Why Nepal floods should worry India

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.