Saturday, April 13, 2024

Let the rivers flow as they should

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Science now recognises that rivers need to be allowed to behave as naturally as possible, writes Zane Moss of Fish & Game.
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By Zane Moss, Southland Fish & Game manager

We Kiwis are passionate about our rivers, and so we should be. While they vary in character, they provide a sense of a wild, natural environment often close to our farms, towns and cities. Whether we fish them, walk the dog, swim with the kids, or just gaze as we cross the bridge, they’re inherently interesting.   

Mana whenua, local authorities, environmental organisations, farmers and wider communities all want the best for our rivers. However, differences can arise when it comes to how things have traditionally been done and what contemporary science is telling us we can do better.

Historically, Kiwis have straightened and manipulated rivers to direct them where we wanted them to go. This has allowed us to develop farmland on river plains that would otherwise be flooded whenever there was moderate rainfall. Similarly, it’s allowed us to build towns and cities in locations where rivers would have either flooded, or perhaps meandered and eroded, were it not for engineering intervention.  

River geomorphologists have termed this approach to river management “command and control”. The scientists who study river behaviour now consider that such approaches are no longer appropriate, and if continued without adaptation, will become increasingly risky.  

Perhaps surprisingly, it’s now recognised that rivers need to be managed in ways that enable them to behave as naturally as possible. This provides for greater natural character, healthier ecosystems, and resilience to the increasing frequency and magnitude of floods we’re predicted to experience through climate change. 

Put simply, our current approach effectively speeds up rivers, causes faster floods with higher energy, so that if a stop-bank fails, the damage is potentially catastrophic for communities. 

Progressively stepping back and broadening river fairways (the area between the stop-banks), although obviously difficult to achieve, would slow down floods, reducing their peaks and lowering the risk of catastrophic failure.     

There’s a temptation to look at a river and see exposed gravel bars that change after each flood. It’s easy to conclude that gravel is building up, and in a few reaches of our rivers, it is. However, it’s just as common for reaches to be “starved” of gravel, caused through decades of extraction for uses such as roading and concrete. 

Because most of the headwaters of the rivers of the Southland Plains are not from highly eroding hills, such as Canterbury or the West Coast, the bedload and migration of gravel is far lower than people appreciate. Generally, movement of gravel is simply a river re-working historic glacial outwash plains, attempting to recreate the same sinuosity and meander patterns that they’ve had in the past.

Often perceived changes in gravel are caused by changes in river channel morphology. A good example is at the Mossburn Bridge on the Oreti River, where Environment Southland (ES) had received complaints about the risk to the bridge, due to an apparent increase in gravel. 

ES re-surveyed the levels and found that while the gravel was higher in some areas, it had eroded against the rockwork they’d placed. This deepening of the channel caused it to deposit gravel on the beach, and seemingly build it up. However, the actual fairway capacity had not changed and nor had the risk to infrastructure.

I’ve heard complaints, primarily from one or two related parties, that the solution to flooding risk in Southland is to dig gravel out of rivers. I’ve also heard repeated allegations that Fish & Game has opposed consents to extract gravel to protect bridges and infrastructure. That is simply not true. 

To the best of my knowledge, Fish & Game has never opposed a consent application in Southland to remove gravel if it poses a risk to bridges or roading infrastructure. 

ES is responsible for managing our river engineering infrastructure. They’ve recognised that our current approach isn’t without its risks in the short and medium term, with predicted increases in flood magnitude and frequency.  

ES has commissioned Ian Fuller, Professor in Physical Geography at Massey University, to help develop a strategic document to inform gravel management on Southland’s rivers. As co-director of Massey’s innovative River Solutions Centre, Fuller’s expertise is second to none. 

It’s unhelpful to continually table-thump about gravel and blame gravel for all that’s apparently wrong with our rivers. Instead, ES needs support for its adoption of a science-led approach to ensure that as a community we make the best investment in future management of our rivers, to ensure we balance the needs of riverside farmers, downstream towns and cities, and instream river values.  

I often remember one of my young sons listening to a work conversation I was having with a farmer about gravel and flooding. Once I was off the phone he asked, “But Dad, isn’t flooding caused by too much rain?”

He’s not wrong, but it’s not quite as simple as that. 

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