[1] The river starts from Lake Bato,[1][2] 6 metres (20 ft) above sea level, and flows 94 kilometres (58 mi) downstream to its estuarine mouth at San Miguel Bay.

On the southwestern side lie the Ragay Hills, which consists of folded and faulted sedimentary formations including limestones, siltstones, conglomerates and shale.

In between these higher areas lies the Bicol plain, which is composed of thick alluvial deposits of sand silt and clay.

The fluvial marine deposits are usually saline at depths of approximately 1 metre (3 ft 3 in) and includes depresses areas along the Bicol River.

The piedmont plain on the fringe of the Ragay Hills consists of fine alluvium clay and reworked colluviums.