Glaciers are moving mountains of ice. There are many of these in Sikkim among which the most important ones are Zemu Glacier, Rathong Glacier and the Lonak Glacier in North Sikkim.
The Zemu glacier is the largest and the most famous glacier of the eastern Himalayas. It is 26 km in length and is situated in a large U-shaped valley at the base of the Khangchendzonga massif in northwestern Sikkim. The Teesta River rises from the snout of this glacier.
Many tributary glaciers feed the trunk glacier. The side valleys in which these glaciers lie open into the main Zemu Valley from different directions. Icefalls and waterfalls have formed at the junction of the tributary glaciers with the Zemu glacier.
Flowing almost right across the length of Sikkim is the River Teesta. 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. Meeting Teesta at the border between Sikkim and West Bengal is its major tributary the river Rangeet which originates from the Rathong Glacier.
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.
The verdant green landscape of Sikkim is broken here and there by waterfalls that leap out of the hillside to the valley floors in plumes of white. Waterfalls are found almost all over Sikkim but there is a profusion of them in North Sikkim specially on the road between Mangan and Lachung as well as in the Dzongu area. Most of the waterfalls are perennial and are spring water fed but there are many that derive their water from melting snow. Except for a few most of the waterfalls are unnamed.