The Awatere River has an opaque hue due to 100-million-year-old rocks from the supercontinent Gondwana.
By Kira Carrington for Local Democracy Reporting
The south Marlborough river had failed to meet the water clarity standards of both the Marlborough Environment Plan and the National Policy Statement for Freshwater Management.
Dr Martin Crundwell told Marlborough councillors on Thursday that ancient greywacke bedrock was the cause of the river's high sediment levels, causing the water to appear cloudy and opaque, and there was little the council could do to clear it up.
Crundwell, an honorary academic at University of Auckland and teaching fellow at Victoria University of Wellington, was commissioned to investigate the persistently high turbidity of the river with Marlborough District Council senior environmental scientist Steffi Henkel.
The greywacke bedrock originated from the Pahau Terrane of the early Cretaceous period, Crundwell said.
"Prior to 100 million years ago, the Pacific Ocean [tectonic plate] was coming down and being pushed down beneath the Gondwanan [plate], and as it happened, sediments from the sea floor were scraped off and piled up along the margin of Gondwana,” Crundwell said.
"That formed a series of sediments that were just pushed up, stacked up, piled up against Gondwana.
"This is where Marlborough sits ... it's caught by these rocks known as the Pahau Terrane, which is the youngest rocks of the tallest terrane."
Today, the Pahau Terrane, mainly made up of the highly erodible greywacke bedrock, made up up to 70% of the rock in the Awatere catchment, particularly along the Inland Kaikōura ranges and the northern Southern Alps, Crundwell said.
"If we move up the Awatere a little, you can see ... these cliffs, very steep, being eroded ... they're soft compacted sediments, so they're prone to erosion."
More than 15% of the "dirty" bedrock was made up of fine grain that eroded through a process called freeze-thaw weathering, Crundwell said.
"That is when there is water within the rock, it freezes and when water freezes it expands ... and then it thaws out, [and] goes through the cycle [again]. And that process actually just breaks the rock apart and it helps release this very fine grain."
The fine grain was then washed down into steams such as the Tone River that fed into the Awatere River.
The erosion of volcanic rocks around Mt Tapuae-o-Uenuku, Marlborough’s highest mountain and believed to be 66-100 million years old, also contributed to turbidity in the Awatere, Crundwell said.
Awatere faultline another key culprit
The Awatere faultline was another key culprit, he said.
"Where the fault goes through, these rocks are crushed, they’re pulverised, the term known as tectonised, so these are tectonically deformed [rocks]."
The tectonised rocks washed down into stream in the eastern Awatere catchment such as the Hodder River, Crundwell said.
Because of how widespread the bedrock is throughout the catchment, Crundwell said it was "neither feasible nor practical to do much about it".
Reforestation in the headwaters of the catchments could cause some improvements, he said, but not enough to meet the Marlborough Environment Plan’s environmental expectations.
Crundwell said the turbidity of the Awatere wasn’t unique, and the same bedrock surrounded the catchment of the Clarence River.
Councillor Barbara Faulls thanked Crundwell for his work, saying his presentation felt like being back at university.
"When’s the exam?" Faulls joked.
Faulls said the council would do what it could to improve the turbidity through the Catchment Care and Hill Country Erosion programmes.
"That’s really good practical support for landowners in that area," she said.
Council rural representative Simon Harvey said farmers would continue to work with the council on mitigation programmes.
"[The report was] a really good knowledge base in terms of what [were] realistic aims for the river," Harvey said.
Councillor Gerald Hope asked Crundwell to “paint the picture” of what the Awatere would have been like 1000 years ago.
"There was certainly more vegetational colour in the Awatere," Crundwell said.
"That vegetation has undergone various iterations, initial fires and things, and then with European settlement, more intense clearing and farming ... it's exposed more of the bedrock to erosion, and that would almost certainly have increased turbidity."
"But when you look at the height of the range and the elevated slopes of those ranges, they have certainly not been impacted to a large extent to what it would have been 1000 years ago.
"I don't think there would have been a great deal of difference."
– LDR is local body journalism co-funded by RNZ and NZ On Air.
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