This is a difficult fact to absorb, standing at her banks across the city of Bhagalpur, in the impoverished hinterland of eastern India.

The subcontinent’s mother river—whose currents sustain, body and soul, some half a billion human beings—slides past in a single, swollen, mile-wide vector: a liquid belt of immense age and power moving east by southeast toward the steamy Bay of Bengal from its source at a shrinking Himalayan glacier called Gangotri. The Ganges’s waters brighten in the morning light. They turn from mud brown to a muzzy white-blue, until, at the horizon, the currents shine and appear to flow palely into a paler white sky.

The directional nature of rivers offers solace. They tell us a familiar story. Biographers of landscape, they have beginnings, middles, ends. Like us, rivers are born and grow and die. Rifling between their banks, they seem to swirl relentlessly in only one direction: to the future. But all of this is false. It is an illusion.

Mahendra Mandal explains in his tomato field.

“Twenty years ago, this field disappeared. The river took it,” Mandal says, squinting over the coarse sands of his riverside farm. “I waited 16 years for the field to come back. All that time, I worked in Khanpur as a laborer. I sold bananas. Now, my field is back. So I am back.”

The farmer points to a spot 50 yards out into the silty currents: the submerged edge of his family’s land. Here, the river slips its banks regularly, flooding farms, inundating villages, snaking laterally, backing up for miles in some instances, retreating, creating new shorelines, and building up colossal new sand islands called chars. No single line can define the Ganges. No one direction. It oscillates vertically, up and down. Over years, its currents yaw crazily sideways, in some cases for miles. The river draws circles through time, blind as the freshwater dolphins that breach its surface.

And yet: Every square inch of the Ganges floodplain remains owned, plotted, accounted for—including the land submerged temporarily under the river itself. A continuum of ancient fields disappear under one bank; they emerge at the other. The farmers pace the banks. They are waiting for the river to move restlessly on.

They wait months. Years. Lifetimes. Their children and grandchildren remember—and wait.

Today, Mahendra Mandal tills the same sediments where, not so long ago, dolphins trailed their flippers in an undertow of darkness, dredging up mollusks to eat. Tomorrow, or maybe next year, it will again be the dolphins’ turn to plow Mandal’s fields. Joined in this way, by ghost waters, the fates of both mammal species, dolphins and humans, intermingle in the dying Ganges.

THERE ARE ONLY 1,200 to 1,800 Ganges dolphins left in the world.

Sunil Kumar Choudhary, an ecologist working at the Vikramshila Gangetic Dolphin Sanctuary—India’s only dolphin reserve—is trying to conserve the animals. To do this he has studied history. He learned that saving the endangered dolphins requires also saving the Ganges’s endangered human fishermen. The local fishing communities had been oppressed for centuries by panidars, rich landowners who charged a crippling tax for access to “their” river waters. The fishermen had begun to starve with the building of dams on the Ganges, especially the giant Farakka diversion downstream, which wiped out the population of hilsa, a key migratory fish. Paying modest stipends, Choudhary has enlisted the people of the river, often the poorest of the poor, to become citizen wardens of the sea mammals.

Choudhary meets me for tea at a restaurant in Bhagalpur. He is friendly and unassuming: “Dolphins eat fish. Human eat fish. Is there competition? Possibly. The resource is shrinking badly. But let us be honest. The villagers have been using the resources of the river for thousands of years. And the sanctuary, with its recent bans on fishing, has caused them great hardship. So if anyone has the right to help manage the dolphins, they do. They know them better than anyone.”

Still, the number of dolphins in the sanctuary has declined in recent years, from about 200 to roughly 150.

The greatest threat to the Gangetic dolphins today?

Straight lines, Choudhary says: The dredging of new, ruler-drawn shipping channels that are destroying the organic warp and bend of the river’s course.

MORE THAN 260 millions gallons of untreated sewage poison the Ganges every day.

By the time the river flows past Varanasi, one of Hinduism’s most sacred pilgrimage cities, where every day dozens of bodies are cremated atop wood pyres and their ashes dispersed into the sacred currents, the fecal bacteria count in the water hits 3,000 times the limit deemed safe for bathing. People still bathe to wash away their sins. Many drink the currents.

Plastic waste and industrial effluent too choke India’s holy river. But the most serious long-term threat to the Ganges is lack of water.

For many years, the flow of the river has been ebbing. Activists blame most of the deficit on unsustainable extraction. Groundwater pumping is drastically lowering water tables in the floodplains. The Ganges also is throttled by more than 300 irrigation and hydropower dams and diversion weirs on its main trunk, and closer to a thousand if you count all the tributaries. Moreover, the climate is changing. The replenishing monsoon rains are growing less predictable. It is a complex problem that has paralyzed a succession of governments.

Last year, a leading environmental advocate for the Ganges, Guru Das Agrawal, conducted a Gandhi-esque hunger strike to protest generations of government inaction. Agrawal wrote Prime Minister Narendra Modi impassioned letters, vowing to starve himself to death unless real conservation steps were finally taken. The letters were unanswered. In a tweet after the activist died 111 days later, the prime minister wrote that Agrawal “would always be remembered.”

IN HIS SHORT STORY “The Location of a River,” the nature writer Barry Lopez imagines a prairie river in frontier Nebraska that literally picks itself up and vacates the landscape, only to reappear later elsewhere.

The disappearing river drives Lopez’s protagonist, a 19th-century white explorer—a man named Foster—mad. Lopez writes: “The Pawnee … told Foster that the earth, the rivers, did not belong to men but were only to be used by them, and that the earth, though it was pleased with the Pawnee, was very disappointed in the white man. It suited the earth’s purpose, they said, to suddenly abandon a river for a while, to confound men who were too dependent on such things always being there.”

I AM WALKING ACROSS INDIA. Nearing the banks of the Ganges in Bhagalpur, my walking partner, the river conservationist Siddharth Agarwal, tells this story, first recorded by a writer traveling the mighty stream a decade ago: For a number of years on the Kosi, a tributary of the Ganges in Bihar state, catastrophic floods erased the villages and crops along its banks, wreaking havoc in the extreme, to the point that the people grew hollow-bellied from hunger and exhausted from rebuilding their houses.

It was the local women who decided, at last, to take matters in hand.

They waded out into the river’s currents, saris billowing about their waists, and cast a jar of vermilion—the red pigment worn by married Hindu women in the parts of their hair—into the surging waters.

“They scolded the Kosi for being too wild,” Agarwal says. “They wanted it to calm down. They told the river to settle down, to stop being so independent, so headstrong and reckless.”

With the smear of vermilion, the women had declared the river married.

IN A MUDDY RIVERBANK neighborhood of Bhagalpur, a woman named Poonam Devi shows me her wares. “Before we used to catch fish the length of your arm,” says the fishmonger. “Today, you are lucky to find anything longer than you finger.”

Devi, who has been selling fish for 35 years, uses both hands to scoop up the catch of the day from a dingy sisal bag: a guppy-size trawl that looks suitable for a child’s aquarium, not a human meal. The fish are not even from her Ganges. She buys the dregs of fish trucked in from Andhra Pradesh, a coastal state hundreds of miles to the south.

“Our river is completely dried up,” Devi says with a weary shrug. “There’s no possibility of our fish coming back. We don’t even think about it anymore.”

I cross the Ganges one last time, in a rowboat, heading north.

I trail my hand for a moment in its warm, gritty currents. The long empty wanderer. I get the sense that, in one life or another, this river has been everywhere on Earth.