Its headwaters are the crater lake of Mount Ruapehu on the central plateau, and it flows into the Tasman Sea eight kilometres southeast of Whanganui.

The sudden collapse of part of the Ruapehu crater wall on 24 December 1953 led to New Zealand's worst railway accident, the Tangiwai disaster.

A lahar – a sudden surge of mud-laden water – swept down the river, significantly weakening the structure of a railway bridge at the small settlement of Tangiwai.

The overnight express train between Wellington and Auckland passed over the bridge minutes later, causing it to collapse into the turbulent waters.

When hot water from the lake spills, it quickly melts the ice and snow in the glacier forming a cave-like tunnel.