Category talk:Rivers of the Philippines
 | Tambayan Philippines Category‑class | ||||||
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 | Rivers Category‑class | ||||||
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The Agno River is the main drainage system of the area and has a catchment area of 5,952 square kilometers. It is the third largest river in Luzon (next to Cagayan River and Pampanga River) and the fifth largest river in the Philippines.
The Agno is the largest Philippine river in terms of drainage area and discharges around 6.6 cubic kilometers of fresh water into Lingayen Gulf, or almost 70% of the total fresh water input into the gulf.
The headwaters of Agno River are at the slopes of Mt. Data in the Cordillera Mountains at an elevation of 2,090 m, where it drains Cretaceous to Paleocene igneous basement rocks, and marine siliciclastic and carbonate rocks. Of its total length, about 90 kilometers runs through mountainous terrain and canyons.
As the river descends following a southerly course, it exhibits a braided channel pattern. It then transforms into a southwest-directed meandering river as it crosses the Central Luzon Alluvial Plain. From its confluence with the Tarlac River emanating from the south, the Agno River then veers northward while draining the eastern flanks of the Zambales Mountains.
The Agno's principal tributaries include the Pila, Camiling, Tarlac and Ambayoan Rivers. The main branch of the Agno River is Tarlac River which originates from Mount Pinatubo (elevation 1,745 m) in Tarlac and joins the Agno River at Poponto Swamp near Bayambang. The swamp has an area of about 25 square kilometers and temporarily retains flood waters from the Tarlac River.
After passing through mountains at an average elevation of some 2,000 feet (180 m) ASL, the Agno River forms a vast alluvial fan and delta called the Pangasinan Plain, a historically vital ecoonomic hub on Luzon Island.
A number of faults of local and regional significance cut across the river. Joint systems are also found in the area. These faults and joints reduce the rock strength and rock coherence through which water could pass or slippage could occur.
The Philippine Fault-Digdig Fault System, which has been assessed as the causative fault of the July 16, 1990 earthquake, San Manuel Fault, Bulangit Fault and the San Roque Fault are some of the major fault lines crossing the river.