Chris Allen has calculated that 60,000 tonnes of gravel has been naturally deposited in the north branch of the Ashburton River in the past year, adding to the 900,000t already there that needs removing.
He knows first-hand how naturally deposited gravel accentuates flooding events but, farming adjacent to the river, he has also discovered that solutions are far from simple.
Allen is part of the Ashburton River Liaison Group, which advises Environment Canterbury on river management.
Consent to remove gravel has become harder to get and logistically it is difficult to extract, but clogged rivers put pressure on flood protection works, as happened on the Ashburton River in 2021 when flood banks failed in several spots.
Allen said there can be unintended consequences when removing gravel, with concerns in the Ashburton River it could expose the foundations of the pillars of bridges and electricity transmission pylons in riverbeds.
He wants a nationally coordinated approach to funding work programmes.
“We need a management plan for the whole river, a plan that tells us who is doing what, and everyone knows their role from farmers to ratepayers, insurers, councils, Transit NZ and anyone else affected.”
Southland Federated Farmers president Chris Dillon said getting consent to extract gravel has become complex and heavily bureaucratic.
Read: Councils seek $150m a year to flood-proof rivers
In recent weeks 3000m3 has been deposited in the Mataura River near his home, and he said when it forms islands it diverts the river flow, eroding banks and releasing sediment.
Dillon spent $2500 “and an enormous amount of time” renewing his consent and pays a royalty for every cubic metre excavated.
Being granted resource consent to extract gravel has become tougher, but Wayne Scott, the chief executive of the Aggregate and Quarry Association, said that is only part of the problem.
Because aggregate is a low-value, high-volume product, it is not economically viable to truck great distances, which is limiting for areas like Ashburton that do not have large, accessible markets.
Gravel is also difficult to store. It cannot be stockpiled in riverbeds due of the risk of being dispersed during high flows, and takes up a vast amount of area on farm or waste land.
Traditionally regional councils survey the amount of gravel in rivers, but Scott said some have let that role lapse.
Consent to extract river gravel is becoming more difficult to get, but data from regional councils indicates volumes have increased, not reduced.
Environment Canterbury rivers manager Leigh Griffiths said reported gravel extraction from Canterbury rivers averaged about 1 million cubic meters per year in the late 1990s, and now averages about 1.6 million cubic meters per year.
ECan is closely monitoring takes to avoid overextraction in rivers such as the Ashley, Waimakariri, Opihi, Pareora and Waihao.
Griffiths said demand is exceeding supply in more places.
“We are paying increasing attention to the state the riverbed is left in when extractors leave the river, with regards to habitat values, natural character, ecosystem health and the potential for exacerbating coastal erosion.”
Randal Beal, Environment Southland catchment operations manager, said the National Policy Statement for Freshwater Management created additional requirements for scrutiny of any activity involving freshwater, including gravel extraction.
“Historically, there has been overextraction of gravel from our Southland rivers,” Beal said.
Extraction has moved to off-channel extraction to a form of habitat ponds, and new consented activities are shifting to beach-skimming type extractions.
“Due to the nature of beach extraction, less gravel can be removed, but it is undertaken in a way which has potential to improve the natural character of the river, with the potential benefits of increased flood capacity and reduced bank erosion,” he said.
Performed in an appropriate manner, gravel extraction is a useful river management tool.
Incidents of annual flooding occurrences in four of Southland’s major rivers from 1982 to 2002 and from 2002-2022 were about the same.
