Positioning Geography: Strategic Issues in
Geographical Education. Field Trip Materials
Fluvial and Volcanic Processes
Lex Chalmers and Janey Nolan
1. Fluvial Processes
Changes in the operation of fluvial processes within the river channel between Lake Taupo and the foot of
the Aratiatia rapids has been influenced by human modifications. These changes have both local and
system wide effects. The river operates differently now compared to the 1830s before the Europeans
settled in this area. See map http://www.zenbu.co.nz/entry/1116402-niwa-turangi
The map shows the that the Waikato River is fed from the Mt Ruapehu, Mt Ngaruhoe, Mt Tongariro and
the Kaimanawa Ranges, noting the changes in the lower Tongariro river regime through the Turangi Power
Development.
Top of Mt Ngaruhoe September 2008 looking out towards Lake Taupo and Mt Tauhara
Diversion of flow for hydroelectric generation in the Tongariro River causes change in volume and
interruption to sediment movement down the river, therefore changing the water sediments entering Lake
Taupo. Effectively these dams act as silt ponds which reduce sediment intake into the lake. The effect of
clearing native ferns and bracken and using urea fertilisers on farms in the catchment of the lake and river
allows soluble nitrogen to get into Lake Taupo with dire effects on water quality.
Sediments present in the lake at present flow out through fluvial transportation into the Waikato River
channel. This flow is controlled, so large peak in-flows in times of flooding are not allowed to pour down
the river and in unusually dry weather flows are decreased to keep the lake level standardised for power
generation.
As stream energy levels can vary abruptly over short time scales. For example, violent thunder-storms often
produce rapid increases in discharge and stream energy in the form of flooding. As these flood events have
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been modified this has important consequences for fluvial erosion and fluvial transportation processes.
Ordinarily, large boulders or rocks can be rolled along the bed of the river by the force of the river current.
The larger the boulders, the more energy the river needs to move them (traction) however if the Waikato
River is forced to operate differently in this environment this process may be slowed down or even
stopped. Other processes such as saltation may be modified in a flood situation.
The traction process is mostly stopped by
the control gates changing water flows so
the river does not flood
The saltation process is modified by the
construction of the Aratiatia dam as the
saltation process is cumulative.
Traction is bedload transport by sliding and
rolling of large particles eg boulders
Saltation is a skipping motion by sand grains
along the river bed
The suspension process is changed as once
particles are in suspension they can travel
long distances but may drop if the river is
slowed at Aratiatia dam
The solution load has been slightly changed
by ground water high in nitrates from farm
runoff and other human activities
Suspension is the transport only of the
smallest particles
Solution is the transport of weathered
minerals in solution
Walking to the Taupo Bungy site, you see the white cliffs below. Look out on the rocky outcrop of Cherry
Island noting fluvial deposition on this island. What is a likely difference between the cliffs where the
bungy jumping is and Cherry Island? HINT: Why has one been eroded and not the other?
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Along the upper reaches of the river past Cherry Island and near Reid’s farm is a rocky outcrop with some
rapids where kayakers enjoy the river. Apart from poles in the ground, human modifications of the fluvial
processes of the river are minor apart from perhaps some further erosion of the banks due to the number
of tourists entering the river at this point.
In terms of the transport process, the load in solution is sometimes more significant than other forms (see
the diagram at http://www.xmind.net/share/cianmm/fluvial-transportation/). The Wairakei geothermal
plant increases the amount of arsenic carried in solution. We need to find out more about this; see the
section on geothermal activities in the field trip area.
Downstream from Reid’s farm the Waikato River enters a chasm and flows through Huka Falls.
At the Huka Falls, the Waikato River which is normally 100m wide, is squeezed through a 20 metre wide
gorge and over a 20m drop. Every second up to 220,000 litres of water gushes through the gorge and
shoots out over 8 metres beyond to create a beautiful blue/green pool. There are many websites with facts
about these falls. For example, see http://www.hukafalls.com/
The speed and volume of this water flow must cause fluvial erosion but do you think fluvial erosion is
being slowed down changed or sped up due to human modifications? The turbulence will cause erosion
at the base of the plunge pool especially near the river banks. What do you think?
The rise and fall of the river level has an effect on the fluvial processes operating both before and after the
falls. As the river is controlled, the changes effect influences river bank erosion and therefore fluvial erosion
is sped up and there is an increase in fluvial transportation and fluvial deposition at the Aratiatia dam. Local
residents have complained to council, upset about riverbank erosion caused by these fluctuations.
