Class 5; Geology for Kayakers, Kaituna River

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Kaituna River, New Zealand

Kaituna River map, draining Lake Rotoiti, NZKaituna River is a glorious water-course that has a bit of everything for kayakers and rafters, from beginner Class 1 and 2 rapids to serious class 5 waterfalls.  It is probably one of the more popular kayaking and rafting rivers in North Island, New Zealand, in part because it is so easy to get to, and close to the Rotorua centre of tourism.

The Kaituna is an outflow of Lake Rotoiti, its headwaters next to the Highway at Okere Falls (just down the Road from Okere Falls Café); it exits at the coast in Bay of Plenty.

 

Rocks etc.

Kaituna River flows over and erodes into volcanic rock known as ignimbrite.  Ignimbrite is the product of particularly violent eruptions; eruptions that send ash clouds many kilometres into the atmosphere, and in the case of this rock type, a gas-charged ash cloud, or pyroclastic flow that moves at great speed across the ground (100 km/hour).  Pyroclastic flows commonly develop from partial collapse of the rising ash cloud.

Cartoon showing an eruption that generates a large plume-like column of ash, and pyroclastic flowspyroclastic flow cartoon

A pyroclastic flow, especially when it is hot (500o-1000oC)  will flatten and incinerate pretty well anything in its path.  Pyroclastic flows (pyro meaning fire, heat) have so much momentum they can move into valleys and climb ridges, spreading many 10s of kilometres from the eruption centre.  The flow ash solidifies as it cools.  If it is hot enough, volcanic glass (like that in obsidian and pumice) will melt and stretch; these are referred to as welded ignimbrites.  Cooling in the ash layer commonly produces a regular array of blocky fractures, or columnar joints.  Not all ignimbrites are hot, but they are all violent and highly destructive.

The resulting rock looks bit like a Christmas pudding where chunks of pumice and sometimes other rock types are encased a much finer volcanic ash.

The ignimbrite beneath Kaituna River is known geologically as the Mamaku Plateau Formation which also underlies most of Mamaku Plateau north of Rotorua.  It arose from a humungous explosive eruption about 240,000 years ago that also resulted in the collapse of a crater-like structure or caldera; now Lake Rotorua (see the cartoon above).

The eastern half of Lake Rotoiti is also part of a much larger and probably younger caldera, the Okataina Caldera that now includes Lakes Rotoiti, Tarawera, Okataina and Rotomahana.  This eruptive center  may have been active since about 500,000 years ago, but it is eruptions in the last 60,000 years that have shaped the modern landscape.  The Tarawera eruption in 1886 is the youngest event in this eruptive centre.

Both calderas are part of the Taupo Volcanic Zone (TVZ), one of the most active regions of crustal rifting in the world (the crust is being pulled apart here).  The southern extent of the TVZ includes the active Ruapehu, Tongariro and Ngauruhoe volcanoes.

Calderas in Taupo Volcanic Zone

TVZ extends northeast and offshore to the White Island active volcano, and eventually links to the large, active volcanic chain, the Kermadec Island Arc.  Unlike the TVZ which is being pulled apart, the Kermadec volcanic chain is located above a zone where the Pacific tectonic plate dives (is being subducted) beneath the Australian plate.

The ignimbrites over which Kaituna River flows is one smallish part of this very grand plate tectonic scheme.  It is a story that hasn’t yet ended.

Creating a landscape

Landscapes in the Rotorua-Taupo volcanic region and the Taupo Volcanic Zone in general are continually modified by eruptions and rivers.  Eruptions provide ash, lava and volcanic edifices. Rivers flow over and erode into the volcanic bedrock and/or sediment; the loose sediment that is produced by erosion is then moved downstream and deposited on broad floodplains or removed to the coast.   Later eruption products fill these valleys or generally blanket the landscape.  The process of construction, erosion, and renewal of landscape is never-ending.  Kaituna River, over its first 30 km or so is incised into (mostly) ignimbrite; in other words it has cut deeply into the bedrock creating steep-sided valleys and gorges.  This is the river section that excites experienced kayakers.

River flow is controlled by headwater floodgates.   Flows of 10-20 cumecs are common, but flows of 30-50 cumecs and higher are what excites many kayakers.  See Whitewater NZ for more details before attempting these runs; also check the local topography online .

Tutea Falls at 7m is the highest commercially rafted waterfall in the world.  Watching kayakers or rafters from the lookout opposite the falls is pretty cool – it is not possible to see the river upstream so boats appear suddenly at the top of the falls and just as quickly disappear in the churning below.  You can also descend Hinemoa’s Steps to get a closer look at the falls; these were carved into the ignimbrite in 1907 as an escape during Maori conflicts. Trout Pool Falls, just downstream of the Trout Pool raft take-out, is a Class 5 run; the plunge-pool is particularly gnarly (see the images at top of page).

Smokey Gorge - the only way to see it is to kayakBelow Trout Pool Falls are three striking gorges: Awesome, Gnarly and Smokey gorges (two shots of Smokey above), all rated as Classes 3 to 5.  The walls of these are frequently vertical over 20-30m, and in places the river itself is only 2-3m wide.  The gorges are draped in native bush, so the rocks tend to be visible close to the waterline.

Check out Sam Ricketts website that has many great kayaking images and videos of the Kaituna., including some night forays over Huka Falls & Trout Pool Falls.

Photos in this post by Jeff Colgrove, Pete Lodge and Sam Ricketts.

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