Lachen river flowing through Chungthang valley before confluence with Lachung river in Sikkim, India. Chungthang, Sikkim Chungthang is a small town at the confluence of Lachen and Lachung rivers. Here the river Teesta, the lifeline of Sikkim is formed. The two most picturesque and common tourist destinations of North Sikkim, Lachen and Lachung, can be reached only through Chungthang. The North Sikkim Highway coming from Gangtok via Mongan bifurcates here.