In the late 1980s, a Japanese scientist named Koji Minoura stumbled on a medieval poem that described a tsunami so large it had swept away a castle and killed a thousand people. Intrigued, Minoura and his team began looking for paleontological evidence of the tsunami beneath rice paddies, and discovered not one but three massive, earthquake-triggered waves that had wracked the Sendai coast over the past three thousand years.

In a 2001 paper, Minoura concluded that the possibility of another tsunami was significant. But Tokyo Electric Power was slow to respond to the science, leaving the Fukushima Daiichi nuclear power plant unprepared for the 15-meter wave that inundated it in 2011. The result was a $188 billion natural disaster. More than 20,000 people died.

For the past several decades, paleo-hydrologist Victor Baker of the University of Arizona has been using techniques similar to Minoura’s to study the flood history of the Colorado Plateau. Like Minoura, he’s found that floods much larger than any in recorded history are routine occurrences. And like Minoura, he feels his research is being largely ignored by agencies and public utilities with infrastructure in the path of such floods.

Earlier this month, when a spillway at the nation’s tallest dam in Oroville, California, nearly buckled under the pressure of record rainfall, the consequences of under-estimating flood risks were brought into sharp relief. Dams aren’t built to withstand every curveball nature can throw — only the weather events that engineers deem most likely to occur within the dam’s lifespan. When many Western dams were built in the mid-20th century, the best science to determine such probabilities came from historical records and stream gauges.

But that record only stretches back to the late 1800s, a timespan Baker calls “completely inadequate.” Today, technology allows scientists to reconstruct thousands of years of natural history, giving us a much clearer picture of how often super-floods occur. “The probability of rare things is best evaluated if your record is very long,” Baker explains.

By combing the Colorado River, the Green River and others in the Southwest for sediment deposits and other flood evidence and then carbon-dating the results, Baker has concluded the short-term record severely underestimates the size and frequency of large floods. On the Upper Colorado near Moab, Utah, Baker and his team estimated the average 500-year flood at roughly 246,000 cubic feet per second, more than double the 112,000 cfs that scientists had estimated drawing on the stream gage record alone. Baker’s calculations put the 100-year flood at 171,000 cfs, also much greater than the previous estimate of 96,000 cfs. In comparison, legendary flooding in 1983 and 1984 that nearly overwhelmed Arizona’s Glen Canyon Dam, just downstream, peaked at just 125,000 cfs. (The dam has been bolstered since then, and today engineers say it can handle flows up to 220,000 cfs.)

In California, too, super-floods may be more common than previously thought. United States Geological Survey hydrologist Michael Dettinger and UC Berkeley paleoclimatologist B. Lynn Ingram have studied the paleo-flood record across a broad swath of California and discovered that such floods happen at least every 200 years, and maybe more frequently. The last one was in 1862. Thousands of people died, towns were submerged and the state’s economy was devastated, yet it was nowhere near the worst: One flood in the 1600s was at least twice as big.

In 2013, Dettinger and Ingram wrote in Scientific American that California was due for another huge water year. Their prediction has proven prescient. So much rain and snow has pounded California this winter that as of Feb. 21, half the state was under flood, rain or snow warnings. Creeks are overflowing their banks and flooding homes, and water managers were forced to spill excess water over the Oroville Dam’s emergency spillway for the first time in the dam’s 49-year history. On the night of Feb. 12, the sediment-choked water began eroding a hole in the spillway, threatening to release a wall of water. More than 180,000 residents fled to higher ground.

Luckily, emergency crews were able to patch the spillway, and people trickled back home. But Oroville isn’t alone — across the country, some 2,000 dams whose failure could cause loss of life are in need of repair, according to the Association of State Dam Safety Officials. And in many ways, Californians dodged a bullet: this winter’s precipitation was nowhere near as heavy as the storms Dettinger and Ingram have studied, and yet if Oroville’s reservoir hadn’t been depleted by years of drought, floodwaters could have easily overwhelmed the dam.

Does this mean dams like Oroville and Glen Canyon need to be fortified to withstand bigger storms? Officials from the Bureau of Reclamation are confident that Glen Canyon, at least, is equipped to handle even “extremely large hydrologic events.” And The U.S. Army Corps of Engineers is reluctant to apply paleo-hydrology research to existing infrastructure, in part because we’ve altered rivers so much that some Corps’ scientists believe ancient flood records are no longer realistic indicators of current risks.

But Baker believes it would be foolhardy to not at least create contingency plans for the possible failure of some of the West’s biggest dams. That Japanese officials were warned about Fukushima and didn’t act is “an embarrassment,” Baker adds. “We may have some similar things occurring in the United States, if we don’t seriously pay attention to this science.”

Krista Langlois is a correspondent with High Country News

over the last two weeks, heavy rains pushed water levels at Santa Clara County’s largest reservoir into the danger zone, with officials warning it could overflow.

That happened over the weekend, sending massive amounts of water into the Coyote Creek, which runs through the heart of San Jose.

