The Lys (Walser German: Liisu or Lyesu) is a small river (classified as a torrente).
The Lys River originates from the Lys Glacier, located on the southern side of the Monte Rosa massif, at the foot of Vincent Pyramid (elevation: 4,215 metres / 13,829 ft). It flows through the Gressoney Valley, which is flanked by a road and passes through the following municipalities:
The Lys converges with the Dora Baltea (French: Doire baltée) as a right-bank tributary at Pont-Saint-Martin.
Its left-side tributaries include: the Avant-Cir, Glassit, Loo, Mos, Niel, Pacoulla, and Tourrison. From the right, it receives: the Rû de Nantay, Stolen, and Valbona.
The river is classified as a torrente, meaning a fast-flowing mountain stream typical of the Alps.