For India's devout Hindus, the sacred River Ganges is always clean and always pure -- even if its waters are a toxic mix of human sewage, discarded garbage and factory waste.
The belief that the Ganges washes away sin entices millions of Hindus into the river each year, and huge crowds of pilgrims are currently passing through the town of Haridwar for the three-month Kumbh Mela bathing festival.
But concern over pollution along the length of the 2,500 kilometre (1,500 mile) river is growing, and the city of Kanpur -- 800 kilometres downstream of Haridwar -- is the site of one of the worst stretches of all.
Factories in the industrial city chug millions of litres (gallons) of polluted water into the river daily, rubbish forms into solid floating islands, and a foul smell wafts over the water's murky surface.
The situation is "acute and critical", said D.K. Sundd, executive director of the Sankat Mochan Foundation, a non-profit group working to clean up the river.
"The problem is worst in Kanpur. The city generates nearly half the volume of sewage and industrial waste as compared to the fresh water flow in the Ganges," he said.
Most communities located on the river from the Himalayas to the Bay of Bengal in Bangladesh lack proper sewage treatment facilities, and the river has for years "been misused as a convenient sink for raw waste," said Sundd.