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:

• Gressoney-La-Trinité
• Gressoney-Saint-Jean
• Gaby
• Issime
• Fontainemore
• Lillianes
• Perloz

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.

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  The sources of the Lys at the Lys glacier in the Monte Rosa mountain massif, below the Vincent Pyramid and close to the giant Lyskamm (4.533 m)
• 
  The river close to Gaby
• 
  The river close to Issime
• 
  In Fontainemore
• 
  The Pont Saint-Martin

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