The Cagayan River Basin


Posted at Oct 23 2009 03:50 PM | Updated as of Oct 25 2009 09:36 AM

This is the map of tributaries and flood-prone areas around the Cagayan River basin from Pagasa’s Flood Forecast Branch. The text below the map gives general information about the Agno River Basin and its environs.
Fast facts:

  • Largest river in the Philippines
  • Drainage area: 27,280 sq. km.
  • Mouth: Aparri
  • Encompasses:
    • Parts of Cagayan
    • Isabela
    • Mountain Province
    • Nueva Vizcaya
    • Quirino
    • Quezon Provinces
  • Elliptical in shape
  • Major axis oriented in a north-south direction
  • Cagayan valley: relatively flat
  • Surrounding mountains in the east, south and west of river basin: up to 2,000 m in elevation


  • Average annual rainfall:
    • 1,000 mm in the northern part
    • 3,000 mm in the southern mountains
  • Floods caused by the Cagayan River flow drown slowly
    • Because of surface retention over the extensive flood plain
      • Extremely gentle slope, retardation of flood by several gorges and river meander
  • Flood target areas:
    • Areas along lower reaches of the river:
      • From Tuguegarao to Aparri
    • The alluvial plain along the river course
      • From Ilagan to Tumauini, Isabela

Connecting water ways and nearby mountain ranges

  • Largest tributary: Magat River (Isabela)
    • Annual discharge: 9,808 million cubic meters
    • Location: southwestern portion of the basin
    • Stretches 150 kilometers from Nueva Vizcaya to Cagayan River about 55 km from the river mouth
    • Extensive drainage area: 1/3 of the whole basin
  • Chico River (Kalinga)
    • Extensive drainage agea: 1/3 of the whole basin
  • Ilagan River (Isabela)
    • Starts from the slopes of Sierra Madre
    • Drains the eastern central portion of the Cagayan River basin
      • Annual discharge: 9,455 million cubic meters
    • Water from Ilagan River flows westward and joins the Cagayan River at Ilagan, Isabela (200 km from the mouth of Bicol River)
  • Siffu-Malig River (Central Cordillera ranges)
    • Water flow is parallel to the Magat River
    • Lower reaches of the river are filled with marshes and swamps

Source: Pagasa Flood Forecast Branch