by Greta Yeoman
Timaru anglers have raised concerns over the state of the Opuha and Opihi rivers.
Angler Barry Stone said the Opuha River was full of Phormidium – often toxic, smelly algae – which had killed off a lot of the insect life and made the water quality unsafe.
“You could smell the river from 20 yards away.”
The Opuha flows into the Opihi and this had affected the water quality of the Opihi downstream of where the two rivers join, he said.
Environment Canterbury’s water quality and ecology principal scientist Shirley Hayward said the council’s science team had noted an increase in algae – both didymo and Phormidium – in the river this autumn.
River conditions addressed
The condition of the Opuha and Opihi rivers have been addressed in Environment Canterbury’s state of the environment report, Shirley Hayward says.
The council’s water quality and ecology principal scientist said the report had been presented to the Orari Temuka Opihi Pareora (OTOP) water zone committee, which had been working on recommendations for improving water quality in the zone. These would be up for public consultation in a few months.
“The minimum flow regime for the Opuha and Opihi rivers is one of the matters being considered.
“[It] acknowledges deteriorations in water quality and includes a recommendation for augmentation of the .. [two] rivers to improve environmental and flushing flows and mahinga kai values,” Ms Hayward said.
Opuha Water Ltd’s environmental manager Julia Crossman said the dam company had been “actively involved” in the OTOP process for the past two years, as well as with the associated Canterbury land and water regional plan.
This included working on proposed flow regimes, including increasing summer minimum flows.
“[We] have been advocates for some flow increases as long as they are based on robust ecological and economic information.”
The proposal, developed by representatives including Opuha Water, Fish and Game, the Department of Conservation and Timaru District Council, would give “greater flexibility” for artificial freshes, including increasing frequency and magnitude.
Ms Crossman said the benefit of the dam on the river ecosystem was that it stored water during periods of naturally higher flow to release it during lower flows. This provided “significant environmental benefit” by limiting the low flow, which had caused parts of the lower Opihi River to run dry occasionally prior to construction of the dam, she said.
The company had been monitoring water quality on the Opuha and Opihi, including one site above and one below the confluence with the Opuha.
“The monitoring shows that Phormidium is typically the main concern in the Opihi, and didymo is the main concern in the Opuha.”
Data from Land Air Water Atmosphere (Lawa) indicates the state of the Opihi River at Salesyard Bridge – considered the measuring point for the state of the flows from the Opuha Dam – has an overall rating of “red” or unsafe for swimming, partially due to toxic algae levels.
This was probably due to recent mild weather and lack of any major amounts of water flushing out the river since Christmas, she said.
While didymo is a non-native algae, Phormidium is naturally occurring and can be toxic to dogs and wildlife in large amounts.
However, Opuha Water Ltd (OWL) environmental manager Julia Crossman said the company, which manages the Opuha Dam, had recorded “unusually low” levels of didymo and Phormidium in both the Opihi and Opuha monitoring sites over the 2018-19 summer.
Its monitoring, undertaken with help from the National Institute of Water and Atmospheric Research (Niwa), had shown that “unfortunately” Phormidium had reached “nuisance levels” in the Opuha in March and April this year.
The company’s most recent survey on April 26 showed the Phormidium was dying off, but that a residual sludge of decaying “algal mats” remained on the river bed, Ms Crossman said.
Mr Stone said Phormidium was smelly when dying, and the angler’s recent visit to the Opuha in late March showed the river was “just rotten”.
“You could smell the river from 20 yards away.”
However, Ms Crossman said the amount of Phormidium in the Opihi had been observed to reach nuisance levels both upstream and downstream of the Opuha confluence.
“[This is] indicating that even without the influence of the dam, it is a significant issue in the Opihi.”
Ms Hayward she had also observed “high amounts” of algae in the Opihi River at Raincliff, which is above where the Opuha River joins the Opihi.
However, she added the health of the ecosystem in the Opuha River had been degrading since the Opuha Dam opened in 1998.
This was indicated by the “excessive algal growth” and a decline in insects, she said.
Ms Crossman said naturally occurring floods, and releases from the dam – also known as freshes – washed things such as didymo and Phormidium from the bed of the river.
While this would grow back, it helped prevent it growing to “nuisance” levels, she said.
However, Mr Stone said the releases had not flushed out the algae, they had only allowed it to spread downstream, into the Opihi.
Upstream of where it joins with the Opuha, the Opihi was “very full” of insect life, Mr Stone said.
By comparison, the anglers had only found six insects in the waterways of the Opuha after two hours of monitoring in early April.
Ms Hayward said research by ECan scientists had also had similar findings during monitoring of the Opuha.
“The state of the Opuha River is a concern because of the deteriorating trend in the invertebrate community, meaning the stream health overall is continuing to deteriorate.
“We need to continue working with Opuha Water Ltd to identify drivers of this deterioration and where improvements can be made.”
Ms Crossman said the dam’s ability to release artificial freshes was currently constrained by the need to store water to supplement low flows in the Opihi, and to supply irrigation.
“Artificial freshes are generally only released in wetter periods, when lake levels are above average.”
Mr Stone reiterated that the anglers were not opposed to the dam, only how it had been affecting, rather than improving, the quality of the rivers.