Agusan River is a river located in the eastern part of Mindanao island in the Philippines, draining majority of the Caraga Region and some parts of Compostela Valley province. It is the third largest river basin of the Philippines with a total drainage area of 10, 921 km² and an estimated length of 350 kilometers from its origin. making it as the longest river in the philippines.<ref name="eearth">Kundel, Jim (June 7, 2007). Water profile of Philippines (English). Encyclopedia of Earth. Retrieved on 2008-09-30.</ref><ref name="adb">Technical Assistance To The Republic of the Philippines For A Master Plan For the Agusan River Basin (PDF). Asian Development Bank (December 2004). Retrieved on 2008-08-09.</ref> The headwaters of the river is found in the mountains of Compostela Valley, near its border with Davao Oriental and east of Tagum City. It flows through the wide Agusan River valley which measures 177 kilomters (110 miles) from south to north and varies from 32-48 kilometers (20-30 miles) in width<ref>Plains of Mindanao</ref>. It finally drains into the Butuan Bay at its mouth in Butuan City. The river is also the longest river in the Philippines.
One of the prominent features in the Agusan River basin is the presence of the Agusan Marsh covering a total area of 19,197 hectares. The marsh serves a flood retention basin for the Agusan River, cutting the high discharge of water causing flash floods in the lower reaches of the river. Aside from that, the marsh harbors unique and pristine habitats like the sago and peat swamps forest and is a home to endangered and endemic flora and fauna. As a consequence, it was declared by former President Fidel V. Ramos as a wildlife sanctuary in 1996.<ref name="adb"/>
The Agusan River basin is divided into three sub-basins on the basis of topographic features: upper Agusan River basin, middle Agusan River basin and lower Agusan River basin. The upper Agusan River basin is the section from its headwaters in the mountains of Compostela Valley province to Santa Josefa, Agusan del Sur, the middle Agusan River basin is the section of the river from Sta. Josefa to Amparo, Agusan del Sur while the lower Agusan River basin is from Amparo to its mouth at Butuan City, Agusan del Norte
- AGUSAN RIVER BASIN. Database of water related projects in the Republic of the Philippines. (Accessed on 28 April 2009).