HUKA FALLS AND ARATIATIA RAPIDS
The Huka Falls are the largest on the Waikato River and lie 3 miles below Taupo and 2 miles above Wairakei. The falls occur when the Waikato River, after flowing over a comparatively wide bed, is abruptly confined for about 250 yards to a narrow rock-bound chasm less than 50 ft wide. At the lower end the whole discharge of the river is precipitated over a cliff 35 ft high into a deep circular basin. The cliff or fall-maker is a band of silicified conglomerate forming part of a sequence of freshwater lake beds (Huka Beds) that cover the Wairakei-Taupo region to a depth of several hundred feet including the region tapped by the geothermal bores in the Wairakei geothermal field. The silicification of the conglomerate is due to ancient thermal activity depositing silica sinter at the ground surface. These hot springs are no longer in existence but shallow drilling has indicated that geothermal heat is still present at depth.
There is a good view of the falls from a loop road just off the main highway. A swing bridge, originally built during the Maori Wars, crosses the river and a path follows the east bank to various vantage points. The name “Huka”, meaning “foam”, is appropriate and the falls are awesome under flood conditions.
The Aratiatia Rapids lie 3 miles downstream of Wairakei where the Waikato River, in a distance of about half a mile, descends 300 ft across a series of hard ribs of rhyolite. Vertical bands of rhyolite line the walls of the gorge and are the skeletal parts of an ancient rhyolite volcano which has been uncovered by the river as it progressively cut down through the overlying blanket of soft pumice. The foaming torrent of the Aratiatia Rapids is now harnessed by the 90 MW Aratiatia Hydro Station situated at the foot of the rapids. The station is supplied from tunnels which have been driven through the hard rhyolite adjacent to the falls. These tap an artificial lake produced by a low dam at the top of the rapids. Except in periods of low water and peak power demand, sufficient water still passes down the rapids to provide a spectacular view for visitors. The best vantage points are the high rock bluffs that dominate the most turbulent stretch of the river.
The name Aratiatia means “Pathway of Stakes” -an allusion to the line of stakes which were driven into the cliff face for the assistance of travellers.
by George William Grindley, M.SC., New Zealand Geological Survey, Lower Hutt.