The Ganges river has joined with a tributary in the Indian state of West Bengal, inundating a large area of land, officials say.
By Subir Bhaumik
BBC News, Calcutta
The Ganges breached an embankment on Saturday and has started flowing into the Pagla.
Local officials, who earlier denied the possibility of the two rivers joining, now say 20 villages are endangered.
"About 100 shops and scores of houses in these villages have been washed away," said district engineer PK Ray.
Mr Ray, executive engineer in the Maldah district of West Bengal, where the breach occurred, said: "It has washed away a huge tract of land."
The Pagla originates from the Ganges and then flows into the Mahananda river on India's border with Bangladesh.
Bengal's leading river expert, Kalyan Rudra, predicted this week that the Ganges could merge with the Pagla.
Government officials had earlier said there was no threat
"The merger has occurred much earlier than I thought," said Mr Rudra.
"The bed of the Ganges has risen and its water started seeping into the Pagla through smaller canals."
Tarikul Islam, the convenor of the Ganges erosion prevention committee, said: "Nothing has been done to stop the erosion, though we kept saying the danger was real.
"We are destroyed, hundreds of peasants will become paupers."
Mr Rudra said there was now a chance of more inundation downstream.
He said the Farakka barrage, which is used to control the flow of river water into India and Bangladesh, might become useless.
Others say rivers such as the Bhagirathi, located downstream in India and fed by the Ganges, will have less water, possibly affecting operations at Calcutta's main port.
West Bengal government officials admit this is an emergency but did not say what measures were being taken to provide relief to the affected villagers.