Australia Weather News

Heavy rainfall that caused flooding across southern Western Australia a week ago has filled Perth's river systems with sediment and nutrients, as well as potentially harmful bacteria.

Earlier this month Perth recorded almost 135 millimetres of rain, which is more than 10 times the monthly average for February and more than three times the summer average of 35.5mm.

Minor flood levels in the Swan Valley and further upstream in the Avon Valley have temporarily turned the Swan and Canning rivers into a murky mess.

"The rivers are so brown because of the unseasonal rainfall and flows into both rivers, it's bringing with it a large amount of organic matter — sediment and some nutrients," Mark Cugley, river systems manager at the Department of Parks and Wildlife, told ABC Radio Perth.

And although the heavy rain has stopped, sediment-laden water continues to pour into the coastal river systems.

"Even this past weekend, we have had some flood peaks coming in from the Avon catchment," Mr Cugley said.

"It's not as high as we had before, but there is still plenty of fresh water coming in with that high organic load and sediments as well."

Mr Cugley said people needed to be wary of swimming in the river systems because high levels of bacteria could also be present.

"The Health Department advises people not to swim after heavy rainfall for at least three days," he said.

"With the significant flows we have seen in the past week, we are certainly urging people to be cautious and not to have primary contact with the rivers in the next few days.

"That means avoiding things like swimming, water skiing, diving — activities where you could ingest some of the water."

The recent rains and high temperatures have also prompted some councils to step up their mosquito control efforts.

The City of Bayswater recently started using an amphibious all-terrain vehicle to treat breeding sites in low-lying areas.

Mr Cugley warned the rivers would likely stay brown for a number of days as water continued to flow through.

"I think it will be the best part of another week while we see this water moving from the upper reaches of the catchment down into the lower estuary into the coast.

"After that the tidal water will really start to dominate the lower estuary again and the discolouration will start to clear."