The Bohorok River runs out of the national park past Bukit Lawang and is one of the small pleasures of the village. It is fast, clear, and cold enough to be a genuine relief from the lowland heat. The classic activity is tubing — sitting in an inflated truck inner-tube and floating downstream for several hours back to the village, through a few short whitewater sections and long stretches of flat water lined with jungle on both banks.
In November 2003 the river produced one of the worst floods in modern Sumatran history when a flash flood from the upper catchment, exacerbated by illegal logging, wiped out much of the original village and killed nearly 250 people. Bukit Lawang has been rebuilt on slightly higher ground and the riverside is again lined with guesthouses, but it is worth knowing the history. The river remains powerful after heavy rain — local guides will tell you when it is unsafe to swim or tube.