River won the Final championship, the second of two in the season, with a five-point margin over Boca Juniors, Estudiantes and Godoy Cruz in joint second place. San Lorenzo won the season's first title, the Inicial.
Unbeaten in their last seven matches, River notched their eighth successive victory at the Monumental, their formidable home form proving decisive in their campaign.
Boca leapt into second place thanks to their fifth successive win, 1-0 over Gimnasia. Estudiantes lost 2-1 at Tigre and Godoy Cruz beat Racing Club 2-1 in Mendoza.
The home side pulled further ahead at a 25th minute corner with right back Gabriel Mercado scoring with a diving header at the far post for his second goal in two matches.
"The team showed great personality, we always took the game to our opponents. The (2-1 away) win against Boca was a turning point and we felt we could fight for the championship," Mercado told reporters after the title ceremony.
Mercado, whose form earned him a place in Argentine's provisional 30-man World Cup squad, added: "If I'm left out of the (definitive 23-man) list I'll feel proud and grateful for having been considered."
Holding midfielder Cristian Ledesma scored a brilliant third goal in the 63rd minute with a shot from 30 metres and Cavenaghi made it four seven minutes later from Teofilo Gutierrez's perfect through-ball.
World Cup-bound Colombia striker Gutierrez put the icing on the cake, finishing off a counter-attack in stoppage time with the fifth from playmaker Manuel Lanzini's pass.
It was a record eighth title as River coach for Ramon Diaz, in his third spell at the club where he shone as a striker in the 1980s.
It was also River's first title since 2008 when Diego Simeone, whose Atletico Madrid were crowned Spanish champions on Saturday, was in charge and will help heal the wounds of the Argentina giants' relegation - their first - three years ago.
Estudiantes had a chance to snatch the title going into the final round of matches but were unable to send veteran captain Juan Sebastian Veron into retirement on a winning note.
Godoy Cruz's victory secured their top flight survival, having flirted with relegation, which is measured by teams' average points over three seasons.
Atletico Rafaela, who won 2-1 at Arsenal, and Colon, who scored in added time to beat Olimpo by the same score, will meet in a relegation playoff on Wednesday after both finished with the same three-season points average.
Argentinos Juniors and All Boys have already been relegated.
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