Running water always erodes rock, but some rocks are more resistant than others. So a waterfall occurs when geological forces have produced either a sudden change in rock types or a steepening of a gradient where a stream is flowing. With a break in elevation, a stream or river becomes a waterfall.
New Zealand’s waterfalls have been formed by five different forces.
Fiordland’s glacial valleys and high rainfall create many long falls. The author of the World Waterfall Database notes: ‘It must be wonderful to live in a region where you can overlook a waterfall that’s nearly 2000' [feet] tall’. 1
A glacier carves its way down a valley, making it much deeper than previously. Left behind are smaller (hanging) river valleys that empty their water down the steep sides of the glacial valley in spectacular falls or cascades. These falls tend to be high, and are often quite thin. Especially common in Fiordland and Westland, they include the Sutherland Falls, draining out of Lake Quill on the Milford Track, and the Bowen Falls at Milford Sound.
Erosion of soft rock
Where there are layers of soft and hard rock, the softer rock is eroded by water, leaving the hard rock exposed and creating a fall. In New Zealand, harder layers of sandstone or limestone often exist close to softer mudstone, and the erosion of the mudstone creates the falls – for instance the Rainbow Falls in Kerikeri. Often a deep plunge pool develops as the soft rock beneath the fall is also cut away. Sometimes a rock shelter forms behind the fall.
Variation in water volume
When small tributaries reach the steep banks of a channel carved by a larger river, they will fall over the edge. A good example is the falls that drop down mudstone banks into the Whanganui River.
Sometimes volcanic activity creates a cliff where waterfalls form. A number of the falls on Mt Ruapehu and Mt Taranaki are of this nature – for example the Taranaki Falls are at the edge of an old lava flow. The Waipunga Falls further east flow over ignimbrite rock deposited in the 200 AD Taupō eruption. Both the Wairua Falls in Northland and the Bridal Veil Falls near Raglan drop over old basalt lava flows.
Underground thermal activity can also cause waterfalls. The Huka Falls drop over a band of silica deposited by thermal activity, while near Rotorua the Kākahi Falls have a temperature of 40° Celsius.
Fall from a wall
The Bay of Plenty’s Tarawera Falls are unique in New Zealand in emerging from a solid rock face. The river rises in Lake Tarawera and then disappears into cooled lava tubes underground before emerging from the rock wall. The fall has been likened to a tap with the faucet knocked off.
Where an active fault produces a severe movement or upthrust of rock, any river flowing along an old channel will suddenly drop. The Wairere Falls, on the west face of the Kaimai Range, pour over the Okauia fault and are the highest falls in the North Island. The valley bottom is sinking at a rate of 2 millimetres per year.