by Greta Yeoman
While most of South Canterbury’s rivers are safe to swim in, according to weekly monitoring, their overall quality rating presents a different picture.
Environment Canterbury monitors the region’s rivers and lakes over summer – from November to February – and while the weekly monitoring of E. coli rates most of them as green or “safe to swim”, the overall bacteria risk rating for the waterways is generally higher.
ECan water quality scientist Shirley Hayward said the overall rating was based on three years’ worth of data, which enabled the regional council to present a wider picture of the river’s safety and quality.
At the time of The Courier‘s deadline, of the 26 green-rated lakes and river monitoring sites in the region – collected on the Land Water Air Aotearoa (LAWA) website – nine had an overall rating of orange or “caution advised” while another five were rated red or “unsafe for swimming”.
There were 12 sites – mainly lakes in the Mackenzie but also two sites on the Opihi River – that rated green on a weekly and overall basis.
The Waihao River at Bradshaw Bridge had a current weekly rating of “unsafe for swimming”, due to high levels of toxic algae (cyanobacteria).
Further upstream on the Waihao, Black Hole was rated orange or “caution advised” due to its bacteria levels on a weekly basis, while it has an overall rating of red.
Bacteria levels found in the Waihi River, upstream of Wilson St Bridge, had prompted a health warning, but this was lifted on Tuesday afternoon.
Ms Hayward said people using South Canterbury’s waterways for swimming or other activities should take both the weekly ratings and overall safety rating into account.
The regional council reminded residents of the advice not to swim in waterways for at least 48 hours after rainfall, as this was often when faecal matter from rural and urban areas would wash into the water.
This was demonstrated by the high levels of E. coli found in the region’s rivers in November and December last year, due to the amount of rain that had fallen before Christmas, Ms Hayward said.
The run-off was why the importance of fencing off stock from waterways had been emphasised, she said.
A 2016 study of the popular-but-often-polluted Black Hole swimming spot in the Waihao River had shown a lot of it came from cattle, deer and sheep, as well as bird life.
Fencing stock away from waterways – and planting natives along the riverbank – gave the faecal runoff a chance to be diluted before it reached the river, she said.
This had been demonstrated by one spot on the Waihi River that had high levels of E. coli in the river before the farmer upstream from the site had fenced off his cattle from the the river.
Because the cattle were no longer defaecating directly into the river, the bacteria levels were a lot lower because it had to filter off the land – and this only occurred when it rained, she said.
While fencing was important, other actions, like the removal of willow trees by a group near the Waihao River, had also improved the water quality, she said.
Willows often caught plants and other matter in the branches, which could result in a build-up, she said.
“[It] chokes the river.”
The removal of the trees – which were being replaced by native plants, many of which act as a natural filter system for runoff – allowed the river to flush out debris.