The Lhonak originates as a small stream from a glacier in the snowy wastes of northern Sikkim. It generally flows south till it joins the Teeta. 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. The upper course is along an ancient glacial valley. While the middle and lower courses have been carved by both the action of ice and running water, Side valleys formed by melt-ice opens into the main valley. No vegetation is found in higher reaches. Alpine pastures occur near the snow line. Sub-alpine and emperate forests is found at lower elevations. Human habitation in the Lhomak valley is very sparse. It is visited by graziers.
The Lhonak Valley is a grassland in the exposed river valley of Goma Chu in northwest Sikkim, with boggy marshes, glacial lakes, barren scree slopes and glaciers. 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 the Green Lake.