How Cutting Down Forests Around Lake Iseo Set Up Two Giant Waves

Tsunamis are not only an ocean event. Under the right conditions they happen in lakes too, and at Lake Iseo in northern Italy they happened twice during the Middle Ages. Researchers who studied the lake say the people living there helped set the stage for both, by clearing the forests that had been holding the lake floor together. The same kind of buildup is now going on at lakes around the world.

The lake floor was gentle until the trees came down

The Oglio River flows into the northern end of Lake Iseo and drops its load of mud and sand there, building a fan-shaped bank under the water. Before the Middle Ages, forest covered that bank. When rain fell, loose soil washed downhill, but dense networks of tree roots and fungi slowed it down and held the ground in place. Underwater, the slopes stayed gentle, tilting slowly toward the lake's deepest point, about 250 meters down.

That began to change around the fall of the Roman Empire. Over the centuries that followed, forests around the delta were removed for pasture and mining, the human population grew, and on average three times more mud and sand was flushed into the lake each year. Across two thousand years of intensifying land use, the amount of sediment reaching the lake tripled. It piled up at the northern end, and the underwater banks grew steeper and less stable, until they were, in effect, waiting to fall.

One collapse in 1117, another in 1222

The lake sits below the Italian Alps, where the same forces that built the mountains still set off the occasional earthquake. In 1117 CE, an earthquake near Verona, magnitude 6.5, shook the steepened banks until they gave way. The collapse sent a huge volume of mud sliding into deep water, pushing the water ahead of it. In the researchers' computer models, waves in the northern part of the lake rose more than 50 meters and reached up to 2.7 kilometers inland, with less than a minute's warning. Near the town of Iseo, waves of about 5.5 meters arrived roughly seven minutes after the landslide. Monte Isola, the island in the middle of the lake, split the waves and partly shielded the towns behind it.

A 12th-century manuscript now held at the Vatican Library describes the disaster: \"The lake overflowed so much that very few of the inhabitants of the nearby villages escaped this danger. Many castles and fortresses also collapsed miserably, with the destruction of their inhabitants.\"

People rebuilt, and went on farming and grazing the same hillsides. Just over a century later, in 1222 CE, an earthquake near Brescia, magnitude 6.0, triggered a second collapse. This time the waves peaked at around 13 meters and ran up to 1 kilometer inland. The town of Iseo itself saw waves of about 2 meters.

Eight centuries of mud later, the shores are crowded

Since that second collapse, more than eight centuries of erosion have rebuilt the delta and steepened it again. Lake Iseo now has the most densely built shoreline of any lake in the Italian Alps, so the researchers say a tsunami comparable to the medieval ones would cause severe devastation today, given how many people live and build along the water.

The problem is not limited to Italy. A worldwide comparison of lakes showed that human-driven erosion has reached its highest levels since the Industrial Era, progressively overloading deltas in many places. The researchers warn that similar catastrophic collapses may become more likely in the future.