The Cagayan River, also known as the Río Grande de Cagayán, is the longest river and the largest river by discharge volume of water in the Philippines. It has a total length of approximately and a drainage basin covering . It is located in the Cagayan Valley region in northeastern part of Luzon Island and traverses the provinces of Nueva Vizcaya, Quirino, Isabela and Cagayan.
The river's headwaters are at the Caraballo Mountains of the Central Luzon at an elevation of approximately . The river flows north for some to its mouth at the Babuyan Channel near the town of Aparri, Cagayan. The river drops rapidly to above sea level some from the river mouth. Its principal tributaries are the Pinacanauan, Chico, Siffu, Mallig, Magat and Ilagan Rivers.
Magat River, is the largest tributary with an estimated annual discharge of 9,808 million cubic meters. It lies in the southwestern portion of the basin, stretching approximately from Nueva Vizcaya d