Nepal's devastating floods have exposed a risk that does not stop at its borders. India and Nepal share the rivers that rise in the Himalayas, but the relationship between a flood in Nepal and a disaster in India is more complicated than a simple story of water flowing downstream, say experts. Some floods can race out of the mountains carrying enormous quantities of mud, rock and debris.
Others may lose much of their force as they reach the plains, leaving behind a more conventional flood. And whether that water becomes a disaster in India depends not only on how much rain has fallen in Nepal, but also on rainfall, river levels, embankments, drainage and development on the Indian side. Nepal's floods are a warning for India - not simply because water flows south across the border, but because the two countries share a vast and increasingly volatile Himalayan river system.
Nepal has more than 6,000 rivers and streams, most draining south into India and the Ganges. Three major systems dominate: the Kosi in the east, the Gandaki (Gandak in India) in the centre and the Karnali (Ghaghara in India) in the west, with the Mahakali (Sharda) along the western border. A dozen smaller rivers rising in the Siwaliks and Chure - including the Bagmati, Kamala, Rapti and Babai - are less famous but can produce faster, more unpredictable floods because their catchments are small and steep.
Seven to nine major rivers flow from Nepal into the Indian plains, and they are responsible for flooding in India, according to Manish Shrestha, a hydrologist at the International Centre for Integrated Mountain Development (ICIMOD). India's Central Water Commission identifies the Kosi, Gandak, Bagmati and Ghaghara as some of the main transboundary rivers. Bihar is by far the most exposed Indian state, with roughly three-quarters of north Bihar officially classified as flood-prone.
Almost all the major rivers that flood the state originate in Nepal, with the Kosi - the "sorrow of Bihar" - the most notorious, followed by the Gandak, Bagmati and Kamala. Eastern Uttar Pradesh is vulnerable to the Ghaghara, Rapti and Gandak rivers. Districts like Gorakhpur, Bahraich, Lakhimpur Kheri and Shravasti are regularly affected.
Uttarakhand state shares the Mahakali-Sharda rivers. North Bengal receives the Mechi and Mahananda. "Bihar dominates in scale.
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