See the animation from Exploring Earth (below) which shows a side view of an area where a river flows into
a lake. As flowing water enters the lake, its velocity decreases and the water's ability to carry sediments
also decreases. Sediments carried by the stream are deposited where the slowing water can no longer
move them. The largest sediments are deposited near the shore. Smaller sediments settle out farther from
the shore where the water is calmer. In this way silt fills up the dams on the Waikato River.
http://www.classzone.com/books/earth_science/terc/content/visualizations/es0604/es0604page01.cfm?chapter_no=visualization
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At Aratiatia, the erosion process has been altered by the river being diverted away from the rapids through
a large pipe. Because much less water flows down the rapids now, fluvial ersion has changed. Think about
the effect of exfoliation – flaking of rock caused by changes in temperature, for example, freeze/thaw.
Photograph taken before excess water spilled over into the rapids above
… and 15 minites later. Photograph taken, December 2009.
Are the rapids still eroding? Why/whynot? Are fluvial transportation processes being changed by taking
the water through an intake tunnel structure shown in the Mighty River’s sites below? See also the
noticeboard in the carpark at the rapids.
http://images.google.co.nz/imgres?imgurl=http://www.mightyriverpower.co.nz/Generation/AboutUs/Hydr
oStations/Aratiatia/Images/AratiatiaDiagram2.gif&imgrefurl=http://www.mightyriverpower.co.nz/Generati
on/AboutUs/HydroStations/Aratiatia/Technical.aspx&usg=__C3VnueouO0POjGI7aW0M5Bq6f0=&h=239&w=590&sz=7&hl=en&start=4&um=1&tbnid=vlVPRFdV00d0AM:&tbnh=
55&tbnw=135&prev=/images%3Fq%3D39%2Bthe%2Bnumber%2Bof%2Bin%2Bthe%2Bwaikato%2Briver%2
BMighty%2BRiver%2Bpower%26hl%3Den%26cr%3DcountryNZ%26sa%3DN%26um%3D1
http://www.mightyriverpower.co.nz/Generation/AboutUs/HydroStations/Aratiatia/
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Since the Aratiatia dam is the smallest of the dams on the Waikato River and the spillway is still
operating what might be its effect on fluvial erosion and transportation compared with the larger dams
further downstream? HINT: After the water has been through the penstocks and turbines it appears to
be clearer than at the dam entrance.
What ideas and or fieldtrip activities do you think would be useful for testing the water above and below
dams? Water samples could be taken at the Arapuni Lake entrance and then again after the water had
been through the tailrace. See the Conference CD disk for fieldtrip ideas.
In conclusion, the human modifications to fluvial processes in the upper reaches of the Waikato are less
significant than those in the lower sections of the river. However, in addition to the flow effects of Aratiatia,
the Wairakei geothermal plant with its two power stations side by side use vast quantities of cold water to
condense steam to make turbines generate almost twice as much electricity as if the steam were simply
discharged into the atmosphere, so we should also explore the interface between the fluvial and volcanic
processes that take place in this region.
Volcanic processes
There are a many published resources that provide good information on volcanic processes, and some of
those with a New Zealand and Taupo focus are included in the references at the end of this resource. As
teaching resources, both 3 and 9 are useful, while parts of 8 have some extraordinary detail about the
volcanic-fluvial ‘interface’ event in the region, the Taupo break-out flood of about 1800 year B.P.
The North Island volcanic zone lies along a major crustal line of weakness sometimes known as the Pacific
Rim of Fire. The zone is evident in volcanic events in a broad band from Ruapehu northeast to White Island
in the Bay of Plenty. While there are recent events like Mt St Helens (1980) and Pinatubo (1991) that
produced 1 and 5 cubic kilometres of material respectively, the size of the major eruption in the volcanic
zone (Taupo in 1800 years BP) is probably the greatest (at 90 cubic kilometres) that is known anywhere.
There are many places where volcanic events occur; we see will see examples of (i) eruptions of viscous
magma (rhyolites and andescites see http://volcanoes.usgs.gov/images/pglossary/andesite.php ), (ii)
extensive volcanic flows of highly mobile magma (such as ignimbrites and basalts), well described in
http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/ignimbrite_page.html and (iii) very
extensive airborne deposits (see http://vulcan.wr.usgs.gov/Imgs/Gif/Hazards/Tephra/tephra_diagram.gif )
known collectively as tephra.
