The Mahakam River is the largest river in East Kalimantan, Indonesia, with a catchment area of approximately 77,100 km2. The catchment lies between 2˚N to 1˚S latitude and 113˚E to 118˚E longitude. The river originates in Cemaru from where it flows south-eastwards, meeting the River Kedang Pahu at the city of Muara Pahu. From there, the river flows eastward through the Mahakam lakes region, which is a flat tropical lowland area surrounded by peat land.
Thirty shallow lakes are situated in this area, which are connected to the Mahakam through small channels. Downstream of the connection with the Semayang and Melintang lakes, the Mahakam meets three other main tributaries - the rivers Belayan, Kedang Kepala, and Kedang Rantau - and flows south-eastwards through the Mahakam delta distributaries, to the Makassar Strait.