A tributary of the left bank of the Durance, the Guil has its source in the scree of the circus formed by the Pointe de Rome, the Pic Gastaldi, and the Pointe Joanne in the Italian Piedmont. However, it only begins to be visible at the level of the Lakes Lestio and Porcieroles. At the height of Echalp and Ristolas in the Hautes-Alpes department, it is known as Haut Guil and flows over a still relatively wide plateau that does not suggest the canyons it has dug a few kilometers downstream.