Nevada's earthquake-prone levees are at risk. And near Tahoe, the state is working to support one.

Earthquakes, whose magnitude is diminished by drought, a warming climate and other more pressing environmental threats, are not on the minds of most Nevadans.

But during the mid-20th century, Nevada was known as an earthquake state. While the state has seen a few large earthquakes since then, recent tremors have caught the attention of those who monitor earthquakes.

On December 5, waves of water in the cave housing the rare Devil's Hole pupfish in Death Valley National Park rose about 2 feet high after a 7.0 magnitude earthquake struck off the coast of California.

Just four days later, a 5.8 magnitude earthquake struck an unnamed fault between Yerington and Silver Springs, shaking northern Nevada and sending objects flying into some buildings.

Neither quake caused any major damage — the California quake was very offshore, while the Nevada quake's epicenter was in a remote area — but they are a reminder that seismic activity can lead to significant damage to the state's aging infrastructure. That's why state officials are proactively shoring up some of Nevada's oldest earthen dams, which if shaken to the point of collapse, could cause contamination of the water supply for tens of thousands of people and deadly flooding.

The strength of an earthquake is measured on a scale of 1 to 10, with damage starting to appear around 5; With a magnitude of 6, buildings could see structural damage requiring repairs. Nevada sees about one magnitude 6 earthquake a year, but it always happens in a remote part of the state, said Christy Rowe, director of the Nevada Seismological Laboratory at the University of Nevada, Reno.

If a quake of magnitude 6 or greater hits Las Vegas, Reno or another densely populated part of the state, “there's going to be a lot of quake.” [of] Rue said.

In the case of recent earthquakes with epicenters in Yerington and off the California coast, urban areas like Reno have been “very lucky,” Rowe said. “If either of them happened in a city, it would be bad news.”

Inspections around the epicenter of the Leon County earthquake revealed cracks in irrigation ditches and collapses on the banks of the Walker River, but no damage to roads or bridges.

The quake also served as a reminder to officials of the state's seismic history and the need to reinforce some of the hundreds of levees across Nevada, most of which were constructed in the days before statewide engineering standards and built of earth and other natural materials.

Damage from earthquakes “is definitely a concern,” said Keith Conrad, chief of dam safety for the Nevada Department of Water Resources.

The state recently received federal money to upgrade the dam on Lake Marlette, one of the oldest in the state. Located on the edge between the Lake Tahoe Basin and Carson City, Lake Marlette is a popular mountain biking and hiking destination known for its fall colors and views of Lake Tahoe. Its reservoir, restricted by an earthen dam estimated to be more than 150 years old, serves as a water source for many Northern Nevada counties and cities.

The state purchased the lake and surrounding land in 1963, but its aging infrastructure, combined with its location in an area of ​​high seismic activity, make it a “high-risk dam.” Annual inspections of the dam indicate there is a “high probability” of a breach in the event of an earthquake of magnitude 6.5 or greater. If breached, it could easily flood State Route 28, which runs between the dam and Lake Tahoe, and with a sewer pipe running under the highway, damage to the highway could release sewage and debris into Lake Tahoe.

Roy said the seismic upgrade being done in Marlette should cover a lot of the vulnerabilities that could impact the lake due to earthquakes or other events.

“If they know the dam is at risk, I'm really glad they're doing preventative work,” she said. “It would be less expensive than if the dam failed.”

High Hazard Dams and Federal Emergency Declarations A map showing jurisdictional dams in Nevada. Green is low risk, blue is high risk, and red is high risk. (Nevada Department of Water Resources/Courtesy)

In 1915, the state experienced its largest earthquake, a 7.3 magnitude tremor near Winnemucca.

But there have been few high-magnitude tremors in urban areas since 1960, with the exception of a magnitude 6 earthquake in the Wells area in 2008. But earthquake faults are still being detected across the state (the Yerrington area quake occurred close to any known faults). . There is still a serious possibility of earthquakes, especially in western Nevada.

Roy said that in Lake Tahoe there are several major fault lines running under the lake, and the area is considered at high risk for earthquakes. While the state has been relatively free of any large earthquakes over the past few decades, Nevada has experienced the third-most frequent number of large earthquakes in the past 150 years.

