Atmospheric rivers - narrow corridors of concentrated moisture suspended in the atmosphere - were responsible for the mysterious mass die-off of wild Olympia oysters in San Francisco Bay in 2011, scientists have found.

This is the first documented case of these 'rivers in the sky' - which can hold 15 times more water than the Mississippi River - directly harming an entire population, and they're expected to increase in frequency, intensity, and unpredictability as the global climate warms. 

"Extreme events are predicted to be more prevalent under climate change," the University of California, Davis team reports.

"We highlight a new mechanism by which precipitation extremes appear to affect a sensitive species, contributing to the near 100 percent mass mortality of wild oysters in northern San Francisco Bay."

If you're not familiar with the natural phenomenon of atmospheric rivers, they're relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapour outside of the tropics.

These suspended moisture plumes are, on average, 400 to 600 km wide, and in California, they can deliver up to half of the state's annual precipitation in just 10 to 15 days. 

While most atmospheric rivers are mild, the ones containing the largest amounts of water vapour and the strongest winds can create extreme rainfall and floods, inducing catastrophic mud slides and drops in salinity.

They have been linked to all seven declared floods on California's Russian River between 1996 and 2007, and all 10 of Britain's largest floods since the 1970s.

Now it appears they've also directly responsible for their first mass die-off event. A population of wild Olympia oysters covered the northern San Francisco Bay with roughly 3,000 oysters per square metre, before almost 100 percent of them suddenly disappeared in March 2011

That's a problem, because not only is it a concern that an entire local population suddenly died out, but oysters also play a particularly important role in ensuring the health of the world's coastal ecosystems.

As Grennan Milliken explains for Motherboard, Olympia oysters are filter feeders that can clean up to 30 gallons (75 litres) of water a day, removing excess nitrogen and other pollutants from the environment.

Since the mass die-off event in March 2011, the University of California, Davis team has been investigating the underlying cause, because, on the surface, it didn't appear that anything catastrophic had occurred in the area.

They first investigated how oysters respond to changes in temperature, salinity, and dissolved oxygen, and found that they are particularly sensitive to low salinity.

Looking at what might have caused a sudden drop in salinity around the time of the great oyster die-off, they found that there were 20 atmospheric rivers between October 2010 and September 2011 in the area, and three of them happened in March 2011.

"In March 2011, a series of atmospheric rivers made landfall within California, contributing an estimated 69.3 percent of the precipitation within the watershed, and driving an extreme freshwater discharge into San Francisco Bay," the team concludes.

"This discharge caused sustained low salinities … that almost perfectly matched the known oyster critical salinity tolerance, and was coincident with a mass mortality of one of the most abundant populations throughout this species' range."

All that freshwater flowing into the environment appeared to hit the oyster population when it was down, because despite being one of the biggest populations of Olympic oysters at the time, over-harvesting and habitat destruction was already chipping away at their numbers.

The researchers report that the population has been recovering since the localised extinction event, but they might never be able to restore the numbers they once had.

"On the one hand, it's good news that despite the mass die-off, a few years later, they came back. But it's not as simple as that," lead researcher Brian Cheng explains in a press statement.

"These new oysters are smaller and less fertile, and that may have consequences for restoring oysters in San Francisco Bay."

Climate change expected to increase the frequency and severity of extreme weather events in the future, and severe precipitation events caused by atmospheric rivers are also predicted to intensify, because of increased evaporation and greater atmospheric water-holding capacity.

The researchers warn that because of this, similar local extinction and mass mortality events could be on the horizon.

Those oysters might have been just the tip of the iceberg for the rivers lurking in the sky.

The research has been published in Proceedings of the Royal Society B.