Mandau River is a river in Riau province of central Sumatra island, Indonesia, about 1,000 km northwest of the capital Jakarta.

The river flows in the central area of Sumatra which has a predominantly tropical rainforest climate (designated as Af in the Köppen-Geiger climate classification).

The warmest month is August, when the average temperature is around 24 °C, and the coldest is January, at 22 °C.

[4] The average annual rainfall is 2899 mm.

The wettest month is November, with an average of 403 mm rainfall, and the driest is June, with 115 mm of rain.