Tsunamis are typically associated with powerful earthquakes beneath the ocean, sending massive walls of water toward unsuspecting coastal communities. However, scientists have found that some of the same destructive forces can unleash catastrophic waves in freshwater lakes, where nearby populations may have little time to escape.
Even more concerning, ordinary human activities such as farming, deforestation, and livestock grazing may be making these disasters more dangerous.
An international research team reconstructed two devastating tsunamis that struck Italy’s Lake Iseo during the Middle Ages, including one that generated waves climbing to approximately 164 feet. Their findings show that centuries of human-induced soil erosion helped create the unstable underwater conditions that made the disasters possible.
The research, published in Nature Communications, shows how seemingly harmless landscape changes can gradually increase the destructive potential of earthquakes.
The implications go well beyond Italy. Researchers identified similar hazards in lakes across Europe, Alaska, and New Zealand, prompting worries that populated freshwater shorelines may face an underestimated natural disaster threat.
How Human Activity Can Turn Lakes Into Tsunami Hazards
Unlike ocean tsunamis, which are frequently generated by sudden movements of the seafloor, lake tsunamis can occur when enormous volumes of sediment collapse along underwater slopes.
These landslides rapidly displace water, generating powerful waves that spread across the lake and surge onto surrounding shorelines.
At Lake Iseo, located in the southern foothills of the Italian Alps in northern Italy, researchers found that human activity had accelerated this process for nearly two millennia. Beginning during the Roman period, expanding agriculture, deforestation, and livestock grazing increased soil erosion throughout the surrounding Alpine watershed.
The Oglio River carried the loosened sediment into the lake, where it accumulated along an underwater delta. Over time, sediment delivery increased to approximately three times its earlier natural rate, progressively building steeper, less stable underwater slopes.
When earthquakes struck, the accumulated sediment became vulnerable to catastrophic collapse. In 1117, a magnitude 6.5 earthquake triggered an underwater landslide that displaced approximately 81 million cubic yards of sediment.
Using geological records, lakebed surveys, and computer simulations, the researchers reconstructed the resulting tsunami. Their models suggest waves exceeded 164 feet near the northern shoreline, with flooding extending roughly 1.7 miles inland in some locations, revealing that the first waves reached nearby shores in roughly one minute.
A Medieval Manuscript Reveals a Devastating Disaster
Remarkably, evidence of the catastrophe may have survived for more than 900 years in a manuscript preserved in the Vatican Library.
A 12th-century account describes a devastating flood that overwhelmed settlements surrounding Lake Iseo.
“The lake (Iseo) overflowed so much that very few of the inhabitants of the nearby villages escaped this danger,” the historical document states. “Many castles and fortresses also collapsed miserably, with the destruction of their inhabitants.”
Although the account cannot independently establish the tsunami’s height, researchers say its description matches their reconstruction.
Archaeological investigations offer additional evidence. The church of Sant’Andrea in Iseo underwent extensive reconstruction shortly after the 1117 earthquake.
Simulations show that tsunami waves approximately 11.5 feet high could have reached the church, although researchers acknowledge that earthquake shaking may also explain the damage.
The team also reconstructed a second catastrophe in 1222, when a magnitude 6.0 earthquake triggered another underwater delta collapse. This time, the resulting tsunami may have produced waves extending about 43 feet.
The fact that two destructive events occurred just 105 years apart remains particularly revealing. Continued erosion and sediment accumulation had apparently rebuilt the unstable underwater delta quickly enough for another catastrophic failure.
A Hidden Threat to Lakes Around the World
Perhaps the most troubling finding is that the conditions responsible for Lake Iseo’s medieval disasters have not disappeared.
Over the past eight centuries, human-driven erosion has continued depositing sediment into the lake, progressively rebuilding the underwater slopes that previously collapsed.
Today, its shores contain densely populated communities, marinas, and extensive tourist infrastructure. A similar landslide could cause devastating flooding with little warning for nearby residents.
Importantly, the danger is not unique to the Italian Alps. Researchers identified comparable underwater delta failures in Switzerland’s Lake Geneva and Lake Lucerne, as well as lakes in New Zealand and south-central Alaska.
In Alaska, for example, underwater delta collapses linked to the powerful 1964 earthquake affected several lakes, showing that the mechanism also operates in seismically active regions of North America.
The researchers also cite a global analysis of 632 lake-watershed records showing that human-driven erosion has reached exceptionally high levels since industrialization, potentially overloading underwater deltas worldwide.
The reconstructed 164-foot wave is a modeled estimate, not a directly measured historical height. Alternative simulations produced smaller waves, and the findings do not establish that another disaster is imminent.
Nevertheless, the research exposes a potentially dangerous blind spot in conventional tsunami preparedness. For communities living beside large lakes, the greatest danger may not come from the earthquake alone, but from centuries of accumulated sediment waiting beneath the water.
“Given that the hidden legacy of human land use can lead to large tsunamis, communities around Lake Iseo and in similar lacustrine settings worldwide can better prepare for these low-probability but high-consequence events,” researchers write. “The cascading effects observed in Lake Iseo serve as both a cautionary tale and a call to action, underscoring that today’s hazard assessment and land management strategies will shape the disasters of tomorrow.”
The team’s recent study, “Human-enhanced tsunami hazard in large lakes,” appeared in Nature Communications.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com