REPORT:
Effects of Mount St. Helens Eruption on Selected Lakes in Washington

Abstract

Water-quality changes were observed only in the lakes situated in the blast zone of the volcano. The changes included increases in most chemical constituents and reductions in transparency. Productivity levels in these lakes, as estimated by chlorophyll a concentrations, probably increased. No chemical or biological changes were observed in lakes that received only ash fall.

Introduction

The major eruption of Mount St. Helens on May 18, 1980, dumped tons of ash, mud, logs, and debris into more than 30 near-pristine lakes in the immediate vicinity of the volcano. Even lakes in parts of eastern Washington received deposits of windblown ash that locally measured up to 7.6 centimeters thick (Sarna-Wojicki and others, 1980).

The objective of this report is to describe the preliminary findings of a long-term study that was designed to (1) describe the present physical, chemical, and biological conditions in selected Washington lakes affected by volcanic activity; (2) compare present conditions in the lakes to pre-eruption conditions; and (3) document the recovery of the lakes. This report summarizes the first 5 months of a 3-year investigation that began in June 1980. The study lakes, whose locations are shown in figure 1, were selected on the basis of the following criteria:

1. All the lakes received significant amounts of ash as a result of the May 18 eruption;

2. Most of the lakes had been studied previously by the U.S. Geological Survey, and some background data are available for comparison;

3. The lakes in the immediate vicinity of the volcano were sufficiently close to be impacted by the deposition of pyroclastic material, ash, ice, mud, or timber; and

4. The lakes distant from the volcano (that is, in eastern Washington) were at least moderately productive of aquatic plants and animals and showed a distinct seasonal succession of the algal community.

General descriptions of the locations and physical features of the eight lakes selected for study are summarized in table 1. Background data for most of the lakes are contained in Washington Department of Ecology Bulletins 42 (parts 2, 4, 5, and 6) and 43 (vol. 4); the detailed references are given at the end of this report. St. Helens Lake had not been studied by the U.S. Geological Survey prior to this investigation; consequently, no background data exist for that lake.

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