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Why is the Brisbane River brown and not blue?

A CityCat crosses the Brisbane River underneath the Story Bridge
There are three main reasons why the Brisbane River is brown.

ABC News: Stephen Cavenagh

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When we put the call out for audience submissions for Curious Brisbane, we were inundated with questions.

But there was one in particular that kept popping up.

Edge of Brisbane River
Edge of the Brisbane River, looking towards CBD, in November 2017.

ABC News: Stephen Cavenagh

It's had creative monikers such as the "brown snake" for its long, winding path and murky colour.

And while they're similar lengths, London's River Thames is considered one of the cleanest in the world, but the same can't be said for the Brisbane River.

While the experts agree on the reasons behind its murky colour, they disagree on whether it's always been brown and whether it can ever be blue.

Let's start with the "why" and break it down into three simple reasons:

1. It's an estuary

By its very nature, the Brisbane River is brown because it's an estuary and influenced by the tide.

Two cyclists ride by the Brisbane River
Urbanisation is one of the three reasons why the river is brown.

ABC News: Stephen Cavenagh

Water flows from the catchment upstream, bringing sediment with it and as the tide comes in from the opposite direction, it causes a lot of turbulence in the water, continually stirring up the sediment.

"Once the sediment is generated into the river, then you get this constant resuspension of sediment just because of the nature of the river itself," Dr Paul Maxwell, principal scientist at Healthy Land and Water, said.

So the fine particles of dirt and mud that make up the sediment never get a chance to settle.

2. Urbanisation

Brisbane has changed dramatically since colonial settlement, and so too has the river.

Erosion from legacy clearing over the last century and a half has caused destabilisation in the water channel, increasing the amount of sediment.

Bathers under the Indooroopilly Railway Bridge circa 1930
Back in the 1930s, bathers would gather under the Indooroopilly Railway Bridge for a dip.

Supplied: State Library of Queensland

The city's heavy rains add to the sediment again as run-off from hard surfaces like roads and roofs enter the river.

After storms, the river will appear browner than usual because of the extra dirt flushed into it by the rain.

3. Dredging

In the 1860s, work began to dredge the river mouth to allow big ships through to the city and beyond.

A big ship on the Brisbane River in the 1930s
You don't see this anymore — giant ships passing through the Brisbane River.

Supplied: Brisbane City Council

The river bed was just 1.5 metres deep, and it was possible to walk from one side to the other.

By 1900, it was six metres deep and now it's about 15 metres deep.

Dr Maxwell said the dredging exacerbated the tidal influence.

"Now [it's] all the way up at Ipswich. So there's a significant change in the hydrology of the river."

Brown doesn't mean it's bad

While the river is dirty, Dr Maxwell stresses it's cleaner than it has been.

"Just because it's brown it doesn't mean it's polluted [or] that it's dangerous necessarily for humans to contact."

That's a legacy from when industry used the river as the city's drain.

Petrie Bight looking across to Customs House Brisbane circa 1890s
Looking across the Brisbane River to Customs House around 1890.

Supplied: State Library of Queensland

Brisbane's Customs House as seen from across the Brisbane River
Looking over to Customs House today.

ABC News: Stephen Cavenagh

Historically, meatworks would dump fat and offal in the waterway. There was waste from fertiliser and paper plants. And Paul Maxwell said blanket factories were particularly bad offenders.

"They used to discharge the dye from the dye pits out into the river so you could tell the day of the week by the colour of the river."

Was the river ever clear?

Experts are split on this one.

As we scanned the ABC archives we came across a story from 1990 on the health of the river.

Enclosed swimming pool in the Brisbane River at Mowbray Park, East Brisbane circa 1925.
People swim in an enclosed swimming pool in the river at Mowbray Park, East Brisbane circa 1925.

Supplied: State Library of Queensland

As the reporter's voiceover remarked:

"They remember relaxing on clean, sandy beaches and swimming uninhibited by fears of pollution."

Paul Maxwell testing water clarity in the Brisbane River
Dr Maxwell tests the clarity of the water.

ABC News: Karina Carvalho

Coastal engineer Jesper Nielsen said those reports were true.

"From an engineering perspective, if we reflect on how the river used to be before engineers essentially came in and changed it so much, it would have been blue," he said.

Dr Maxwell was circumspect.

"You've got nutrients coming through with sediment, it's going to go from brown to green.

"So when we get floods, it goes a little bit brown and in the dry time over winter we go green."

Can the river ever be blue?

Jesper Nielsen firmly believes the answer is "yes" — he's close to completing his PhD which looked for a solution to the river's brown colour.

"Surprisingly there's actually sand in the middle of the river and the mud is all on the mud banks and it's this mud that gets re-suspended continuously by the tide, giving the river its brown colour," he said.

He said the cure laid with the river's mudbanks and a native freshwater plant.

Bacopa monnieri sprung up along the river after the 2011 floods and despite the saltier conditions, it took to the river's muddy shores with earnest.

Mr Nielsen and his team have been replanting the bacopa in Jindalee.

Bacopa monnieri
Bacopa monnieri by the Brisbane River.

ABC News: Karina Carvalho

"These plants will help keep the mud here [on the banks] so the river will put the mud onto the mud banks — we'll work to plant these plants, keep the mud here and the river will clear itself up."

He has an ambitious timeframe.

"It would be a small army. Maybe 10 to 15 people."

Jesper Nielsen replanting bacopa at Jindalee
Mr Nielsen replanting bacopa at Jindalee.

ABC News: Karina Carvalho

Dr Maxwell applauded the engineer's efforts but doesn't believe it will work.

"It's very ambitious. I think the work they're doing is fantastic," he said.

"I think the impact on the river is far greater than the sediment that's on the side of the river itself.

"We've got big issues in the upper catchment and the legacy clearing."

Meet the question asker

Calum McKinnon with his fishing gear surrounding him, standing by a body of water
Curious Brisbanite Calum McKinnon.

Supplied: Calum McKinnon

Calum McKinnon is a 28-year-old public servant with the Queensland Police Service.

He's a keen fisherman, having moved to Brisbane from the Sunshine Coast in 2010.

He often goes fishing at the Wivenhoe Dam with bass, salmon and snapper among his catch.

As a result, Calum is keenly interested in the river and its water quality.

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