The river flows in the northeastern area of Borneo island with predominantly tropical rainforest climate (designated as Af in the Köppen-Geiger climate classification).[4] The annual average temperature in the area is 22 °C. The warmest month is April, when the average temperature is around 23 °C, and the coldest is December, at 22 °C.[5] The average annual rainfall is 4020 mm. The wettest month is October, with an average of 438 mm rainfall, and the driest is June, with 263 mm rainfall.[6]