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Lahars and Debris Flows at Mount Baker

Deposit of the largest lahar from Mount Baker, Washington, exposed ...
Deposit of the largest lahar from Mount Baker, Washington, exposed near confluence of Middle and North Forks of the Nooksack River, about 30 km (20 mi) from source at Roman Wall. (Public domain.)

Mount Baker has the necessary ingredients to produce lahars and flank failures – an abundance of altered rocks coupled with lots of fragmental material (loose rock and soil) and plenty of water from rain and melting snowfall. Lahars can be triggered by an eruption or by non-eruptive events such as regional earthquakes causing failure of water saturated material, slope failure, increases in subsurface heat (like in 1975, which may cause snow and ice on the flanks to melt) and sudden release of glacial water (i.e., glacial outbursts).

Small lahars occur frequently and typically without an eruption. They travel only a few kilometers from source and have moved down all drainages that originate on the volcano. Intense rainfall or small landslides are the most probable cause of these types of lahars.

Baker Lake lies south of Mount Baker, Washington, view to the east. (Scurlock, John. Public domain.)

Moderate-sized lahars have occurred both during times of eruptive and non-eruptive activity. These flows have traveled between 10 and 14 km (6 to 9 mi) from the summit, and affect valley bottoms just beyond the flanks of the volcano. Lahars on the east and southeast side of the volcano could affect the Baker Lake and Lake Shannon reservoirs. Moderate-sized lahars that are either eruption-induced or triggered by failure of the east wall of Sherman Crater may enter Baker Lake and displace enough water to cause a wave to overtop Upper Baker Dam and impact Lake Shannon and the Lower Baker Dam. Failure of Baker Dam would result in catastrophic floods down the Skagit River. However, if water levels are low enough in the Baker Lake and Lake Shannon reservoirs, and the volume of lahars entering Baker Lake do not exceed reservoir capacity, then the lahars may be contained. If monitoring data suggest that unrest is occurring and an eruption is possible, it would be advisable to lower reservoir water levels as was done in 1975.

Large lahars (traveling farther than 15 km from source) almost always occur during eruptive unrest and activity. About 6,700 years ago, several large landslides occurred during a period of unrest and eruption off the northeast, east, and southeast sides of Sherman Crater. The landslides transformed to lahars that flowed at least 12 km (7.5 mi) down the Sulphur Creek and the Middle Fork of the Nooksack River drainages. The lahar was at least 100 m (325 ft) deep as it moved down the Middle Fork, and based upon the travel distances of similarly sized lahars at Mounts Rainier and St. Helens, it is likely that the flow continued downstream to the Puget Sound.