A leading geoscientist says two popular Cairns beaches will become mudflats over the coming decades in a revelation that could shock waterfront homeowners.
The modern Cairns Esplanade is famous for its man-made lagoon, rich wading birdlife and long, brown mudflats – but tourists a century ago encountered a markedly different scene.
Historical photos and accounts depict an esplanade lined by white sand as recently as the 1920s, with some contemporary reports speculating that decades of shipping-channel dredging are to blame for the transformation.
James Cook University adjunct professor Jonathan Knott disagrees and says the shift from beach to mudflat was due to entirely natural causes.
"One of the most telling pieces of evidence is a map of Trinity Bay and Trinity Inlet from 1878," he says.
"It shows a beach with no mangroves.
"There were mangroves on the airport side of Trinity Bay — or at least what is now the airport side.
"But there was just a sandy beach running along where the mudflats are now."
Mother Nature's changing moods
The map shows the mouth of the Barron River used to be further south than its current location — and the delivery of sand into Trinity Bay was its responsibility.
"We know that in the 1930s, the Barron switched its mouth to where it now comes out at Machans Beach. It moved north," Dr Knott explained.
"That effectively cut off the sand supply to the embayment of Cairns.
The sandy beach along the Cairns Esplanade was never a deep, long-reaching foreshore like tourists enjoy along Williams Esplanade at Palm Cove – at least not while photography or accounts from European holiday-makers were a factor.
Dr Knott says it is a fairly narrow strip of sand that separates the city from the ocean.
However, it was significant enough to elicit some heavy lashings of verbosity from one visiting writer way back in 1922.
Historian Timothy Bottoms, writing in A History of Cairns: City of the South Pacific 1770–1995, described the beach as stretching "like a band of pearl and saffron along the blue water's rim, dotted with palm trees, with giant fig trees, and flame trees just bursting into flower".
By the next year, a correspondent to the Northern Herald newspaper had a very different assessment, noting the popular sandy strip along the esplanade had become "anything but pleasant" because of the roughness.
"Previously they galloped along the soft sand beside the road, but now against the wall."
Whatever the case, Dr Knott says the beach's vanishing act is natural.
"There's a sort of balance between the amount of sand being delivered and the amount of mud being delivered," he said.
"That ratio has changed, whereby there's now more mud and less sand."
A history of coastal transformation
Coastlines are a fluid thing, and history shows what is now considered the norm is by no means static.
Sand deposits can still be found a long way back from the Cairns Esplanade — although most of the natural topography has been obscured by development.
"The whole of the Cairns CBD area is built upon sand deposits," Dr Knott said.
The best place to see those natural beach ridges is at the Pioneer Cemetery on McLeod Street, a few blocks inland from the city's public hospital.
"Because the cemetery is sacred ground, so to speak, it hasn't been built on," Dr Knott explained.
"It's made up of really coarse ground sand particles, and those sand ridges were actually placed there by tropical cyclones.
"Thousands of years ago, that's where the beach was."
The hospital had to be evacuated during Cyclone Yasi in 2011 because of its proximity to the ocean.
The coastal shapeshifting routine is still happening today — and it could have significant implications for residents of two beach suburbs within this lifetime.
Muddy prediction for two Cairns beaches
The mouth of the Barron River currently empties into the ocean just to the south of Machans Beach.
It also feeds into Thomatis Creek, which in turn reaches the ocean about 5.5 kilometres further north along the coastline, between Yorkeys Knob and Holloways Beach.
Nature is a big fan of seeking the path of least resistance — and Dr Knott expects the Barron River to make another significant shift in the not too distant future.
"In future, the Barron will switch to go out to the northern end of Holloways Beach of the southern end of Yorkeys.
"That makes it a shorter path for the Barron to the ocean.
"I don't know exactly what the time period will be, but there's potential for it to happen in the next 50–100 years."
Dr Knott predicts Thomatis Creek will eventually become the Barron River's main channel, and its flow down towards Machans Beach will gradually decrease until it peters out entirely.
That means less sand delivery to counteract the natural build-up of sediment in that area.
And that means the emergence of mudflats where the sandy foreshores of Machans and Holloways beaches currently stand.
Ongoing battles against the mudflat march
There are ways to artificially counteract the natural abatement of sand delivery, but Dr Knott believes they are futile in the long-term.
"That's what the [Cairns Regional] Council has been trying to do, really," he says.
"They've been trucking in sand from Half Moon Bay, just north of Yorkeys Knob, and dumping it on the foreshore to make an artificial beach [at the far northern end of the esplanade].
The council and the local port authority, Ports North, are also battling nature to stop the Cairns mudflats from becoming a mangrove forest.
Contractors are employed to pull out any saplings that have the nerve to extrude from the mire — otherwise the glitter strip's panoramic vista would soon become obscured by vegetation.
Bringing beaches back to the esplanade
Dr Knott says sandy beaches could theoretically be forced back to the esplanade, through major engineering works to re-route the Barron River to its former course.
However, it would be about as popular as a snake in a sleeping bag.
"There's a thing called the 4 o'clock club where everyone meets at the Esplanade each day to watch the mud-wading birds," Dr Knott said.
"I actually did some consultancy work with the council on this, because sand was starting to be deposited on the mudflats — and it wasn't encouraging the wading birds to come there anymore."
It turned out it was the council's own stormwater drainage outlets that were the culprits.
"When we get heavy rain, the water flushing out of the pipes washes the sand out into a sort of big delta, and gets pushed onto the mudflats," Dr Knott said.
At any rate, the veteran geoscientist doesn't believe there is any appetite to artificially re-contour a major river for the sake of easy beach access.
Cairns Regional Council has ruled out considering any realignment to the Barron River but noted it had recently adopted a coastal hazard adaptation strategy.
"The strategy assesses the potential risks to council, community and private assets, and proposes a number of measures to mitigate those risks," a council spokesman said.
"Importantly, the strategy identifies the assets most at risk, what the community in each locality values most, and what adaptations need to be prioritised."
Myth-busting on the Mulgrave River
He already ruled out dredging as a cause for the esplanade beach's disappearance, but Dr Knott wanted to pour water on another oft-repeated claim as well.
"A lot of people in Cairns put up an argument that the Mulgrave River used to flow into Trinity Inlet, and it's now switched and gone further south," he says.
"That's a myth. It never occurred."
Only time will tell whether the predictions come true, and Machans and Holloways beaches are doomed to a sandless future as the mudflat march continues its northbound incursion.
