The Ilagan River originates from the western slopes of the Sierra Madre and drains the eastern central portion of the Cagayan River basin.

It has an estimated catchment basin size of 3,132 square kilometres (1,209 sq mi)[3] and an estimated annual discharge of 9,455 million cubic meters/s.

It flows westward and joins the Cagayan River in Ilagan, Isabela.

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