Heavy downpours across the state has led to the Yarra River becoming murkier and browner than usual, with health authorities urging Victorians to avoid the water at all costs.
Torrential rainfall this week caused some areas in Victoria to receive more than three months' worth of rain in a day.
The deluge has sent clay, dangerous debris and other sediment from the upper catchments towards the city's river.
Water quality warnings have been issued across the city urging would-be swimmers to avoid the river and parts of Port Phillip Bay.
River expert and member of the Yarra River Keeper Association, Janet Bolitho, said it was important to steer clear as the debris piled up.
"There's pesticides, there's fertilisers, there's oil that's been left on the road, all swept into the river," she said.
Swimmers have also been told to avoid beaches from Port Melbourne to St Kilda, with on-flow affecting water quality.
But the question that's surfaced for many is what causes that infamous brown colour in the first place?
According to Professor Vincent Pettigrove, an eco-toxicologist from RMIT, the colour is caused by clay within the water.
"The clay particles are like spaceships in their shape, when the light hits it, it disperses the light so you can't see into the water and it looks muddy and murky," he said.
It's expected the river and beaches should return to normal in about a week.
