The elderly man was tending to horses in Perosa Argentina, a small town in the foothills of the Alps west of Turin, when the bank of the river gave way beneath his feet, causing him to fall in.
His son, who was with him, told the civil protection agency his father had been carried away by the surging waters.
Several hundreds of people had to be evacuated from their homes and many roads, schools and businesses were closed across the northwestern regions of Piedmont and Liguria as the River Po and its tributaries burst their banks in numerous places.
Production of Nutella and other chocolate treats was stopped at Ferrero Rocher's factory in Alba as a precautionary measure because of the high level of the nearby Tanaro river.
The Po was a metre (3.25 feet) above its danger level in Turin and most bridges in the city had been closed on safety grounds.
Two boats normally used for tourist trips were ripped from their moorings and drifted downstream before coming to rest against a bridge.
With heavy rain expected to continue over the weekend, the rising waters have revived painful memories of 1994, when 70 people were killed by flooding in the region.