A series of pyroclastic flows from Redoubt Volcano in Alaska between December 1989 and April 1990 rapidly melted snow and ice that generated lahars in Drift River (valley in photo). The lahars swept 40 km to Cook Inlet. Most of the pyroclastic flows were caused by the repeated collapse of a lava dome growing high on the volcano's north flank.

This view across the upper Drift River valley is to the SW; the north flank is on the right side of the volcano, but the dome is not visible. As each pyroclastic flow swept down the volcano's snow- and glacier-covered north flank, the hot lava-dome fragments eroded and mixed with the snow and ice to form a torrent of water that swept into Drift River. The sudden surges of water eroded loose sediment on the valley floor and transformed into lahars.

In this part of the Drift River valley, many of the lahars covered the entire valley floor, but they were generally no more than a few meters thick. The lahars provided an excellent opportunity to test a new lahar detection system.