The Whangaehu River in New Zealand's Lower North Island owes its distinctive character to its origins: it is shaped by lahars and flows out of an acidic crater lake, giving it properties unlike most other rivers in the region. The water is acidic and notably murky, concealing numerous rocks just below the surface throughout the run. This combination of hidden rocks, extensive boulder gardens, and narrow passages with drops makes for a technically demanding paddle that demands constant attention.

The main run extends from an abandoned quarry to Colliers Bridge and is rated a meaty grade 3, typically taking around four hours to complete. Getting to the put-in at the quarry is itself a challenge, requiring a descent of roughly 100 metres down a steep slope through brambles and other dense vegetation. An alternative put-in further upstream is available, adding approximately 45 minutes to the overall paddle time. For those who want to shorten the trip, there is an intermediate take-out roughly midway through the run, though reaching it involves a 40-minute walk uphill.

The river's murky, acidic water is one of its defining features. While reports that the water melts eyeglasses are noted to be exaggerated, the acidity is real and contributes to the river's unusual appearance and feel. The hidden subsurface rocks throughout the run mean paddlers cannot rely on reading the water surface alone and must remain alert for obstacles at all times.

The Whangaehu is considered a grade 3 run overall, making it suitable for paddlers with solid intermediate skills. The combination of boulder gardens, drops, and poor water visibility means it is not a river to underestimate. The take-out is at Colliers Bridge, providing a defined end point for the main run.