Effects of Mount St. Helens eruption on selected lakes in Washington

Abstract

On May 18, 1980, after more than a month of earthquakes and eruptions, Mount St. Helens, in southwestern Washington, exploded in a volcanic eruption more violent than any in the conterminous .United States during the 20th century. A lateral blast of hot gas and rock particles devastated an area of about 150 square miles on the northern side of the mountain knocking down trees to a distance of 15 miles. Several minutes later, a giant ash cloud rose to about 60,000 feet. Winds then carried the ash cloud across the United States, with heavy fallout and deposition in eastern Washington and parts of Idaho and Montana. Earlier, smaller eruptions deposited ash in western Washington and parts of Oregon and Canada.The hydrologic effects of the May 18 eruption have been both widespread and intense. During the eruption, a massive debris avalanche moved down the north flank of the volcano depositing about 3 billion cubic yards of rock, ice, and other materials in the upper 17 miles of the North Folk Toutle River valley. The debris deposits are about 600 feet thick in the upper reaches of the valley. Following the avalanche, runoff from the melted glaciers and snow, and possible outflow from Spirit Lake, caused an extraordinary mudflow in the North Fork Toutle River. The mudflow shattered and uprooted thousands of trees, destroyed most of the local bridges, and deposited an estimated 25,000 acre-feet of sediment in the Cowlitz River channel. A considerable amount of additional sediment was conveyed through the lower Cowlitz into the Columbia River where it was deposited and formed a shoal that blocked the shipping channel. Mudflows also occurred in the South Fork Toutle River and in tributaries on the east flank of Mount St. Helens which enter Swift Reservoir.As part of a concerted Geological Survey effort to study the volcanic event and to identify potential hazards, Survey hydrologists have mounted an intensive program to document the hydrologic effects of the eruptions. The major initial hydrologic findings are reported in this circular series. Quick, useful assessment was made possible only because the Survey has long conducted extensive water-resources investigations in the affected areas of Washington, Oregon, and Idaho. Hence, there was a well-defined basis for identification and documentation of the types and magnitudes of hydrologic changes.The Geological Survey Circular 850, "Hydrologic Effects of the Eruptions of Mount St. Helens, Washington, 1980," consists of individually published short chapters that emphasize data collection activities, field observations, and initial comparisons of preand post-eruption conditions. The series will cover hydrologic events occurring on May 18 in the Toutle and Cowlitz River; physical alteration of the Toutle River system; the chemical and physical quality of precipitation, streams, and lakes affected by volcanic ash fall; ash-leaching studies; and Mount St. Helens glaciers.

Effects of the 1980 eruption of Mount St. Helens on the limnological characteristics of selected lakes in western Washington

“…The severe alterations to the thermal and DO characteristics of Spirit Lake immediately after the eruption are discussed in detail in Dion and Embrey (1981). In May 1981 temperatures within the water column were relatively the same after spring overturn, and DO concentrations near the surface and near the bottom were 5.2 and 3.2 mg/L (milligrams per liter), respectively.…”

“…Two types of optical conditions were observed that deviated from the clear-blue, preeruption condition in the study lakes: (1) water severely turbid and gray in color from suspended particulate matter, typified by St. Helens and Venus Lakes; and (2) water relatively clear but intensely brown-stained from dissolved organic material, typified by Spirit Lake. Increased concentrations of total and dissolved organic and inorganic chemicals dramatically changed the chemical character of Spirit, Fawn, and Venus Lakes, but essentially did not change the chemistry of Walupt Lake, 37 miles northeast of the volcano (Dion and Embrey, 1981 The posteruption physical, chemical, and biological conditions of Spirit, St. Helens, Venus, and Deadmans Lakes varied according to their distance from the volcano and the initial degree of effect in 1980. Varying conditions from 1980 to 1982 in the lakes were probably due in part to the water-exchange times (time required for the water volume to be replaced by an equal volume of water from inflows) for each lake.…”

“…Data collection on Fawn, Walupt, Warden, Sprague, and Amber Lakes ended in October 1980 and on Spirit, St. Helens, and Venus Lakes in August 1982. Dion and Embrey (1981) summarized the results of the first 5 months of the investigation for the eight study lakes. Evaluation of the data in October 1980 indicated that the eruption did not have major, immediate effects on the study lakes in eastern Washington, nor on Walupt Lake in western Washington, compared to the degree of effect on Spirit, St. Helens, and Venus Lakes.…”

Changes in the organic material in lakes in the blast zone of Mount St. Helens, Washington

“…The increase in the concentration of dissolved organic carbon (DOC) in blast-zone lakes has been discussed by Dion and Embrey (1981), Wissmar, Devol, Nevissi, and Sedell, (1982), Pereira and others (1982), and McKnight and others (1982). During the summer of 1980, DOC concentrations were greatest in South Fork Castle, Coldwater and Spirit Lakes.…”

“…The volume of the lake increased by about 14 times during this interval, which indicates that dissolution of volcanic deposits in the watershed continued to be a source of dissolved inorganic constituents. The changes in the inorganic chemistry of lakes in the second group varied with the position of the lake relative to different blast deposits (Dion and Embrey, 1981;Wissmar, Devol, Nevissi, and Sedell, 1982). Lake, March 28, 1980, andJune 30, 1980 [Date from Dion and Embry.…”