When Washington State's Mount St. Helens erupted in 1980, the 57 resulting deaths were the most recorded for a volcanic eruption in the United States. Yet a report has warned these could be a tiny fraction of the true toll if nothing is done to tackle the legacy the outburst left behind. The danger lies in catastrophic floods that could be unleashed through a failure of a natural dam created during the eruption.

When St. Helens blew its top, it triggered an avalanche that dammed the Toutle River. Water could no longer drain from Spirit Lake, raising the water level. If this dam was to fail suddenly, the flood that would sweep down the river system would threaten 50,000 people living downstream.

Authorities were aware of this threat and quickly put measures in place to reduce the risks, including a 2.5-kilometer (1.5-mile) tunnel to allow some of Spirit Lake's water to escape. However, as time has gone on, the danger has faded in people's minds. A report to the US Forest Service by the National Academies of Science warns that engineering works to protect areas below the dam are in urgent need of updating.

Rumbles from St. Helens earlier this year gained attention as an indication the mountain might be about to erupt again. That seems to have been a false alarm, but even if it was real, the ash and lava of a repetition of the 1980 explosion would be unlikely to produce anything like the damage of failure by the Spirit Lake dam.

Such a failure is, unfortunately, a real risk. The pressure of water can cause dams to collapse, and the location's high sediment loads increase the chances of this, particularly given the frequent floods from higher up the mountain. Moreover, given its proximity to the Cascadia Seismic Zone, and the local shaking from even small eruptions, earthquakes are to be anticipated.

The report recommends repairs to the tunnel and to sediment retention structures above the dam. It also suggests consideration be given to the creation of a spillway or a second drainage tunnel.

 As dangerous as the Spirit Lake Dam is, it has nothing on another natural blockage half a world away. In 1911 an earthquake in Central Asia triggered a landslide that blocked the Murghab River and formed Sarz Lake. The lake is hundreds of meters deep, and if the dam were to fail as a result of a new earthquake, the resulting wall of water would wipe out 28 villages in the valley below. In the worst case scenario, more than 5 million people living further downstream would be in peril. Geologists disagree, however, on the likelihood of such an event.