"Immersion of idols not only increases pollution levels, it is threatening aquatic creatures including dolphins," said R.K. Sinha, an expert on dolphins.
"Heavy metals in the river water are highly dangerous for rare dolphins and fish," Sinha said.
Guddu Baba, who leads a movement to clean the Ganga, said modern idols made of non-biodegradable material had just made matters worse, noting that traditional clay idols did not pose such threats.
"It is an age-old practice to immerse idols in the river but modern idols pose a serious threat to the river and its aquatic life," he said.
Till a decade ago, idols were made of clay and decorated with vegetable colours and other biodegradable material. "Now idols are made of Plaster of Paris, synthetic colours and decorated with non-biodegradable material that are not at all friendly to the river," Guddu Baba noted.
He said the immersion of hundreds of idols had added around 5,000 litres of paint, hundreds of kg of Plaster of Paris and toxic synthetic material into the river.
Sinha expressed serious concern over the government neglecting the issue and not prohibiting idol immersion in the river. The authorities should create awareness of the need to immerse idols elsewhere to check river pollution, he said.
"Polythenes, plastic items and synthetic clothing of the idols are dumped into the river, which was not the case earlier," he pointed out.
A recent study on the Ganga revealed that apart from the threat to the rare dolphins, pollution has led to a decline in the number of fish as well. The water has become unfit for bathing, leave alone drinking.
A report released by international NGO WWF earlier this year said that the Ganga was among the 10 big rivers in the world facing extinction.