Past eruptions have severely affected river valleys that head on Glacier Peak.
Pyroclastic flows mixed with melted snow and glacial ice to form rapidly flowing slurries of rock and mud known as lahars. Also, landslide-generated lahars have occurred at Glacier Peak, but not as frequently as at Mount Baker. The Suiattle, White Chuck, Sauk, and Skagit river valleys are all at risk from lahar inundation.
About 13,000 years ago, several eruption-generated lahars churned down the White Chuck, Suiattle, and Sauk Rivers, inundating valley floors. Lahars then flowed down both the North Fork Stillaguamish (then an outlet of the upper Sauk River) and Skagit Rivers to the sea. In the Stillaguamish River valley at Arlington, more than 95 km (60 mi) downstream from Glacier Peak, laharsdeposited more than 2 m (7 ft) of sediment. Shortly after these eruptions ended, the upper Sauk's course via the Stillaguamish was abandoned, and the Sauk River began to drain only into the Skagit River, as it does today.
Between 5,000 and 7,000 years ago, dome-building eruptions generated lahars that extended to the sea. In small eruptions since 2,000 years ago, lahars have extended the entire length of the White Chuck River and part way down the Suiattle.
In river valleys downstream from the volcano, lahars could block transportation routes, destroy highways and bridges, bury houses in mud, cover farmland with debris, choke river channels, and increase the severity of floods for years or decades after the eruptions stop. These effects will be most frequent in the White Chuck and upper Suiattle River valleys. They will be less frequent, but potentially more damaging due to greater population and infrastructure, in the Sauk and Skagit River valleys. Still less likely would be lahars in the Stillagumish River valley, which would occur only if the Sauk River became choked with enough debris to be diverted west into the Stillaguamish River valley. When a lahar takes place, residents of communities along these rivers should move to high ground as quickly as possible.