Mount St Helens lahar. Aerial photograph of a solidified lahar (volcanic mud flow), one of several that followed the eruption of Mount St Helens on 18 May 1980. Mount St Helens is in Washington State, USA. The lahars consisted of material from the collapse of the northern flank of the volcano, mixed with glacier ice, snow and meltwater from the eruption. The lahars flowed down valleys in the surrounding area, demolishing trees, bridges and houses, and then solidifying like concrete. The lahars covered distances of nearly 30 kilometres at speeds of many kilometres per hour. This is the South Fork Toutle River, that drained the western flank of Mount St Helen's. Photographed on 4 October 1980.