The bodies were found shot and tied up in a suburb of Aleppo.
Reuters reported Tuesday that at least 65 are believed dead, though the New York Times reports the toll may be up to 80 people — mostly men in their 20s and 30s.
In a YouTube video believed to be filmed in Bustan al-Qsar, after the bodies were transported from the river, a man claims the men were killed because they were Sunni Muslims, but the motive is still unclear.
Aleppo is divided between Syrian government and opposition forces. The mass killing is the latest in a conflict that has left at least 60,000 people dead.