Lake Tahoe was created by seismic activity, where a seismic fault caused part of the mountain to fall, creating a giant bowl, and volcanic sediments blocked the bowl on its north side.

About 5,000 years ago, the western shore of Lake Tahoe was hit by an earthquake large enough to trigger a tsunami — and the rifting caused by the tsunami in the basin would have overdue a quake, scientists said more than a decade ago at an annual meeting of the American Geophysical Union.

Earthquakes occur every 3,000 to 4,000 years along the West Tahoe Fault, which runs from the bottom of the lake to the Echo Peak area. This fault witnessed the last major earthquake 4,500 years ago. The cliff fault, located in the northeastern part of the lake, witnessed a magnitude 7 earthquake about 575 years ago.

Roy said that during earthquakes, attention first focuses on injuries and loss of life. After that, the impacts on basic services and communications are a serious concern, she added.

“Community resilience depends on the resilience of water supplies and food supplies,” she said. “It could take days or weeks to restore this type of service.”

It's the focus on infrastructure that prompted the state to apply for a $10 million federal grant to upgrade Lake Marlette Dam to reduce the risk of a dam breach during an earthquake. The Federal Emergency Management Agency (FEMA) agreed that the dam, listed as “high hazard,” warrants the work and issued funding.

The Nevada Department of Water Resources, the department where Conrad works, oversees about 660 dams across the state. Additional dams, such as those managed by the Bureau of Reclamation, the Army Corps of Engineers, and the Bureau of Indian Affairs, do not fall under the department's jurisdiction.

A few are concrete or stone dams – most are earth dams. Many are part of Las Vegas' stormwater detention basins or are small, privately owned structures used for irrigation in rural parts of the state (nearly half of the state's dams are privately owned) but some are larger and in areas that can hold significant amounts of Dams. Downstream effects if it fails, such as the dam at Lake Marlette.

Dams are classified into “low”, “substantial” and “high” risks in terms of how disastrous their failure would be to downstream residents and infrastructure. While failure of low-risk dams is unlikely to have any significant economic impacts or cause fatalities, failure of high-risk dams is more likely to cause significant economic impacts; A high-risk dam failure could potentially result in death.

Nearly a quarter of the dams monitored by the state, especially in the Reno/Tahoe and Las Vegas areas, are considered “high risk.” (The designation does not reflect the safety or condition of the dam.)

Nevada did not begin regulating dams until 1955; “Anything before that, we don't know much about these dams, and they probably weren't engineered at all,” Conrad said. When it comes to the Lake Marlette Dam, which was built nearly a century before the state began regulating dams, “who knows what techniques they used to build it. There's a big old question mark over that part of the dam.”

Conrad said that the general condition of dams within the state of Nevada is very poor.

A 2018 report by the American Society of Civil Engineers rated Nevada's dams a low grade of “D+,” or “vulnerable: vulnerable.”

A separate report released by the National Dams Inventory in 2023 echoed the 2018 findings, listing the condition of the bulk of the dams included in the report as “fair” or “poor.”

“It's hard to say exactly what the probability of failure is,” Conrad said.

In 2003, the state began formulating emergency action plans for all high-risk and large-risk dams in the state.

“We're kind of ahead” compared to some other states, Conrad said.

Conrad said those plans have been triggered from time to time, as happened earlier this year at Angel Lake near Wells when cracks were found in the nearly 150-year-old dam. The leaking cracks created gaps at the top of the dam, complicating the repair process.

There has never been a federal emergency declaration in Nevada due to a dam failure in the state, but Nevada has seen several dam failures over the years.

The first documented dam failure in Nevada occurred in 1876, when an irrigation dam across the Humboldt River failed 22 miles east of Battle Mountain, releasing a large amount of water across the valley and flooding several farms downstream.

In 1955, a severe December storm dumped between 10 inches and 13 inches of rain in northern Nevada, causing flooding along the Walker, Carson, and Truckee Rivers. The Derby Dam on the Truckee River failed, and the Hobart Dam outside Carson City failed and released water, causing extensive damage to US Route 395. Nearly $4 million in damage (about $44.2 million in 2022 dollars) was incurred and one person died.

Hobart Reservoir, also part of the Lake Marlette water system, is next on the list of state dams to be repaired with federal funding, Conrad said.

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