The river Ganges' long decline.

• Author: Sampat, Payal

In the basin of a half-billion souls, purification and pollution swim together in unholy wedlock.

According to Hindu mythology, the Ganges river of India - the goddess Ganga - came down to the earth from the skies. The descent was precipitated when Vishnu, the preserver of worlds, took three giant strides across the Underworld, the Earth, and the Heavens, and his last step tore a crack in the heavens. As the river rushed through the crack, Shiva, the god of destruction, stood waiting on the peaks of the Himalayas to catch it in his matted locks. From his hair, it began its journey across the Indian subcontinent.

Whatever one makes of this myth, the Ganges does, in fact, carry extraordinary powers of both creation and destruction in its long descent from the Himalayas. At its source, it springs as melted ice from an immense glacial cave lined with icicles that do look like long strands of hair. From an altitude of nearly 14,000 feet, it falls south and east through the Himalayan foothills, across the plains of northern India, and down to the storm-lashed Indo-Bangladesh delta, where it empties out into the Indian Ocean.

Another version of the myth tells us that Ganga descended to earth to purify the souls of the 60,000 sons of an ancient ruler, King Sagara, who had been burnt to ashes by an enraged ascetic. Today, the river symbolizes purification to millions of Hindus the world over, who believe that drinking or bathing in its waters will lead to moksha, or salvation. It has been esteemed by other religions and cultures, as well: the ancient Greeks, the first century Jewish scholar Josephus, and the medieval Christians all believed the Ganges to be Phison, the first river of Eden. And the 16th century Moghul ruler Akbar revered Ganges water as the water of immortality, refusing to drink from any other source.

Curiously, some evidence suggests that there may be a scientific as well as religious basis for the beliefs that this river can bring purification. Rivers continually purge themselves. Fresh infusions from rain or groundwater dilute their streams. The flow of water flushes solid materials downstream rather than letting them settle to the bottom. (Damming a river reduces its flow, and hence obstructs its flushing capacity, but free-flowing rivers keep washing out the garbage put into them.) The cleansing process is further aided by aquatic microorganisms that break down waste materials. These microbe-collaborators require dissolved oxygen (DO) in the water to do their job, and a high level of DO is normally a sign of a healthy river.

The more organic waste there is in the water, the more oxygen is needed by the microorganisms that break it down. The measure of this need is biological oxygen demand, or BOD. The organic wastes that drive up a river's BOD include sewage, dead aquatic life, agricultural wastes, and plant-based industries like food-processing, alcohol distilling, and paper production.

According to studies reported by environmental engineer D.S. Bhargava of the University of Roorkee, the Ganges decomposes organic waste 15 to 25 times faster than other rivers. Bhargava conducted his studies in collaboration with the Indian Institute of Technology in Kanpur. He monitored water at various locations along the river at different seasons over a five year period. Bhargava's findings tally closely with those of the government's Central Board for the Prevention and Control of Water Pollution in New Delhi. This finding has never been fully explained.

Both sets of findings indicate that the Ganges has an extraordinarily high rate of reaeration, the process by which it absorbs atmospheric oxygen. When organic waste is dropped into it, as much as 60 per cent of the BOD is processed within an hour. The water quality samples also suggest that the Ganges retains DO much longer than does water from other rivers. If this is true, it could explain why bottled water from the Ganges reportedly does not putrefy even after many years of storage. Many Hindus keep water from the Ganges in glass bottles as a sacred relic, or for use in religious ceremonies.

The most life-giving qualities of the Ganges, however, are not unique, but universal: they are characteristic of rivers everywhere on the planet. Great rivers have always been the fountainheads of civilization. The Nile, the Huang He, the Indus, the Tigris, and the Euphrates were all centers of ancient societies. All were ascribed mythical qualities by the peoples they supported. These rivers, like hundreds of others around the world, brought life where they flowed, feeding agriculture, fisheries, trade, and culture, and taking away what civilizations discarded. The Ganges is venerated in India as a mother, Ganga Ma, for her capacities to create, preserve, and destroy life - reflecting the same powers as those of the Hindu trinity of Brahma, Vishnu, and Shiva.

If Ganga originally came to bring salvation to Sagara's 60,000 sons, as the legend has it, the poor goddess has now ended up with a burden ten thousand times greater than she bargained for. India's fertility rate has declined since the days of that prolific procreator, but its population has nonetheless grown vastly. The river supports a staggering 400 million people along its 2,510 kilometer (1,560 mile) course. If the delta it shares with the mouth of the Brahmaputra River is included, the number of people it supports rises to half a billion, or nearly one-tenth of all humanity, making it the most populous river basin in the world. To put that in perspective, note that the Amazon basin, where the incursions of human activity have galvanized the concern of environmentalists, is still one of the most thinly populated parts of the world. The Ganges, by contrast, is one of the most densely populated, at about a thousand people per square mile. Its total population is projected to rise to 750 million by the year 2020, and to almost a billion ten years later.

The Ganges plains were settled by invading Aryan tribes around 1200 BC. In the 3,200 years since then, the landscape of the region has been completely transformed. The fertile alluvial soils that first attracted those settlers have supported a hundred generations...