India, Aug. 26 -- Authorities in Uttar Pradesh, Bihar, and Uttarakhand were put on high alert after a sudden flash flood in neighbouring Nepal swept through mountain valleys, killing dozens of people and leaving nearly 400 tourists, including 105 Indians, missing.

The flash flood about 200 km north of the India border may result in water levels rising in rivers flowing from Nepal, such as the Gandak in Uttar Pradesh and parts of north Bihar. Trishuli river, which takes in most of the Bhote Koshi river that originates from Tibet, flows into the Gandak.

Heavy rain and the opening of barrage gates in Nepal could also increase flow into the Ghaghara and Rapti rivers in Uttar Pradesh and the Kosi in Bihar.

Citing an advisory from India's Ce...