A portion of the lock gate no. 17, which was being repaired, got broken and fell into the river.
Along with it, two labourers also fell down into the water but one of them was rescued, the police said.
The second labourer, identified as Bhudeb Mondal, could not be found and efforts were on to trace him.
The 2245 metre long barrage with rail cum road bridge across the river connects south Bengal and north Bengal.