A series of four lahars (volcanic debris flows and their deposits) occurred in rapid succession about 2,500 radiocarbon yr ago in the river system that drains the northwest sector of Mount St. Helens. The huge initial lahar had an instantaneous peak discharge near that of the Amazon River at flood stage, and the third in the series was the second largest in the history of the watershed. The deposits of the flows form a widespread terrace underlain by as much as 12 m of deposits, but its distribution reflects the fact that the lahars were the middle segments of flood waves beginning and ending as streamflow surges. These flood waves originated as lake-breakout surges analogous to those that would have been released, without engineering intervention, from the outlet-blocked Spirit Lake and several new lakes formed in 1980.

The lahar deposits are granular and noncohesive, reflecting the origin of the entrained sediment as stream alluvium beyond the base of the volcano. Megaclasts consisting of blocks probably derived from an ancient debris avalanche are abundant in the initial flow and locally form a diamicton with the laharic diamicton as matrix. Although fine-pebble angularity resulting partly from cataclasis during flow is diagnostic of the lahars, a high content of rounded clasts and a channel facies with zones of clast support may explain why similar deposits have not been more widely recognized. The lahar channel facies superficially resembles alluvium.

The depositional record of lahars and their distally evolved, hyperconcentrated lahar-runout flows many tens of kilometers from their source can provide a remarkably detailed history of volcanic activity. It may, in fact, provide the only evidence of some important events, of which the magnitude, frequency, and behavior are vital to the assessment of future volcanic hazards. Lake breakouts, meltwater surges caused by large pyroclastic flows, and forms of catastrophic volcanic ejection are examples of events that can create large downstream lahars but may leave little evidence of their true magnitude preserved on and near a volcano. The largest lahar in the river system, for example, began as a streamflow surge that bulked to a debris flow only after more than 20 km of flow.