The Pasig-Marikina-San Juan River System, of which total catchment area is 635 km2, runs through the center of Metro Manila and flows out to the Manila Bay. Its main tributaries, the San Juan River and Napindan River, join the main stream at about 7.1 km and 17.1 km upstream from the Pasig River mouth, respectively. The three largest waterways contribute largely to the flooding in the metropolis brought about by the riverbank overflow of floodwaters. Metro Manila, which encompasses 16 cities and 1 municipality having a total projected population of over 11 million, is the economic, political and cultural center of the Philippines.
However, even though the completion of Mangahan Floodway, flood damages along the Pasig-Marikina River have been frequently experienced in last 25 years between 1986 and 2010; 1986, 1988, 1995, 1998, 1999, 2000, 2002, 2004 and 2009. Especially, Tropical Storm 'Ondoy' brought a heavy rain and caused devastating flood disasters in Metro Manila, its surrounding area and Laguna Lake area on September 26, 2009. The heavy rainfall of 453 mm/day observed at Science Garden in Quezon City brought a huge volume of flood discharge along the Pasig-Marikina River, resulting in the death/missing of about 500 people and causing massive damages.
To cope with such existing flood problems in Metro Manila, the necessity of river channel improvement of Pasig-Marikina River has been studied. The Department of Public Works and Highways (DPWH) conducted a updated Master Plan (M/P) for flood control and drainage improvement in Metro Manila and a Feasibility Study (F/S) on the channel improvement of the Pasig-Marikina River from January 1988 to March 1990, under a technical assistance from the Japan International Cooperation Agency (JICA), called "The Study on Flood Control and 2 Drainage Project in Metro Manila".
Based on the F/S, the "Pasig-Marikina River Channel Improvement Project" from river mouth to Marikina Bridge (29.7 km long) is being proposed for the implementation in the following four phases: Phase I (Detailed Design), Phase II (Channel Improvement of the Pasig River), Phase III (Channel Improvement for the Lower Marikina River) and Phase IV (Channel Improvement for the Upper Marikina River)