The incident happened around 2.30 pm at Ranisundari ghat when the boat was on its way from the west to the eastern bank of the river, which is a major tributary of the Manas river, police said.
All the passengers swam safely to the shore. However, a motorcycle and around 25 bicycles which the passengers had loaded onto the boat sank.
The reason for the incident has not been established as yet, police said.