India's Ganges river at the city of Allahabad has been flushed with fresh water from upstream to improve bathing for millions at a key Hindu festival.
Some 70m devotees are expected over the 45-day festival
Some holy men, or sadhus, had threatened to boycott bathing in waters they considered too polluted.
Five million devotees have already gathered on the banks of the river for the 45-day Ardh Kumbh festival.
Sunday and Monday are considered auspicious bathing days as the sun enters the Tropic of Capricorn.
Hindus believe that bathing at Sangam, the confluence of three holy rivers - Ganges, Yamuna and the mythical Saraswati - can wash away their sins.
Water from canals and dams upstream was released to clean the river at the northern city of Allahabad to pacify the sadhus, some of whom had threatened to kill themselves.
Some Sadhus were angry at the pollution of the water
Local official RN Tripathi said: "We are trying to win them over. There is enough fresh and clean water in the Ganges and it is fit for bathing."
Offerings cast into the river and waste and sewage dumped into it have polluted the river over many years.
Astrologers are divided on the most auspicious time of alignment but millions of Hindu devotees are expected to bathe late on Sunday and early Monday.
The Ardh Kumbh Mela, which began on 3 January, is one of the largest gatherings on Earth, with more than 70m Hindus expected to attend over the 45-day period.
It is a smaller version of the Kumbh Mela, or Pitcher Festival, which takes place every 12 years.
According to Hindu mythology, gods and demons fought a celestial war over a pitcher of divine nectar. Allahabad is one of the four towns where drops of nectar fell during the battle.