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

Introduction

This event provided a unique opportunity to describe the posteruption physical, chemical, and biological characteristics in affected lakes. One or more of the following criteria were used to select eight lakes for study:

1. the lake received ash deposits from the May 18 eruption;
2. background data were available from previous studies by the U.S. Geological Survey for comparison with current data; and
3. lakes in the immediate vicinity of the volcano were sufficiently close to be affected by deposits of pyroclastic material, ash, ice, mud, or timber.

Data collection began in June 1980 at the following study lakes: Spirit, St. Helens, Venus, Fawn, and Walupt Lakes in western Washington, and Warden, Sprague, and Amber Lakes in eastern Washington (figure 1). Data collection of 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. At the same time, new lakes were forming in the valleys of streams that, prior to the eruption, were tributary to the North Fork Toutle River and were dammed by a 17-mile-long plug of debris deposited in the upper valley of the Toutle River. The decision was made to discontinue data collection on the three eastern Washington lakes and to substitute for study, two newly created lakes in western Washington, Coldwater and Castle Lakes. Walupt Lake was also dropped from the study and replaced by Deadmans Lake.

The purpose of the investigation was to describe the posteruption physical, chemical, and biological conditions in the western Washington study lakes near the volcano; determine the changes in the water quality and physical characteristics of previously existing lakes brought about by the eruption and relate them when possible to distance from the volcano; to monitor the recovery of these lakes; and to document the developing characteristics of the two new lakes.

This report discusses the effects of the May 18, 1980, eruption on the physical, chemical, and biological characteristics of selected wester Washington study lakes: four lakes within the blast zone of the volcano -- Spirit, St. Helens, Venus, and Fawn Lakes -- and two lakes outside of the blast zone -- Deadmans and Walupt Lakes. The limnology of the two new lakes -- Coldwater and Castle Lakes -- is also included in the report. The 3-year study began in June 1980 and ended in August in 1982. Data collection at Deadmans, Coldwater, and Castle Lakes began in Spring 1981.

Limited background data for Spirit, Venus, Fawn, and Walupt Lakes are contained in Washington Department of Ecology Bulletin 43, v.4 (Bortleson and others, 1976). Additional data are available for Walupt and Spirit Lakes in Washington Department of Ecology Bulletin 42, part 2 (Bortleson and others, 1974) and part 5 (Dion and others, 1976), respectively. No documented studies of St. Helens and Deadmans Lakes were available prior to this investigation.

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