The Kosi, a river older than the Himalayas that keeps jumping its banks
The Kosi flowed before the Himalayas rose, and it has cut its gorge deeper as the mountains lifted around it. Downstream it drags so much silt onto the plains of Bihar that it abandons old channels and carves new ones. That habit earned it a grim nickname: the Sorrow of Bihar.
The river gathers water from both sides of the world's highest range. Its catchment holds Everest, Kangchenjunga, Lhotse, Makalu and Cho Oyu, and falls from above 8,000 metres to under 100 metres across six climate belts, from the Tibetan Plateau to the lowland Terai. One sub-basin alone contains 36 glaciers and nearly 300 glacial lakes. Its three main branches, the Tamur, the Arun and the Sun Kosi, meet at Triveni in Nepal, and from there the combined stream is called the Sapta Koshi, the seven rivers, after its upper tributaries.
Squeezed through the narrow, steep Chatra Gorge, the river gains tremendous force, then spills onto the flatlands and slows. Monsoon floods load it with sediment that it dumps as it goes, building one of the largest alluvial fans on Earth, about 15,000 square kilometres, laced with more than a dozen wandering channels. Over roughly 250 years the main course has migrated over 120 kilometres; in the eighteenth century it passed near Purnea, and today it runs west of Saharsa. Some writers described a steady westward march, but a study of 28 old maps from 1760 to 1960 found the swings were irregular and oscillating.
Engineers have tried to pin it down. After severe floods in 1954, India and Nepal signed the Kosi Agreement that April, and the Kosi Barrage straddling the border was built between 1959 and 1963 at India's expense. Embankments stretching 246 kilometres were set 12 to 16 kilometres apart so the silt could settle between them. The capstone, a tall dam upstream in Nepal, has been discussed for decades without being built.
The embankments failed dramatically on 18 August 2008, when the river broke through at Kusaha in Nepal and swung back into a channel it had left a century earlier, taking 95 percent of its flow with it. Around 2.7 million people were affected, prompting India's largest flood rescue in more than half a century.
Source: Kosi River
