Some of the corpses were half-burnt, and many just skeletal remains, said Saumya Agarwal, the chief of the riverside district of Unnao where the bodies were recovered. "Most of them are believed to be very old."
In Hinduism and some other Indian faiths, funerals are conducted by cremation. The ashes are then scattered into rivers.
The Ganges, Hinduism's holiest river, is dotted with numerous cremation spots along its shores.
A full Hindu funeral costs up to 7,000 Rupees (around $110), Agarwal said. Sometimes, poor people cremate their dead with limited firewood as a ritual and float the partly-burnt bodies into the river, she said.
Many people also bury dead children and unwed women in graves along the river.
"These bodies and decomposed skeletons from dismantled graves surfaced when the water level dropped as low as to the riverbed," she said. All remains have now been buried in the riverbed at the same site, Agrawal added.
The Ganges, India's most sacred and polluted river, originates from a glacier in the Himalayas.
It passes through Varanasi, one of the most sacred Hindu centers.
As part of his executive plans, India's Prime Minister Narendra Modi has vowed to clean up the Ganges -- called the Ganga in Hindi -- with a range of measures, including a special ministry and a fund.
Efforts to contain river pollution date back to 1986, but little success has been achieved.
India has also launched what it calls a National Mission for Clean Ganga involving various stakeholders, including religious figures and environmentalists.