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

Geological Survey

Library of Congress catalog-card No. 81-600041

OREGON

inch (in.) foot (ft) mile (mi) acre

f

TABLES EFFECTS OF MOUNT ST. HELENS ERUPTION ON SELECTED LAKES IN WASHINGTON

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) docu-

Gl ment 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
2. Most of the lakes had been studied previ-
3. The lakes in the immediate vicinity of the
4. The lakes distant from the volcano (that is,

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. ash as a result of the May 18 eruption;

ously by the U.S. Geological Survey, and some background data are available for comparison ;

volcano were sufficiently close to be impacted by the deposition of pyroclastic material, ash, ice, mud, or timber; and

in eastern Washington) were at least moderately productive of aquatic plants and animals and showed a distinct seasonal succession of the algal community.

',fp:..-..- ------l.O-~CHLAND

EFFECTS OF VOLCANIC ERUPTION

The four study lakes in the blast zone of Mount St. Helens (Spirit, St. Helens, Fawn, and Venus Lakes) received varying amounts of pyroclastic material, mud, ice, ash, and downed timber. The blast zone, as used in this report, is defined as the area immediately north of the volcano that was affected by the violent outburst, or explosion, that marked the beginning of the eruption. A detailed description of conditions in the blast zone at the time of the eruption was given by Korosec and others (1980).

St. Helens Lake is immediately north of the volcano and was in the direct line of the initial blast. Unlike Spirit Lake, St. Helens Lake did not receive a direct flow of volcanic material, so it was affected differently from Spirit Lake. Aerial photographs taken before (fig. 5) and

GS after (fig. 6) the eruption depict the devastation resulting from the volcanic blast. As shown in figure 6, much of the surface of the lake is covered by logs, and much volcanic material undoubtedly fell into the lake or slid into it from the steep slopes that surround the lake. The outlet of the lake, which allows water to flow across a bedrock lip and into Spirit Lake, appears to have been unaffected. Water was observed flowing out of the lake when the area was visited in July and August 1980. Because bathymetric maps are not available for St. Helens Lake, the effects of the eruption on the depth of the lake cannot be accurately determined. Sporadic soundings of the lake with a handline suggest that the post-eruption lake is more than 250 ft deep.

Fawn Lake, about 9 miles north-northwest of the volcano, and Venus Lake, about 10 miles north-northeast of the volcano, were also impacted by the eruption; but, because they are farther from the volcano, they were less impacted than Spirit and St. Helens Lakes. Both Fawn and Venus Lakes received heavy deposits of ash, and most nearby trees were uprooted, blown down, or completely stripped of their bark, branches, and leaves (figs. 7 to 10). The stages of the lakes have not changed significantly because of the eruption, and when they were visted in August 1980, water was draining from both lakes. Comparisons of preand post-eruption bathymetric maps (not shown in report) indicate that the basins of Fawn and Venus Lakes have not changed significantly, despite the deposition of ash in the lakes.

The configurations of the basins of other study lakes also were unchanged. For this reason, photographs and bathymetric maps of these lakes have not been included in this report. Readers desiring that information are referred to reports by Bartleson and others (1974, 1976), Dion and others (1976, 1980), and McConnell and others (1976).

The May 18, 1980, eruption introduced huge quantities of volcanic debris into nearby lakes and varying quantities of ash into all lakes beneath the ash plume depicted in figure 1.

The leachate also contained high concentrations of ammonia, nitrate, phosphate, and up to 225 mg/L (milligrams per liter) of dissolved organic carbon. Taylor and Lichte theorized that the source of the organic material in the ash was the atmospheric condensation of the products formed by the burning of vegetation surrounding the volcano.

In order to determine the effects of the debris and ash on the chemical characteristics of lake water, comparisons were made of chemical conditions in the study lakes before a1',r1 after the eruption. Those conditions are summarized in tables 2-9, at the end of the report. Post-eruption vertical profiles of water temperatures and dissolved-oxygen concentrations were completed for most of the lakes studied and were compared with pre-eruption profiles, if available. These comparisons indicated that, except for Spirit Lake, no significant changes harl occurred in the temperatures and dissolved-oxygen concentrations of the lakes ; therefore, the profiles have not been included in this report. Because of the lack of either pre- or post-eruption data, however, this comparison was not made for St. Helens Lake.

Most of the water samples were collected over the deepest part of each lake ; however, for logistical reasons, some of the samples from lakes in the blast area of the volcano were collected from the littoral (nearshore) zone of the lake. The amount of pre-eruption chemical data available for Fawn and Venus Lakes is lim-Constituent

Calcium Chlorine Magnesium Manganese Potassium Sodium Sulfate Concentration (micrograms per gram dry ash)

Chemical analyses of the leachate of volcanic ash collected near Moses Lake (Taylor and Lichte, 1980) indicate that the ash contains the following, and possibly other, soluble constituents:

Arsenic Barium Beryllium Cadmium Copper Iron Lead Mercury Selenium Zinc

ited to that collected as part of a reconnaissance investigation of Washington lakes.

