Course
The Loa's sources are located on Andean mountain slopes at the foot of Miño Volcano. The upper Loa basin is flanked on the west by a ridge with elevations that reach above 4,500 m, whereas to the east lies a volcanic chain, which separates it from endorrheic basins as that of Salar de Ascotán.
The river flows south on an elevated plateau, for about 150 km, to the oasis of Chiu Chiu (CHEE-oo CHEE-oo). The upper courses of the river are at a considerable elevation above sea level and receive a large volume of water from the Andes, mainly of two major tributaries: San Pedro de Inacaliri River and Salado River. The former joins the Loa near Conchi reservoir and the latter about 3 km south of Chiu Chiu. The water of its upper course and tributaries is fresh. However the lower course, as in all the rivers of this region becomes brackish.
From Chiu Chiu, the Loa flows west and north in a great curve to Quillagua (kee-YAH-gwah). After flowing for 115 km in this portion of its course and having run through the city of Calama, the Loa receives the waters of San Salvador River at the locality of Chacance. Thence the river flows north for about 80 km to Quillagua.
From Quillagua, its dry channel turns westward again and marks the border between the regions of Tarapacá and Antofagasta. Along its lower course, the Loa flows through a 500-m-deep canyon, which crosses the Chilean Coastal Range. The river reaches the Pacific Ocean in lat. 21° 26' S., a few miles south of the old port of Huanillos (Wan-EE-yōs).
The hydrologic regime of the river basin is rain-dominated. The increase of its water volume occurs mainly in January and February, as a consequence of the phenomenon known locally as Bolivian Winter.
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