For reference, Maungatautari is an older (Miocene)
andescitic cone, and there are places where intrusive
rhyolitic domes like Pohaturoa (photograph on right)
are found near Atiamuri.
The Pleistocene era (from about 2.6m years BP up
until the Holocene 12k years BP) saw almost
continuous volcanic activity in the field trip area.
There were massive outpourings of ignimbrite (in
sheets 10s metres thick) that we encounter first at
Piarere, and extensively after Atiamuri. The most
extensive outpourings of ignimbrite in the region are
found in the Mamaku series extending over perhaps
150sq km.
Lahars (http://www.geology.sdsu.edu/how_volcanoes_work/Lahars.html) and nuée ardente
(http://library.thinkquest.org/17457/volcanoes/hazards.nuee.php) are also associated with volcanic
eruptions.
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In terms of volcanic processes, the final feature that we will see in the field area is geothermal activities
(http://en.wikipedia.org/wiki/Geothermal_power ) created when ground water comes in contact with
volcanically heated subsurface materials.
Teacher activity. Using the material above and the text and diagrams of resources 1 and 2 to produce a
schematic diagram of the processes involved in volcanic eruptions. Include subsurface and surface
features, two forms of eruptions, air-fall and related processes.
Interacting volcanic and fluvial processes
The following text describes a volcanic event that had dramatic consequences in terms of fluvial processes
in the field trip area. The text is directly adapted from the abstract and text of resource paper 8.
Sudden releases of impounded water from lakes in volcanic regions constitute a major and frequently
repeated hazard. An outburst flood from Taupo caldera, New Zealand, released ~20 km3 of water, within
decades following an ignimbrite-emplacing eruption, ca. 1.8 ka. Lake Taupo has a modern area of 620 km2
and a volume of ~60 km3, and is the largest lake in the Taupo Volcanic Zone. The lake overflows with a
mean discharge of ~130 m3/s into the 320-km-long Waikato River.
The emplacement of 30–40 m of non-welded ignimbrite blocked the caldera outlet channel via the Waikato
River. Remnants of this fill survive along the valley walls and in side tributaries at elevations declining from
~390 m near the outlet to ~360 m at Aratiatia, 12 km downstream. The lake basin subsequently refilled
from precipitation runoff. The post–1.8 ka high-stand of the lake is marked by a semi-continuous wave-cut
bench and shoreline deposits at elevations of 28–43 m above the modern lake level of 357 m above sea
level.
Reconstruction of the Taupo flood provides estimates of peak discharge at the outlet in the range 17 000–
35 000 m3/s. Analysis demonstrates that (1) failure of the barrier was essentially instantaneous, (2) the
event may be treated hydraulically as a dam break, and (3) the peak discharge was a function of outlet
geometry rather than lake volume or breach formation rate.
Reconstructions based on empirical relations derived from historic dam breaches yield only order of
magnitude estimates of peak discharge. Calculations based on the physical dimensions of the outlet
channel at Taupo and hydraulic principles are likely to be more accurate and are in close agreement with
computer-implemented dynamic-flow–routing models. The latter give peak discharges and maximum stage
levels similar to constraints imposed by field evidence and estimates of flow depth and velocity. The long
duration of the Taupo flood and the relatively narrow, confined flood route resulted in minimal attenuation
of the flood wave compared with modern dam breach events, and flood deposits can be traced as far as
232 km downstream. The existing river falls 337 m in the first 180 km downstream of the outlet, through a
steep, narrow valley confined between late Pleistocene fluvial terraces and gorges incised in ignimbrite.
Teacher activity. Caldera lake breakout floods may be among the most far-reaching hazards associated
with volcanism. How would you use the Taupo break-out’ flood as an example of an extreme natural
event?
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Geothermal processes and human modification
On the field trip we will view the Karapiti (Craters of the Moon) tourist attraction where the geothermal
activity has been made into a tourist attraction. Reference 9 has details that are useful in understanding the
geothermal processes; this is available for purchase at the entry kiosk.
The more important activity is the visit to the Wairakei geothermal field. Pages 10-12 of resource 3 describe
the key processes, and the two images on the following page provide supporting evidence. One important
question is how do humans modify the volcanic (geothermal) process? Page 25 of resource 3 describes how
the steam flow has been drying up, and page 26 contains the important water “re-injection” process that is
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now being undertaken to sustain the life of the geothermal resource. Page 32 describes how the generation
of power from geothermal heat also uses the Waikato River for cooling purposes.
