The Río San Francisco is the principal river of Jujuy Province in northwestern Argentina and the most voluminous tributary of the Río Bermejo. Formed at the confluence of the Ríos Lavayén and Grande de Jujuy near the city of San Pedro de Jujuy, the river flows approximately 155 kilometers in a generally northeastern direction before crossing into Salta Province and emptying into the Río Bermejo near the town of Pichanal. The river drains a watershed of roughly 25,700 square kilometers and carries an average annual flow of approximately 103 cubic meters per second — measured at Caimancito — with recorded highs near 285 m³/s and lows around 37 m³/s. This wide range of flows reflects the river's dynamic, seasonally variable character. The surrounding landscape is one of subtropical jungle and yungas forest, making the Río San Francisco one of the most ecologically rich river corridors in all of Argentina.