The Lys (Walser German: Liisu or Lyesu) is a small 40 kilometres (25 mi) river (classified as a torrente).
It flows from the Lys Glacier on the south side of the Monte Rosa massif, at the foot of Vincent Pyramid, elevation 4,215 metres (13,829 ft), and runs through the Gressoney Valley, flanked by a road, through the following municipalities:
It converges with the Dora Baltea (fr. Doire baltée) as a right tributary at Pont-Saint-Martin.
Its left-side tributaries are Avant-Cir, Glassit, Loo, Mos, Niel, Pacoulla, and Tourrison. From the right it receives: Rû de Nantay, Stolen and Valbona.
