Asahan River

The Asahan drains Lake Toba to the Malacca Strait, and the section below Tangga is regularly listed among the three most difficult commercially rafted rivers anywhere, with continuous grade four and five water through a steep basalt gorge - the standard run drops sharply over a short distance with named rapids that give little recovery time between them, and it is not run for beginners under any circumstances. The Asahan has hosted international rafting championships. The flow is regulated by hydroelectric releases from the dam above, which serves the Inalum aluminium smelter downstream, so the level is set by industry rather than by rain and operators work around the release schedule. The Sigura-gura waterfall is on the same river above the run.

What you actually see

• Continuous grade four and five water - among the hardest rafted rivers in the world.
• A basalt gorge - the steep confined section below Tangga.
• Dam-regulated flow - releases set by the hydro scheme and the smelter downstream.
• Sigura-gura falls - the waterfall on the river above the rafting section.

When to go

All year, subject to release schedules. Two days.

Honest expectations

This is expert water - operators require rafting experience and swimming ability, and refusing unqualified customers is normal. Releases dictate when trips run. Access is a long drive.

Nearby

Lake Toba and Parapat are upstream; Medan is north.