The river flows in the northeastern area of Borneo island with predominantly tropical rainforest climate (designated as Af in the Köppen-Geiger climate classification).[3] The annual average temperature in the area is 24 °C. The warmest month is August, when the average temperature is around 26 °C, and the coldest is January, at 22 °C.[4] The average annual rainfall is 4020 mm. The wettest month is December, with an average of 393 mm rainfall, and the driest is June, with 251 mm rainfall.[5]