Northern Luzon PRSD
 
CRB
The Cagayan River Basin is the largest river in the Philippines having a drainage area of 27,280 sq.km at the river mount in Aparri, Cagayan. It occupies the major part of Cagayan, Isabela, Qurino, Kalinga, Apayao, Mountain Province, Ifugao and Nueva Vizcaya provinces and small portion of Aurora province. The basin is chararacterized by a large central valley oriented north to south. The valley is bounded on the east by the Sierra Madre Mountains, on the west by the Cordillera Mountains, on the south by the Caraballo Mountains, and on the north by the coastline of the Babuyan Channel, The river has a length of 505 kms.
 
The climate of the basin is characterized by no pronounced maximum rain period and a short dry period. The average annual precipitation varies from less than 2,000mm in the northern part of the basin to more than 3,000mm in the southern mountainous part.