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In a February 2017 incident, failures in the spillways of Oroville Dam forced the evacuation of 188,000 people and caused $1 billion in damage repairs.

According to scientists, a warmer climate might create more dangerous events like this.

Researchers at Scripps Institution of Oceanography at the University of California San Diego and the University of Colorado analyzed the event to understand why there was such a large inflow of water into the reservoir behind the dam leading up to the breach. They were interested in the potential role of intense “atmospheric river” storms, phenomena that transport large amounts of water vapor in focused “rivers” of precipitation to coastal areas.

A team led by Brian Henn, a former researcher at the Center for Western Weather and Water Extremes (CW3E) at Scripps Oceanography, found that the February 2017 atmospheric river sequence, while intense, was not extraordinary in terms of the amount of rain and snow it delivered. 

What was extraordinary, say the researchers, was the unusually deep snow recorded in the northern Sierra Nevada mountains before the storm event. Subsequently, several records were set for how much snowmelt occurred during the atmospheric river. The melt took place because of unusually warm and wet conditions during the atmospheric river, and it increased water available for runoff by 37 percent over rain alone, straining the capacity of California’s second-largest reservoir.

The researchers found that snowmelt in the Lake Oroville watershed reached 200-400 millimeters (8-16 inches) in some areas that additionally received up to 500 millimeters of rain (20 inches). Collectively, the large influx of water (71-91 centimeters [28-36 inches] in places) overwhelmed soil storage and resulted in tremendous runoff.

“Our findings suggest that without the unusual warmth that caused  extreme snowmelt from the atmospheric river, the inflows to Lake Oroville would have been less and the situation around the spillway failures may have been less critical,” Henn said.

The study appeared July 16 in the journal Geophysical Research Letters. The authors received support from NASA and the California Department of Water Resources.

Atmospheric rivers, phenomena that have only been widely understood in the past 20 years, will become more variable and play larger roles in extreme flood events in places around the world like California as the climate warms and changes, researchers believe. The storms can deliver torrential rains or no precipitation at all for reasons scientists are trying to understand.

The risk is highest, say scientists, when several factors converge, such as in February 2017, when deep and extensive existing snowpack -- itself the product of colder atmospheric rivers earlier in the winter -- was followed by four to five days of warm, wet, windy conditions. This sequence of storms contributed to the second-largest runoff into Lake Oroville in the last 30 years.

Multiple studies have projected that the risk of extreme snowmelt similar to what was seen above Lake Oroville will increase in response to climate change as atmospheric rivers become warmer and wetter.

After a series of intense atmospheric river-driven storms at the beginning of 2017, dam managers noticed breaches in Lake Oroville’s primary concrete spillway after heavy outflows. As high flows into Lake Oroville continued and water began to flow over the emergency spillway, officials briefly issued evacuation orders to 188,000 people living downstream on Feb. 12, 2017. Costs of repairs to the spillways, completed in November 2018, totaled $1.1 billion.

"We know that climate change is expected to increase the intensity of storm events in the Sierra Nevada, including extreme melt of deep mountain snowpack,” said University of Colorado researcher and study co-author Keith Musselman. “With our Oroville Dam case study, we highlight an example of potential threats to public safety and infrastructure associated with a warmer and more variable climate.”