The Kosi’s origin in Nepal is near Kathmandu. But it largest tributary, the Arun, originates in Tibet where it is called the Phung Chu, makes a mighty arc in Tibet and cuts through the Himalaya between Mt Makalu and Mt Kangchenjunga to meet its siblings Sun Kosi and Tamor at Tribeni.
Geologists say rivers like the Arun that slice through the Himalaya are older than the mountains, and cut their way through as the terrain rose over millions of years with the collision of the India and Eurasian tectonic plates.
The Kosi watershed therefore drains an enormous vertical landscape stretching from Mt Xixapangma which lies just 80km on the other side of the Himalaya in China, both sides of Mt Gauri Shankar (Chomo Tseringma), Mt Everest and Mt Makalu, as well as the western slopes of Kangchenjunga. Six of the world’s 13 peaks above 8,000m are therefore a part of the Kosi catchment.
These mountains block the monsoons, and in eastern Nepal their southern slopes get more than 3,000mm of rain a year, weathering them down relentlessly, making the sedimentation load of the Kosi the heaviest for any river in the world.
The topography of this mighty river has shaped Nepal’s history and culture, and can determine the country’s future prosperity as well. But only if we respect its geology, hydrology and the climate of its basin.
Watershed expert Madhukar Upadhyay says, “We have not even begun to understand the hydrological and geological specificities of our mountains and rivers and yet are rushing headlong into ever-larger infrastructure development. Nepalis will still be here 500 years from now, what kind of country and landscape are we leaving behind for them?”
Last month, landslides blocked the Melamchi-Indrawati, unleashing a deadly debris flow that destroyed infrastructure, homes and livelihoods. It imperilled the Melamchi project, and its tunnel supplying water to Kathmandu was saved only because the gates were closed for maintenance just hours before the onrush.