Underwater rivers running along Australia's continental shelves are unlike any others in the world, according to a group of scientists at the University of Western Australia. Researchers claim the scale of the underwater rivers is unprecedented.

"This is the most significant discovery for coastal oceanography in recent decades, not only in Australia but globally," Chari Pattiaratchi, professor of coastal oceanography, said in a news release.

Researchers detailed their discovery of these unique underwater rivers this week in the journal Scientific Reports.

The scientists used a fleet of underwater autonomous vehicles, part of Australia's Integrated Marine Observing System, to observe the underwater rivers as they shifted through the seasons.

"The data spanned more than a decade and is the equivalent to spending more than 2500 days at sea," said Tanziha Mahjabin, who completed the research as part of her doctoral thesis at University of Western Australia. "We were able to examine data from different areas of Australia and also look at the seasonal variability."

Scientists typically use satellites to study river plumes, the flow patterns formed where large amounts of freshwater meet bodies of saltwater.

But the large, unique flow patterns found along Australia's continental shelves are hidden beneath the surface, invisible to satellites.

Unlike coastal freshwater inflows in other parts of the world, where freshwater remains buoyant and flows on top of the water, Australia's freshwater is quickly evaporated as a result of its hot, dry summers. As a result, coastal waters are actually denser than surrounding ocean water.

Scientists estimate the underwater rivers are fueled by these differences in density, sediment load and temperature of coastal and deep ocean water masses. As temperature and composition differences in the adjacent water masses shift through the seasons, the rivers ebb and flow in size and strength.

"The coastal ocean is the receiving basin for suspended and dissolved matter that includes nutrients, plant and animal matter and pollutants and represents an important component of the ocean environment, connecting the land to the deeper ocean," said UWA researcher Yasha Hetzel.

According to the study's authors, their findings suggest underwater rivers play an underappreciated role in the transportation of sediments, pollution and marine nutrients along the coasts of continents all over world.

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