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The Cagayan River Basin

abs-cbnNEWS.com | 10/23/2009 3:50 PM

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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

Flooding

• 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