What is a lahar?
When melting snow and ice combine with ash and debris on a volcano, the mixture can form a volcanic mudflow known as a lahar. These flows rush down river valleys at speeds of about 40 miles per hour and can travel up to 50 miles away. Lahars can occur at any time and are not always triggered by a volcanic eruption. The video below shows a lahar flowing down a riverbed in Semeru, Indonesia.
Lahars have traveled down river valleys to Puget Sound in the past. Parts of Tacoma that lie in the floodplain of the Puyallup River sit atop lahar deposits. Lahar deposits are also found under parts of Burlington, Trout Lake, and Castle Rock. About 500 years ago, a rock avalanche on the flanks of Mount Rainier triggered the Electron Mudflow, which traveled 60 miles down into the Puget Lowland, reaching as far as Sumner. The image below shows areas around Mount Rainier that could be affected by volcanic blasts, pyroclastic flows, lava flows, and lahars.
How to prepare for lahars
- Take some time to find out if areas where you live, work, or recreate are in lahar hazard zones. A map of lahar hazard zones is available on our interactive Geologic Information Portal.
- Plan ahead. Identify evacuation routes, put together an emergency kit, and discuss evacuation plans with your family.
- Sign up for alerts offered by your community. For example, the Volcano Notification Service (VNS) will send you notification emails about volcanic activity at monitored volcanoes in the United States. Listen to media outlets. If a lahar is detected, notifications are issued on TV and radio.
- Listen for lahar sirens. Pierce County has sirens to alert affected towns that a lahar from Mount Rainier may be approaching.
- If a lahar warning is issued or there is a volcanic eruption, avoid river valleys and other low-lying areas. Get to high ground immediately. If you feel unsafe in your home or have been told to evacuate, go to a designated shelter or evacuation area.
- Stay informed. Areas impacted by lahars are often flooded repeatedly by mud long after the initial event. Continue to avoid low-lying areas until notified that it is safe to return home.