Nicholas Grill, deputy head at Churcher's College in Petersfield, Hampshire, was walking along the flooded stream at West Harting with his three boys Jonathan, 10, Christopher, 7, and William, 4, when the youngest fell in, upstream from a bridge and a tunnel carrying flood water.
Fearing that his son had been washed into the tunnel Mr Grill, 39, began searching for him, only to fall in and be washed back through the tunnel and into a weir pool below.
"Jonathan said his father had fallen in while trying to find William and had been sucked under the tunnel," a neighbour, Ray Heather, said.
"He said he had been holding his daddy's hand for a long time while he was in the water. But he couldn't hold on to him any longer. He had tried to get him out but he was too heavy.
Christopher ran home to alert his mother, Judy. She drove to the scene to find her husband lying unconscious face down in the water.
Ambulance paramedics and a doctor tried to revive him but he was found to be dead on arrival at St Richard's Hospital, Chichester. A spokesman for Sussex police described the incident as "a tragic accident".
As the south of England was soaked with another day of continual rainfall the National Rivers Authority put a number of rivers on flood alert last night. About 12 rivers in the South-west were on yellow alert, with the River Dart in south Devon on amber alert.
The River Avon burst its banks after a dead bullock got caught under a hump-back bridge in the already swollen water. NRA staff had to pull the animal's carcass out of the river at Loddiswell near Dartmouth, Devon.Reuse content