One of the rivers that almost flow right across the length of Sikkim is the Teesta. It's major tributary is the Rangeet which originates from the Rathong Glacier, meets it at the border between Sikkim and West Bengal. Teesta originates from the Cholamu lake where it is hardly a stream. No one can imagine that this innocuous looking stream would transform into a thundering mighty river less than a hundred kilometers downstream.

During monsoons the otherwise innocuous looking rivers of Sikkim become swollen, swift, muddy and dangerous. The rivers are narrow, serpentine and full of rocks and hence are not navigable. Because of swift currents hitting rocks, the rivers are very noisy and can be heard for miles together. The Teesta finally joins the Bhramaputra in Bangladesh.

The rivers are fed by snow melting on the mountains as well as rain that accumulates in the catchment areas during the monsoons. Human settlements usually must exist above the level of rivers and hence even if flooding takes place life and property remain safe.