From WikiPilipinas: The Hip 'n Free Philippine Encyclopedia
The Bicol River is a medium-size river in Southern Luzon and drains an area of 3,771 square kilometers. Situated in the Bicol Peninsula, it embraces the central portion of Camarines Sur, the northern portion of Albay, and a portion of Camarines Norte. The basin is oriented in a northwest-southeast direction and bounded on the east by a chain of volcanoes and on the west by highlands and lowhills. The flat alluvial land in the Bicol Plain occupies the area between the Eastern Bicol Cordillera and the Ragay Hills. The runoff which is estimated annually at 5,100 million cubic meters, starts from the Mayon Volcano with an elevation of 4,421m, and meanders in a generally northwestward direction. After being regulated through lakes Bato, Buhi, and Baao, the streamflow reaches the mainstream of the Bicol River which owing to its very gentle slope, is affected by tides as far as upstream of Naga City.
The principal tributary of the Bicol River is the Sipocot River. Unlike the main river, the Sipocot River cuts through mountainous terrain and has a steeper slope. The Bicol River after joining the Sipocot River, widens to more than 1,000 meters across at the estuary before discharging into San Miguel Bay.
The average annual rainfall varies from 2,000mm in the southwestern area to 3,600mm in the southwestern section of the basin. Flood-producing rains in the basin from May to February are mainly due to tropical cyclone passages, northeast monsoon and orographic effect.
Flooding from the sea in the alluvial plain near the mouth of the river is caused by storm surges associated with the passage of slow-moving typhoons near or over San Miguel Bay particularly during high tides.
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