San Juan River (Argentina)

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The San Juan River is, together with the Jáchal, the most important river of the Argentine province of San Juan. Both join the Desaguadero/Colorado system that end at the Atlantic Ocean.

The river starts near the town of Calingasta, in the southwest of the province, from the confluence of three main rivers: the Calingasta River, the Castaño Viejo River, and the Río de los Patos. These rivers are born at an altitude of around 4,000 metres above mean sea level in the west and southwest of the province (in contrast, the Jáchal receives its Andean tributaries from the north of the province).

From Calingasta the river flows eastwards feeding the Quebrada de Ullum Dam (Embalse Quebrada de Ullum) located near San Juan city, from which it continues south-eastwards receiving a few more tributaries, including the Mendoza River, to later join the Desaguadero River near the Salina Pampa de las Latas (or Salina Bermejo), at the common border of the provinces of San Juan, Mendoza and San Luis.

The river has an average flow of 56 m³/s. Its course is 500 km long, and its drainage basin covers an area of 39,906 km². The Ullum Dam, at 768 m AMSL, covers an area of 32 km² and has a volume of around 440 million m³. The upper course of the river is used for water sports such as rafting and fishing, and windsurfing and swimming at the Ullum Dam. Along its whole length, the San Juan River is heavily used for irrigation.