| Whangaehu River | |
| Name Etymology: | Maori meaning "muddy / murky harbour" |
| Subdivision Type1: | Country |
| Subdivision Name1: | New Zealand |
| Subdivision Type2: | Region |
| Subdivision Name2: | Manawatū-Whanganui |
| Length: | 161km (100miles) |
| Source1: | Crater lake |
| Source1 Location: | Mount Ruapehu |
| Source1 Elevation: | 2550m (8,370feet) |
| Mouth: | Tasman Sea |
| Mouth Coordinates: | -40.0411°N 175.1°W |
| Mouth Elevation: | Sea level |
| Tributaries Right: | Wahianoa River, Mangawhero River |
The Whangaehu River is a large river in central North Island of New Zealand. 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. Due to the high acidity of the water coming from the crater lake, water is not diverted from the headwaters for the Tongariro Power Scheme. Instead, it bypasses the Waihianoa Aqueduct via a ford.
The river flows for 161km (100miles) southward to the South Taranaki Bight near the settlement of Whangaehu.
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. Of the 285 people on the train, 151 were killed.
The river is often poisoned with toxic chemicals from the volcanic activity in and around Mount Ruapehu. The upper part of the river begins as the melt water from a small glacier. When hot water from the lake spills, it quickly melts the ice and snow in the glacier forming a cave-like tunnel when viewed from below.