The eruption of Mount St. Helens in southwestern Washington had a pronounced effect on the chemistry of water in rivers draining these areas affected by the debris deposits, pyroclastic flows, mudflows, and airborne ash. Comparison of pre-eruption and post-eruption data show that there were marked increases in selected rivers of the levels of sulfate, chloride, turbidity, total nitrogen, total organic nitrogen, and total iron, manganese, and aluminum. In streams affected solely by ash, the changes were shortlived and the intensity of change decreased with distance downwind from Mount St. Helens. Streams in the blast and mudflow-affected areas showed more persistent effects.
After 100 years of dormancy, Mount St. Helens, in southwestern Washington (fig. 1), erupted with moderate intensity on March 27, 1980. Ash and steam were emitted, and a crater opened on the flank of the cone. Eruptions of ash and steam continued for several weeks, the crater enlarged daily, and continual seismic activity was recorded. On April 23, a bulge was detected on the north flank overlooking Spirit Lake. The bulge continued to grow outward at the rate of about 5 feet per day.
On May 18 at 8:32 a.m., Mount St. Helens erupted violently. After an explosive north-trending lateral blast, ash and steam were shot vertically to about 50,000 feet. In the immediate vicinity north of the volcano, a once-productive forest was completely pyrolyzed or burned; with increasing distance from the mountain, trees were uprooted and stripped of bark, branches, and needles or leaves. Ash was carried easterly by the prevailing winds and deposited to depths of up to 5 inches over much of the agricultural lands of eastern Washington. Watersheds immediately east of the volcano received heavy ash fall, and streams draining to the south, tributary to the Columbia River, received only traces of ash.
At the onset of the eruptive activity, the major part of the north flank of the volcano collapsed, depositing a massive plug of debris, including rock, ash, pumice, and ice in the upper 17 miles of the North Fork Toutle River valley. Following the initial blast, a mudflow quickly developed in the South Fork Toutle River, and several hours later a much larger mudflow originated from the massive debris avalanche deposit in the North Fork Toutle valley. This mudflow caused widespread destruction as it moved downstream through the lower Toutle and Cowlitz Rivers, and a considerable part of the sediment was conveyed through the Cowlitz into the Columbia River.
The massive physical alterations caused by the eruption could be expected to have marked effects on river water quality. This paper describes some of the effects caused by (1) ash deposition in rivers east of the volcano, and (2) the massive debris deposit and mudflows in rivers draining to the west and south.
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