By Tuesday, the creek was overflowing at numerous locations, inundating neighborhoods, flooding hundreds of homes and forcing the frantic evacuations of more than 14,000 residents, who remained out of their homes Wednesday.

The worst flooding to hit Silicon Valley in a century left San Jose reeling and residents angry about why they were not given more warning that a disaster was imminent. Even city officials on Wednesday conceded they were caught off guard by the severity of the flooding and vowed a full investigation into what went wrong.

“If the first time a resident is aware that they need to get out of a home is when they see a firefighter in a boat, then clearly there has been a failure,” said San Jose Mayor Sam Liccardo. “There is no question that we’ll need to do things differently next time.”

Late Wednesday, Assistant City Manager Dave Sykes said officials had learned that the information they had on the capacity of Coyote Creek channel was not accurate. He also said the city was working with the Santa Clara Valley Water District to determine whether debris caused blockages that contributed to flooding.

“The creek spilled over the banks faster and higher than anybody expected,” said city spokesman David Vossbrink.

Residents told harrowing stories of water flowing into homes and flooding streets. Many had to be rescued by boat. Some said they were surprised they did not get urgent warnings about the extent of the flooding.

“They didn’t say it was going to go up as high as it did,” said Louis Silva, 48. He said that his possessions were swallowed up in the flood and that the city should have warned people about the scale of the disaster with a cellphone text alert or by knocking on doors.

“They should’ve put the footwork in to show the urgency of the situation,” Silva said. “It hurt everyone. ... When Mother Nature shows up, she shows up.”

Dawn Rogers, 47, said she was in the mandatory evacuation zone but decided to hunker down instead of leave. She watched as firefighters took a boat down the street to rescue residents in homes that were flooded.

By 1 p.m. Tuesday, residents were rushing to fill up their cars with priceless valuables.

“It was scary,” Rogers said. “Being in a drought for all these years, you don’t ever think you’re ever in danger of a flood.”

Rob Souza, 49, thought he was prepared. He knew exactly where the floodwaters had previously risen on his William Street property, just west of Coyote Creek.

He spent eight hours Monday painstakingly building 3-foot walls of sandbags to protect his newly renovated cottage and his two-story home.

But by Tuesday morning, rising waters burst through the first sandbag wall, wrecking the cottage. Then Souza watched as the water rose to two feet above his home’s windowsill.

“It was like I was looking at an aquarium,” Souza said.

Then a window broke.

And then, Souza said, “it was all over.”

Anderson Reservoir, which is located in Morgan Hill about 22 miles south of downtown San Jose, had been releasing as much water as possible through its main outlet since Jan. 9, said Rachel Gibson, a spokeswoman for Santa Clara Valley Water District. The district was releasing water at a rate of 420 cubic feet per second through the reservoir’s outlet.

“We were trying to flood out as much water as we could in advance of any storms,” Gibson told reporters. “We have been pummeled by a number of storms since Jan. 9, so Anderson Reservoir was slowly filling up because more water was coming into it than we could practically let out of that outlet.”

Santa Clara water’s chief operating officer, Jim Fiedler, said his agency had been working in recent days with San Jose city officials on possible flood control options in case of a major event. He said the district had been in regular contact with city officials.

The situation came to a head over the weekend, when another round of heavy rain sent Anderson Reservoir over its tipping point, causing water to spill out of the lake and into Coyote Creek.

The first major flooding occurred Tuesday in the Rock Springs area of San Jose. San Jose firefighters paddled on rafts and waded through the chest-deep deluge, rescuing hundreds of residents trapped in homes and in trees.

Evacuation centers were set up at two community centers, where more than 300 residents stayed overnight. Two high schools were converted into overnight shelters, with dry clothes, food and cots.

Meanwhile, a damaged levee allowed water to flow onto U.S. 101 on Tuesday, forcing its temporary closure.

Coyote Creek slithers its way northwest from the reservoir to San Jose’s doorstep, where it proceeded to flood neighborhood after neighborhood, carving a destructive path through the heart of the city.

The creek crested to a height of 13.6 feet at a South San Jose river gauge point on Tuesday evening — nearly four feet above flood stage. The height shattered a previous record that had stood since 1922.

“This is a once-in-a-100-year flood event,” National Weather Service meteorologist Roger Gass said, referring to Coyote Creek’s surging height in South San Jose.

By Wednesday, the creek was no longer rising, but it was too late for some evacuees. 

City officials said some residents could be allowed home as early as Wednesday night, though Liccardo had warned earlier in the day that the water was highly contaminated with fuel, oil and possible sewage and posed a potential health risk. 

Officials said that on Thursday they would focus on assessing the damage and getting residents back home.

The approximately 14,000 people under mandatory evacuations hailed mostly from central San Jose. Evacuation advisories were also issued to 36,000 residents in a zone that covered a business and industrial area along a roughly seven-mile stretch of Coyote Creek.

By Wednesday evening, city officials had lifted some mandatory evacuations for homes north of Interstate 280. They also revised the number of residents impacted by evacuation advisories down to 22,000.

“We haven’t really had anything quite like this before,” Vossbrink said.