Beachgoers and boaties have witnessed an engrossing sight in the Pumicestone Passage.
Two vastly different colours collided in the waterway, creating a dynamic effect.
Drone photographer Damien Lange was on hand to capture the scenes last week, when it was probably at its peak.
He was unsure what could have produced the phenomenon.
“There was a large amount of black, dirty water coming out of Pelican Waters (mainly from Lamerough Canal) into the passage,” he said.
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“The tide was coming in, but the fresh water couldn’t get in (to the bank).
“It was like chalk and cheese.”
Competitors in the Caloundra Triathlon swam through the black water into the clear water and back.
One of the racers, Wendy Miller, said via social media that it was an obvious distraction.
“You could see the distinct line where sea water was meeting canal water,” she said.
“After the swim, I heard so many (competitors) say it felt like they had hit a wall and couldn’t swim through it.”
Mr Lange wondered if the water was problematic.
“There must be something wrong with this. It never used to be this bad,” he said.
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University of the Sunshine Coast lecturer in environmental science and engineering Dr Luke Verstraten clarified the situation after examining the photos.
“It looks like a stormwater plume meeting ocean water,” the expert in stormwater and drainage said.
“The lower salinity of the fresh water compared to the ocean water can reduce mixing and cause stratification where the less dense/heavy fresh water from the creek runoff sits on top of the salt water.
“Tannins and sediments in the stormwater make the separation obvious by the different colours.
“(It’s) common after rain and occurs at other creeks on the coast.
“As stormwater runoff can potentially contain pollutants and microbes that can be harmful to human health, it is typically advised to avoid contact with it as a precaution.”
The organisers of the triathlon, The Event Crew, have been contacted for comment. Among the sponsors of the race are Australia’s peak body for triathlons AusTriathlon and Sunshine Coast Council.
A council spokesperson also commented on the sight.
“The change in water colour is a result of the fresh/brackish water coming out of the Pumicestone catchment and meeting the salt water,” the spokesperson said.
“It’s also referred to as the fresh/saltwater interface. It’s a result of recent rainfalls.”
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