Devastating floods in Nepal have revealed a risk that extends beyond its borders, serving as a warning for India due to their shared and increasingly volatile Himalayan river system. While water flows from Nepal into India, experts state that the relationship between a flood in Nepal and a disaster in India is complex.
Nepal's more than 6,000 rivers and streams largely drain south into India and the Ganges. Major systems include the Kosi, Gandaki, and Karnali, with several smaller rivers also contributing to potentially rapid floods due to their steep and small catchments.
Bihar is identified as the most exposed Indian state, with roughly three-quarters of north Bihar officially classified as flood-prone. Eastern Uttar Pradesh is also vulnerable, with districts like Gorakhpur and Bahraich regularly affected. Uttarakhand and North Bengal also share transboundary rivers.
Hydrologists describe the process as a three-part chain: rainfall, the river's characteristics, and conditions when the water reaches India. The peak flow at the border is largely determined by rainfall in Nepal, but the severity of flooding in India depends significantly on local conditions such as rainfall, Ganges river levels, embankments, and drainage.
The 2008 Kosi disaster, which resulted in nearly 400 deaths in Bihar, illustrates this complexity. The catastrophe was caused by an inadequately maintained embankment breaching, rather than an exceptionally high flow, with the river carrying only about a tenth of its capacity, according to Professor Rajiv Sinha.
India can receive roughly 12 hours to two days of warning for ordinary monsoon floods, depending on water travel time. Flash floods from Chure rivers can arrive within hours. India and Nepal share real-time rainfall and river-level data, but gaps remain, including insufficient monitoring stations in fast-flowing Chure catchments and limited routine sharing on embankment conditions.