[1] The Lhonak originates as a small stream from a glacier in the snowy wastes of northern Sikkim.

The main Himalayan wall with its high snow-clad peaks forms the northern boundary of the catchment.

Small glaciers or tongues of perpetual ice descend from the depressions between the ridges into the valleys.

They give birth to small streams which join the main channel of the Lhonak.

Goma Chu rises in North and South Lhonak glaciers and runs across the valley to join Zema Chu, a glacier at the southern end of the valley, as is Green Lake.