These photographs before (top) and after (bottom) the first rainy season following the 1980 eruption of Mount St. Helens show the erosive power of running water.

Within a year of the eruption, thousands of gullies developed on steep slopes (bottom)  as the eruptive products from the enormous directed blast were removed and swept into stream channels. Many of the gullies cut into the underlying pre-1980 soil and tephra layers. Scientists measured erosion rates higher than 40,000 m³ per km² per year in some locations.

Erosion on hill slopes around the volcano slowed markedly in the second year following the eruption. On average, erosion rates during the second year were only 5 percent of the average first year rate. 

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