The river flows in the eastern area of Kalimantan 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 September, when the average temperature is around 26 °C (79 °F), and the coldest is January, at 22 °C (72 °F).[4] The average annual rainfall is 2,835 millimeters (111.6 inches). The wettest month is December, with an average of 332-millimeter (13.1 in) rainfall, and the driest is August, with 154-millimeter (6.1 in) rainfall.[5]