Mahakam River, river of east-central Indonesian Borneo (Kalimantan). It rises in Borneo’s central mountain range and flows east-southeast through southern East Kalimantan province for about 400 miles (650 km) before emptying into Makassar Strait in a wide delta. The chief town along its course is Samarinda, capital of East Kalimantan, situated about 30 miles (48 km) above the river’s mouth.