Volcanic and Fluvial Processes Field Trip January 19 2009

Changes in the operation of fluvial processes within the area between Lake Taupo and the area just below the Aratiatia rapids has been influenced by human modifications made both before the river is allowed to flow into the river and within this area. The river operates differently now compared to the 1830s before the Europeans settled in this area.

See Map

This shows the source of the Waikato River is fed from the mountains of Mt Ruapehu, Mt Ngaruhoe, Mt Tongariro and the Kaimanawa Ranges but changes in the lower Tongariro river regime have been caused by the Turangi Power Development.

Top of Mt Ngaruhoe September 2008 looking out towards Lake Taupo and Mt Tauhara

Diversion of flows for hydroelectric generation in the Tongariro River, is more recently causing changes in flows and interruption to sediment movement down the river therefore changing the water sediments entering Lake Taupo. Effectively these dams act as silt ponds which reduces 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 however controlled so naturally large peak 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, a violent thunderstorm often produces rapid increases in discharge and stream energy in the form of flooding. As these flood events have 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

If you were to walk to the Taupo Bungy you can see the white cliffs below. Also look out on the rocky outcrop of Cherry Island where there has been some 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?

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 negligent apart from perhaps some further erosion of the banks due to the number of tourists entering the river at this point.

Frequently the solution of the load is more significant than the others?

Here is a link of a diagram that could be used (http://www.xmind.net/share/cianmm/fluvial-transportation/)

The Wairakei geothermal plant does increase the amount of arsenic carried in solution. We need to find out more about this.

The Waikato River then enters a chasm and then flows through Huka Falls

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?

There are many websites with facts about these falls eg http://www.hukafalls.com/

The rise and fall of the river has an effect on the fluvial processes operating in the area both before and after the falls. As the river is controlled this rise and fall effect influences river bank erosion and therefore fluvial erosion is sped up and therefore there is an increase in fluvial transportation and fluvial deposition at the Aratiatia dam. Local residents have complained to council on countless occasions upset about riverbank erosion caused by these fluctuations.

See this animation from exploring earth 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. Increasingly smaller sediments settle out farther from the shore where the water is calmer. http://www.classzone.com/books/earth_science/terc/content/visualizations/es0604/es0604page01.cfm?chapter_no=visualization
Silt therefore fills up the dams on the Waikato River just before the edge of the dam.

Photograph taken before excess water spilled over into the rapids above and 15 minites later photograph taken below (December 2009).

The operation of the erosion process has been altered by the river being detoured mostly away from the rapids through a large pipe. Due to the fact much less water flows down the rapids now than in earlier years fluvial ersion has changed. Think about the effect of exfoliation - flaking of rock caused by changes in temperature, for example, freeze/thaw.

Are the rapids still eroding? Why/whynot?

Are fluvial transportation processes being changed by taking the water through an intake tunnel structure? Look at the Mighty River Power sites below and the Aratiatia rapids site link or look on the noticeboard in the carpark at the rapids.
An Interesting animation of deposition.
http://www.mightyriverpower.co.nz/Generation/AboutUs/HydroStations/Aratiatia/
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 for sedimentation?
Water samples could be taken at the Arapuni Lake entrance and then again after the water had been through the tailrace. See the disk for fieldtrip ideas.
In conclusion the human modifications to fluvial processes in the upper reaches of the Waikato are somewhat insignificant compared to the human modifications in the lower sections of the river however 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 need to find out the effect this has on the operation of fluvial processes in this area.
Groundwater flowing into the river has been altered as have the water tables by the geothermal plant.