The Control Gates Bridge regulates the flow of water from Lake Taupō to the Waikato River. Photo / Dan Hutchinson
The Control Gates Bridge regulates the flow of water from Lake Taupō to the Waikato River. Photo / Dan Hutchinson
Near-maximum lake levels over the past four months have led to calls for tighter control over how much water is released from Lake Taupō into the Waikato.
On December 20, the lake measured less than two centimetres under the maximum allowed level, with overall levels sitting in the top quarter of the acceptable range continuously since July 12.
Water levels in the lake are controlled by the conditions of a resource consent held by Mercury, which has nine hydro power stations spread along 188 kilometres of the Waikato River, between Taupō and Karapiro. It owns the the Control Gates Bridge at Taupō, which regulates the lake outflow.
Mercury must keep the lake level between 355.85-357.25 metres above sea level. Outside of those levels, other resource consent conditions kick in to protect lakeside and downstream communities and the environment. For the past year - the wettest year on record for Taupō - those levels have been close to the maximum.
The level of Lake Taupō (blue line) has been consistently close to the maximum permitted level (orange line) for several weeks. Graph / Mercury
Dr Earl Bardsley, School of Science research associate at the University of Waikato, says water levels in the lake require careful management.
“The balance of any lake is a balance between the water going in and water going out.”
He has renewed his position that Lake Taupō should be subject to special regulation, with a target level in the middle of the current tolerances, rather than the minimum and maximum levels typically used in managed lakes.
“By keeping it in the middle, you have that extra bit of flexibility”, particularly when levels of rainfall are high.
The outflow from Lake Taupō has varied widely, while the lake level remains high. Graph / Mercury
He said that it was in Mercury’s interest, however, to keep the lake levels higher when possible to ensure levels of energy production.
This wasn’t necessarily of benefit to the lake itself.
“There’s something of a conflict, as always, between the economic and environmental interests of the lake.”
Running the lake’s levels as “income-optimal, as opposed to environmentally optimal” could potentially pose dangers to communities living close to the lake shore in periods of unstable weather, he said.
“When you have a high lake level and there’s a big storm event coming in, that’s when you risk major floods.”
His sentiments were echoed by Hamish Brookie, a spokesperson for lobby group Friends of Lake Taupō, which has concerns about the effects of the water level on lakeside communities.
“Prior to Mercury getting relaxed Consent Conditions in 2010, the maximum lake level would have been 100 millimetres lower than the current allowable maximum of 357.25m.
“That 100mm is what once protected lakeside communities from the effects of frequent extreme summer rainstorm events, which cause flooding and erosion of the lakeshore.”
He said Mercury’s operational decisions are unacceptably increasing the flood risk, and accused them of “playing Russian roulette”.
“If we do get a major storm on top of a full lake, then lakeside infrastructure and property will likely be impacted by flooding and erosion.”
However, Dr Bardsley said he does not “see Mercury as being bad people here”, simply a company operating optimally within the market.
It’s that market, said Bardsley, that should shift to prioritise the environment and security of the lake.
In response, Tim Thompson, head of wholesale markets at Mercury, said it has been the wettest winter since records began in 1905 and at no time have they exceeded the maximum control level of the lake.
“We work within rules designed to protect the lake and river and their users and environment.”
He said Mercury uses “a highly experienced team” to manage the water levels in the lake, and the control gates were fully opened between December 11 and 30 “to proactively manage the level of Lake Taupō”.
Despite the lake levels being just 15 centimetres below the maximum level on January 10, as the remnants of Cyclone Hale were passing across the Central North Island, the floodgates were not fully open.
“They are not fully open today [January 10] as the current forecast indicates that Lake Taupō will not receive significant rain.”
Having the floodgates fully open also has risks for anyone swimming downstream of the Control Gates Bridge. Some years hundreds of people drift down the river, emerging at Hipapatua Recreational Reserve, upstream of the Huka Falls.
The exceptionally fast currents resulting from fully open flood gates has prompted numerous warnings this summer not to enter the river from Taupō District Council, Taupō harbourmaster Jamie Grant and Mercury.
“My advice would be not to do a float trip at these high lake levels. It’s really dangerous, and Mercury can’t slow [the river] down; the lake is too full, and for pretty much the last three months it has been running at maximum.”
Tūwharetoa Māori Trust Board also released a statement on December 21, reminding residents and visitors alike that Waikato can be translated as “the surging currents”.
“The name ‘Waikato’ is as much a warning as it is a recognition of the power and lifeforce that flows from it.”
