Lahars, along with debris flows and debris avalanches, are masses of rock, mud and water that travel rapidly downslope and downstream under the action of gravity. Volcanoes are a perfect setting for these events because of an abundance of steep, rocky rubble and a ready source of water in the form of rain, snow or ice. Lahars can flow many miles downstream from the volcano, making this the most threatening hazard in the Cascades.
Lahar is an Indonesian word describing a mudflow or debris flow that originates on the slopes of a volcano. Small debris flows are common in the Cascades, where they form during periods of heavy rainfall, rapid snow melt, and by shallow landsliding. These relatively small debris flows seldom move more than a few miles down valleys. In the Cascades, the word lahar is typically reserved for larger events that occur in conjunction with volcanic eruptions, and travel many miles down valleys and affect local communities. Lahars can occur by rapid melting of snow and ice during eruptions, by liquefaction of large landslides (also known as debris avalanches), by breakout floods from crater lakes, and by erosion of fresh volcanic ash deposits during heavy rains. During and immediately following volcanic eruptions, lahars can pose the most severe hazard to populated valleys downstream from Cascades volcanoes. Visit individual volcano websites to learn more about specific Cascade lahar histories and hazards (volcano drop down from CVO home page).
About 500 years ago, the collapse of weakened rocks caused a large lahar at Mount Rainier. Called the Electron Mudflow, the lahar traveled through the Puyallup River valley. A 2009 USGS study by Wood and Soulard noted that 78,000 residents lived in the area covered by Electron Mudflow deposits. Today, those numbers are greater, and the next lahar in the Puyallup Valley could create significant impacts on the community and economy.To learn more about general lahar hazards and their effects, visit the Volcano Hazards Program lahars webpage.
To understand lahars, scientists observe and quantify what happens when these events occur naturally, derive equations to describe lahar behavior, and perform controlled experiments in a 300-feet (100-m) long debris flow flume. The results help us to understand flow dynamics and develop improved technologies for mitigating the destructive effects of debris flows.
Because of the danger presented by lahars, scientists are ready to deploy lahar-detection systems when eruptions are imminent. Officials near Mount Rainier maintain a permanent lahar-detection system and accompanying public notification system. If you are on the floor of a valley that heads on a volcano when officials announce the presence of an oncoming lahar, seek high ground. Find out more about lahar safety in the preparedness section of this website.