In addition to the logs covering its surface, Spirit Lake contains large amounts of organic debris that impart an unpleasant odor to the water and give it a dark color. In fact, the water looks and smells very much like the effluent liquor from a pulp mill. The lake is rich in organic compounds (H. E. Taylor and J. M. Klein, U.S. Geological Survey, oral commun., November 4, 1970), especially gases (R. A. Rasmussen, Oregon Graduate Center, oral commun., October 10, 1980), as a result of the thermal, chemical, and biological alteration of organic matter introduced at the time of eruption. When visited on September 4, 1980, the surface of the lake was emitting large quantities of gas, some of it in "boils" 2 to 3 ft in diameter and some of it in almost continuous "streamers" of gas bubbles. On the October 1980 visit, the gas activity of Spirit Lake was considerably less. According to Rasmussen, nlore than 50 gases have been identified, including H 2S (hydrogen sulfide), CS2 (carbon disulfide), and COS (carbonyl sulfide). Many of the gases identified are toxic, and some are flammable. The concentrations of the gases within the lake, however, are unknown. Rasmussen also reported that there is a large amount of chlorine gas present.

Concentration (nanograms per gram dry ash)

Gll

On May 19, 1980, one day after the eruption and the subsequent influx of hot volcanic material into Spirit Lake, the temperature at the water surface (fig. 11) was 32.7 °C (T. J. Casadevall, U.S. Geological Survey, written commun., October 8, 1980). By October 16,.19SO, the temperature at the water surface had decreased to 12.7 °C and was only slightly higher than on October 9, 1974. The maximum temperature observed in 1974 was 16.1 °C, in August.

The heating of Spirit Lake and the introduction of organic matter have contributed to the depletion of the DO (dissolved-oxygen) concentration of the water. On October 16, 1980, the DO concentration in the epilimnion (top

w zw

w w

w z w

w

w

May

z

w

June July

G14 water layer) of Spirit Lake was only 0.8 mg/L; on October 9, 1974, before the eruption, the DO concentration in the epilimnion was 9.2 mg/L.

The chemical data in table 2 indicate that, as a result of the eruption, there were significant increases in the dissolved-solids concentration, specific conductance, hardness, and alkalinity of the water in Spirit Lake. The concentrations of dissolved iron and manganese also greatly increased, although the iron concentration had decreased by June 8, 1980. Significant increases were also noted in the concentrations of metals (calcium, magnesium, sodium, and potassium), nutrients (nitrogen and phosphorus), sulfate, chloride, and silica.

Sept. Oct. en ::::> 30 ~UJ

en

z

15 UJa.. -- -- -- -- -- --

Because no pre-eruption data were available for this lake, it was assumed that the quality of the water prior to the eruption was similar to that of Spirit Lake. Previous studies (Dethier and others, 1979) have shown that most alpine lakes without significant cultural development have similar chemical and biological characteristics. A comparison of pre-eruption water-quality conditions in Spirit Lake (table 2) with post-eruption conditions in St. Helens Lake (table 3) suggests that the changes brought about by the volcano were similar to those observed in Spirit Lake, but not as great. This generalization holds true for such characteristics as dissolved solids, specific conductance, hardness, metals, phosphorus, sulfate, and chloride. The concentrations of silica and DO in the epilimnion may have decreased, whereas the nitrogen, iron, and manganese may have increased. Assuming that the transparency of pre-eruption St. Helens Lake was as great as that of Spirit Lake, the transparency of St. Helens Lake, as measured by Secchi disc, was substantially decreased because of the suspended mud and ash. On August 28, 1980, the water was grayish brown and transparency was only 1 ft. No significant change in the water temperature has been observed.

The pre-eruption water-quality data for this lake are sparse, making comparisons difficult. The small amount of available information (table 4) suggests that the specific conductance, hardness, and concentrations of metals, sulfate, chloride, dissolved solids, organic nitrogen, iron, manganese, and chlorophyll a increased while the transparency and concentration of dissolved silica decreased as a result of the

eruption. The decrease in silica concentration may have been the result of changes in the algal diatom population and not a direct result of the eruption. The water transparency observed on August 27, 1980, was 7 ft, but it had increased to 12 ft by October 28, 1980.

The paucity of chemical data available for this lake prevents a complete discussion of the effects of the eruption. However, the observed increase in the specific-conductance value, from 9 to 233 micromhos (table 5), suggests that the lake water has undergone some type of alteration because of the eruption.

