Drainage

The most important rivers of the Philippines are the Cagayan, Agno,Pampanga, Pasig, and Bicol on Luzon and the Mindanao (Río Grande de Mindanao) and Agusan on Mindanao. The northern plain between the Sierra Madre and the Cordillera Central is drained by the Cagayan, while the central plain is drained in the north by the Agno and in the south by the Pampanga. The Pasig, which flows through the city of Manila, was once commercially important as a nexus for interisland trade but is no longer navigable except by small craft; heavy pollution has required significant cleanup efforts. Most of the Bicol Peninsula lies in the Bicol basin. On Mindanao the Agusan drains the fertile lands of the island’s northeastern quadrant, while the Mindanao River drains the Cotabato Valley in the southwest. One of the Philippines’ most unique waterways lies underground, emerging directly into the ocean at Puerto Princesa Subterranean River National Park on the island of Palawan; the park was designated a UNESCOWorld Heritage site in 1999.

The largest lake in the archipelago, with an area of 356 square miles (922 square km), is Laguna de Bay, on the island of Luzon. Also on Luzon and just to the southwest of Laguna de Bay is Taal Lake, which occupies 94 square miles (244 square km) inside a volcanic crater; a volcanic cone emerges from the lake’s centre. Lake Sultan Alonto on Mindanao is the country’s second largest lake, covering an area of 131 square miles (340 square km).

Source: http://www.britannica.com/EBchecked/topic/456399/Philippines/23713/Pre-Spanish-history