East Kalimantan sits near the equator and is warm and humid year-round, with temperatures ranging between 24°C and 34°C. Rainfall is spread across the year rather than concentrated in a single monsoon season, though the wettest months tend to be October through March.
The best time to visit for Mahakam river travel is the dry season, roughly May to September. River levels are lower in the dry season, which makes some stretches of the upper Mahakam easier to navigate by speedboat and reduces the risk of flooding. During the wet season, the river can rise dramatically and some smaller tributaries become impassable or hazardous.
For wildlife — particularly proboscis monkeys and orang-utans in the forests along the Mahakam — the wet season actually has its advantages: the jungle is lush and animals are more active. If you are willing to accept muddier trails and the possibility of itinerary changes, the wet season can be rewarding.