Mount St. Helens
As well as the effect of the fast-moving hot gasses and stones from the explosion, the collapse of the northern side of Mount St. Helens caused lahars, or volcanic mudflows. These were mixtures of volcanic ash with melted ice and snow. The lahars went many miles down the Toutle and Cowlitz rivers, destroying bridges and killing many trees. A total of 3.9 million cubic yards (3.0 million m³) of material was carried 17 miles (27 km) south into the Columbia River by the lahars.