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The Indo-Gangetic Plain
The debris of the Himalayas, and a billion people living on it
The Himalayas are still rising, and the plain below them is the debris. As the Indian plate grinds into Asia, rivers strip the young mountains and dump the sediment into a subsiding trough at their feet, laying down alluvium that in places is kilometers thick — flat, fertile and almost stone-free. That trough is the Indo-Gangetic Plain, sweeping some 2,500 kilometers from the Indus in Pakistan across northern India and Bangladesh to the Bay of Bengal. It carries perhaps a billion people, which makes it one of the most densely populated large regions on the planet and a large part of the reason South Asia can feed itself. The mountains made it; the monsoon keeps it alive.
Three river systems drain it: the Indus in the west, the Ganges through the center, and the Brahmaputra sweeping in from the east to join the Ganges in Bangladesh and build the largest delta in the world. The plain is remarkably level — the Ganges falls only a couple of hundred meters over more than a thousand kilometers — so its rivers wander, flood and change course, and the surface rises to just about 300 meters where it meets the Siwalik foothills. The summer monsoon delivers most of the year's rain in a few months. Toward the Thar Desert in the west it is dry enough to require irrigation, and canal systems built under the Raj still shape Punjab.
Cities appeared here early: Harappa and Mohenjo-daro stood on the Indus more than four thousand years ago, and the Ganges plain later carried the Mauryan and Mughal empires, along with Varanasi, Delhi, Lucknow and Patna. The Green Revolution of the 1960s turned Punjab and Haryana into a granary using tube wells, fertilizer and high-yield wheat, and the price is now visible from space: groundwater in the northwest is among the fastest-depleting anywhere, and the burning of crop residue each autumn helps make the plain's winter air, in Delhi and Lahore alike, some of the filthiest on Earth. Roughly a billion lives hinge on getting the water and the air right.