The information below will be of particular interest to fishos asking for more information about the varying Tongariro River levels and flows. Thank you to AFTR for asking the hard questions:

Genesis staff met at AFTR (Advocates for the Tongariro River) committee meeting on 2nd August. 

The comments below were summarised by Eric Wilson, (photos from TRM library)

”At our recent committee meeting on August 2nd, we were joined by Genesis staff Gareth Gray (Renewable Energy Control Centre Manager and Principal Hydrologist) and Royce Hargeaves(Hydro Operator Maintainer- Tokaanu Power Station). 

The initiation of this visit was driven by the desire for a deeper understanding of how the power scheme interacts with the workings of the river, and in particular sedimentation.  The relation between flow rates and the amount of flushing of the river required to maintain the health of the riverbed and surrounds, is of particular note. 

Gareth and Royce provided an overview of parts of the Tongariro power scheme relevant to the Tongariro River, and how they are managed to satisfy the requirements of the respective resource consents. Cumecs= Cubic Meters per Sec 

– A minimum flow of 16 cumecs is to be maintained below the Poutu intake. 

– A maximum of 80 cumecs of water to be taken at Poutu intake- practical maximum circa 65 cumecs 

– The gates of the Poutu intake are to be closed if the flow above intake exceeds 160 cumecs (this takes around 2 hours as a process). 

 There was detailed and robust discussion around the impacts of power scheme operations may or may not be having on the river in terms of sedimentation in the lower river and gravel infilling in the middle sections. The river needs periods of high flow to clear itself for the health of the river system. In recent years, including the wild 2023 summer, average flows in the river have been at well below average levels. 

The AFTR questioned what a possible cause was for the abrupt drop in flow rates following rainfall events, where the rivers would rise naturally, but then drop suddenly. The explanation was that once the flow rate above the intake falls below the 160 cumecs then the gates would be re-opened and the flow rate downriver could drop quite rapidly. Bridget noted that this seems to have been occurring at flow rates less than the 160 cumecs required to close Poutu gates. It was agreed that Bridget would provide some screenshots of the flow graphs to Gareth. Gareth stated that he would then look at operations data for those dates and check if nothing else was going on, and if there was anything else that could be done better. Gareth then noted that recently, there was a tendency for short rainfall events, or weatherbombs, and maybe these were a factor. 

 We also explained our concern at the rapid expanse of invasive vegetation on the riverbed and surrounds. There are many reasons why this has become prolific in the last few years. It certainly hasn’t been helped by the lack of persistent high-flow events, as these would not have enabled much of it to take hold. The power scheme curbing the natural flow of the river does have an impact. We asked Genesis if they would consider partnering with financial support to assist community projects to help rectify the current situation. Gareth and Royce said they would assist us by liaising with the appropriate people at Genesis and advise us on how best to approach for assistance”. 

 The Advocates for the Tongariro River is a charitable organisation dedicated to preserving and enhancing the values of the Tongariro River and the surrounding environment. It relies on membership fees and donations for its work.

For more info go to: https://tongariroriver.org.nz/