The river flows in the eastern area of Borneo with a predominantly tropical rainforest climate (designated as Af in the Köppen-Geiger climate classification).[3] The annual average temperature in the area is 23 °C (73 °F). The warmest month is May, when the average temperature is around 24 °C (75 °F), and the coldest is July, at 22 °C (72 °F).[4] The average annual rainfall is 2932 mm. The wettest month is February, with an average of 362-millimeter (14.3 in) of rainfall, and the driest is September, with a 132-millimeter (5.2 in) of rainfall.[5]