk prior to the incident and were making their way home after 3am.
Robertson was approached by the victim and they got into a struggle and Munro, who had been on the phone, became involved.
Witnesses reported seeing the men apparently “play fighting” before the pair grabbed the victim by the legs.
Nightclub security manager Kevin Gibney said he saw the victim disappear off the bridge. He said: “One minute he was there and the next he was gone. The two gentlemen ran across the road.”
Robertson had admitted being involved in the initial struggle with the victim, but claimed he was walking away towards Falkirk when the man went into the water.
Defence counsel Derek Ogg QC, for Munro, said he wished to pay tribute to the family of the deceased for “their dignified conduct throughout the case and since”.
He said Munro had expressed “great remorse”. He added that the father-of-two was “a hard-working, good family man”.
He said the fatal incident was “an example of a momentary and catastrophic error of judgment” by Munro which had been triggered in the first place by going to the aid of his friend.
Mr Ogg said: “He collapsed in shock on hearing of the death of Mr Robertson and showed great stress thereafter in his dealings with the police.”
The defence counsel said that the author of a background report on Munro had concluded that he was at low risk of re-offending.
Lorraine Glancy, counsel for Robertson, said his wife was expecting their first child in the next two months.
She said: “A period of imprisonment will result in his wife being alone when she gives birth and he will be an absent father for a significant period of his child’s early years.”
But she accepted that given the tragic death of Mr Robertson there could only be one sentence in the case.
Miss Glancy said: “The location of the incident caused events to unfold in a way that had catastrophic consequences for the deceased.”