R OUND AND round the baggage carousel at London’s Heathrow airport goes a battered cardboard box waiting to be claimed by its owner, a passenger from Delhi: “PLEASE KEEP THIS SIDE UP! GANGA JAL—HOLLY WATER.” For many Hindus the world over, nothing is more holy or pure than Ganga jal, or water from the Ganges.

The whole river—from Himalayan glaciers across the vast North Indian plain to the filigree delta on the Bay of Bengal—is worshipped as a life-affirming goddess. The spiritual potency comes not from the Ganges’s 2,500km length, which falls short of the world’s longest rivers. Rather, its basin supports half of India’s population of 1.3bn (plus nearly the entire population of Nepal and much of Bangladesh’s). For its water and fertile sediment, no river is more important to humanity. And so for centuries Ganga jal has marked births, weddings and deaths. Scores of cremations take place daily on the riverside ghats in the city of Varanasi alone. Between January and early March, a temporary city sprang up on the banks of the river near Allahabad (recently renamed Prayagraj) for the Kumbh Mela festival, in which a staggering 240m devotees took to the river to wash away sins and human ailments.

Yet the Ganges is likelier to add to the ailments than cure them. For decades, declining water volumes have been a growing worry, as hydropower dams have proliferated, wanton irrigation and industrial schemes have drawn water away and the annual monsoon has become more fickle. Three months before this summer’s monsoon, the Ganges is a thin meander, much of its bed exposed, as it passes through Kanpur, the biggest city along its course.

Low flows not only harm the livelihoods of fishermen and farmers downstream. They also degrade water quality. Sewage is pumped raw into the stream. Levels of fecal coliform bacteria are off the chart. Tests from the Yamuna, a tributary which flows through Delhi, have found 1.1bn such bacteria per 100 millilitres—nearly half a million times the officially recommended limit for bathing. No wonder “Delhi belly” is so prevalent. Victor Mallet describes in “River of Life, River of Death” how the Ganges system appears to be a conduit for bacteria increasingly resistant to antibiotics.

Alarmed at the state of the Ganges, some holy men have spoken out. In October G.D. Agrawal, an environmental engineer turned guru, fasted to death as a protest. Despite such dramatic gestures, too few Hindus accept that the Ganges’s holy waters are sullied. Civic pressure to clean up the river remains slight.

To his credit, Narendra Modi, the prime minister, declared a clean Ganges a priority when he came to power in 2014. It was a nod to his Hindu-nationalist following. He promised $3bn and new plants to treat sewage and industrial waste. Five years on, progress is disappointing. In Varanasi, the focus is on razing a rambling old quarter to provide vistas for visiting VIP s, rather than on cleaning up the river.

As for Kanpur, a city of Dickensian leather factories, the picture is dystopian. The river stinks. It is not just sewage that goes untreated into the Ganges, among whose pools children play. So, too, do effluents from the 300-plus tanneries, most notably chromium, a toxic heavy metal. When Banyan visited, the tanneries were supposedly closed to spare bathers at the Kumbh Mela 200km downstream. Yet in one ancient factory, huge wooden vats were still turning, and workers were carrying slopping buckets of chemicals around. Meanwhile tens of thousands of Kanpur’s poorest live in slums drawing groundwater laced with chromium, which is known to cause cancer, liver failure and early dementia. Kanpur has facilities to recycle industrial wastewater and extract the chromium. The process is said to add no more than nine rupees (13 cents) to the cost of a pair of shoes. Yet a blind eye is turned to environmental breaches.

Too often, says Shashi Shekhar, a former senior water official, state governments and their business cronies are more interested in constructing treatment plants than ensuring their long-term use. New forms of public-private partnership may start to bear fruit in a few years’ time, Mr Shekhar predicts, and water quality at last improve. Yet deeper change is needed. The Ganges is abused in search of short-term gain. Meanwhile, neither politicians nor the press lay out the scale of the environmental problem. As Mr Shekhar puts it, if a river in which millions of devotees bathe “is full of shit, then people are required to be told”.