Notes on Geothermal Power at Wairakei
View of Geothermal Power Harnessed at Wairakei
Teacher activity. Make a diagram that explains the geothermal volcanic process, along with the capture
of steam and the human modification of the process through the re-injection of water to sustain the
system.
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Supporting documentation
1.
Ansell, R and Taber, J (1996) Caught in the Crunch: Earthquakes and Volcanoes in New Zealand. Harper
Collins; Auckland. Covers earthquakes first as part of the plate tectonics background required. The section on volcanic
events starts on p. 126, with good line drawings of the processes. This is followed by a section on Auckland, then south
Auckland to include (p.152-164) the Taupo Volcanic zone.
2. Cox, G J and Hayward, B W (1999) The restless Country: Volcanoes and Earthquakes of New Zealand.
Harper Collins; Auckland. Good overall coverage of physical processes of land formation, from plate tectonics to
volcanicity. Taupo volcanic zone is treated on p. 34-39, but section on geothermal material (pp. 40-41) especially good. See
map on page 40. Also covers earthquakes.
3. ECNZ Geothermal Group (2009) Power from the Earth: Geothermal Development in New Zealand.
Geothermal Group, ECNZ; Taupo. This is a simple but highly effective description of the processes of geothermal power
generation based on volcanic process at Wairakei. Suitable for year 13.
4.
Ell, G (2000) Volcanic Places. Bush Press; Takapuna. Good photography and simple text, with maps and diagrams.
5. Encyclopedia of New Zealand (2007) Life on the Edge: New Zealand’s Natural Hazards and Disasters. The
Encyclopedia of New Zealand. David Bateman: Auckland. Pp. 76-115. Good chapter on Caldera Volcanoes and
the Taupo Volcanic zone. There is text on Taupo/Oranui eruptions (26500 B.P) through to the most recent Taupo eruption
(1800 B.P.). There is also text on Okataina caldera. Good text on tephra, and recent volcanic events in the region north and east
of Ruapehu.
6. Froggatt , P (1997) Volcanic Hazards at Taupo Volcanic Centre. Ministry of Civil Defence. Volcanic
Hazards Information Series No. 7. Government Printer; Wellington. This is a dense text, but much useful
information, and very specific to Taupo Volcanic Zone. Good tephra map (p. 14)
7. Houghton, B F (1982) Geyserland: A Guide to the Volcanoes and Geothermal Areas of Rotorua.
Geological Society of New Zealand; Lower Hutt. Good introductory text including simple line diagrams. Good on
processes and includes wider region.
8. Manville, V White J D L Houghton B F Wilson C J N (1999) Paleohydrology and sedimentology of a post–
1.8 ka breakout flood from intracaldera Lake Taupo, North Island, New Zealand. Bulletin, Geological
Society of America; 111 (10) 1435–1447. This is one of the best scientific papers on the relationship between volcanic
and fluvial events in the region. It deals with the Taupo ‘breakout flood’.
9. Mongillo, M A (2003) Karapiti: Craters of the Moon Thermal Area. What’s the Story Publications;
Pirongia. This field guide to the site has some good graphics of volcanic processes, some maps and photographs and a Year 13
comprehension level.
10. Nairn, I A (1991) Volcanic Hazards of Okataina Volcanic Centre. Ministry of Civil Defence. Volcanic
Hazards Information Series No. 2. Government Printer; Wellington. Nairn provides a detailed description,
with some useful information, specific to recent (9000 B C to present) events in Okataina Volcanic Zone (Tarawera-EdgecumbeRotoiti). Good pyroclastic map (p. 6)
11. Thompson, B N Kermode, L O and Ewart A (1966) New Zealand Volcanology: Central Volcanic Region.
Department of Scientific and Industrial Research. Government Printer; Wellington. The text is a tour
handbook for 1996 conference in Auckland. The book has excellent maps and high level of technical information. It includes
chapter on Aratiatia rapids, pp. 73-74. There is also a good (if historic) chapter on Wairakei (pp. 156-171).
12. Volcanic Hazards of New Zealand. Special Issue of Tephra 14 (2) 1-32. Published by the Ministry of Civil
Defence. Government Printer; Wellington. The text has a useful map of Taupo Volcanic zone and descriptive accounts
of different volcanoes in the North Island. Good chapter on Ruapehu (18-21). There are very good process-related
photographs, and coverage of impacts; pp. 22-26.
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