Effects of volcanic ash on the benthic environment of a mountain stream, northern Idaho

The May 18, 1980, eruption of Mount St. Helens deposited about 15 millimeters of volcanic ash on the Big Creek basin in northern Idaho. Much of the uncompacted ash remained on hillsides a year after the eruption. Physical and chemical analyses of water samples from Big Creek collected from December 1980 to December 1981 showed no anomalies attributable to ash. Qualitative collections showed benthic invertebrates to be abundant and diverse in Big Creek. Experiments conducted in an unimpacted mountain stream revealed a small quantity of volcanic ash may be beneficial not detrimental to invertebrate communities. Benthic invertebrates were most abundant on ash-covered artificial substrates, with detritovores dominating the communities on all substrates. (USGS)