On the basis of available chemical data, the eruption had no effect on the water quality of Walupt Lake, about 37 miles northeast of the

G17 volcano. The relatively low transparency observed on June 23, 1980 (3.5 ft--table 6), occurred at a time of high runoff and inflow, and the exact source of the inorganic suspended sediment that caused the low transparency is not known. The source of the sediment may have been ash that was deposited either directly in the lake or on snow in the drainage basin of the lake. The sediment may also have been the result of normal spring runoff, although the minimum transparency value observed prior to the eruption was 20ft (in 1971).

For the most part, on the basis of chemical and physical characteristics documented in this report, the study lakes in eastern Washington appear to have been unaffected by the volcanic eruption, despite the deposition of up to 3 inches of ash in some areas. The chemical data (tables 7-9) suggest that the dissolved-iron concentration in the hypolimnion (bottom water layer) of Amber Lake and the dissolvedmanganese concentration in the hypolimnion of both Warden and Amber Lakes have increased. However, the data for Sprague Lake suggest that the iron and manganese concentrations in that lake have decreased. Given this set of seemingly contradictory data, it is difficult to ascribe changes in the water chemistry of the lakes to the erpution with any degree of certainty.

Very little pre-eruption biological information is available for lakes in the blast zone of Mount St. Helens, and, therefore, the biological effects of the eruption can only be surmised. Qualitative and quantitative data pertaining to populations of phytoplankton and zooplankton

G18 are available for the post-eruption period, but, because corresponding pre-eruption data are not available, the post-eruption data have not been included in this preliminary report. Productivity levels can be approximated, however, by the concentration of chlorophyll a in the water. Chlorophyll a is a green photosynthetic pigment present in all groups of algae.

Chlorophyll a concentrations in Spirit Lake in 1974 ranged from 0.3 to 1.3 f-tg/L (micrograms per liter). If the assumption can be made that pre-eruption concentrations in other alpine study lakes (St. Helens, Fawn, Venus, and Walupt Lakes) were similar to that in Spirit Lake, it appears that the productivities of all lakes mentioned were increased as a result of the eruption. The possibility exists, however, that the very fine suspended sediment in the water samples interfered with the optical technique of determining chlorophyll a

'"'"'co

The high temperatures and low DO concentrations observed in Spirit Lake after the eruption most likely killed all fish in the lake. Fish conditions in other blast-zone lakes have been described by B. A. Crawford of the Washington Department of Game (oral commun., November 24, 1980). In September 1980, according to Crawford, gill nets were set in three blast zone lakes not studied as part of this investigation. Live fish were found in all three lakes. The fish were fat and in good condition, but most had been feeding on terrestrial, as opposed to aquatic, insects. Crawford believes the long-term implications of this diet are significant. The fish will most likely survive the winter season by becoming semidormant beneath the ice cover of the lake and by utilizing stored fat reserves. However, the spring season will be especially critical for the fish, until such time as the terrestrial and (or) aquatic insect populations become large enough to support the fishery. Crawford thinks all fish in St. Helens Lake were killed by the high turbidity, although gill nets have not been set in that lake.

The study lakes of eastern Washington (Warden, Sprague, and Amber) are considerably more alkaline and productive than the lakes discussed above. A comparison of chlorophyll a concentrations before and after the eruption indicated that the concentrations generally have not changed. An algal bloom observed in Sprague Lake on August 13, 1980, resulted in a chlorophyll a concentration of 133 p.g/L. Sprague Lake has a history of summer blooms, and there is no evidence to indicate that the bloom observed on that particular date was a result of ash deposited in the lake.

The speed and extent of biological recovery of lakes in the blast zone of the volcano will depend in large measure on the specific biological effects of the physical and chemical changes observed in this study. It is likely that the lakes will not recover to their exact pre-eruption biological levels, but rather to new levels of productivity that are determined by prevailing physical and chemical eonditions. Because algae are at the base of the aquatic food chain, and because they produce organic tissue from inorganic substances dissolved in water, the recovery and well-being of the algal population is of prime concern in discussions of lake recovery in general.

A prediction of the likely biological effects of the observed changes is difficult; not all the changes affect algal growth in the same manner. Some of the changes, such as increased concentrations of nutrients, dissolved solids, silica, alkalinity, and hardness, would tend to encourage algal growth. Other changes, such as reduced transparency and the presence of potentially toxic trace metals (cadmium, copper, and zinc) might discourage growth. The effects of still other changes, such as increased temperature and increased concentrations of organic substances, iron, manganese, sulfate, and chloride, are more difficult to assess. Further complications arise from the fact that some elements, such as copper, are considered vital for algal growth, yet the same elements in excessive concentrations act as deterrents to growth.

Recent experiments by D. M. McKnight, G. L. Feder, and E. A. Stiles (U.S. Geological Survey, written commun., September 20, 1980) investigated the possible toxicity of leachate from volcanic ash collected in eastern Washington. The investigators determined that, although the leachate from ash collected near Richland was toxic to cultures of the bluegreen alga Anabaena fios-aquae in dilutions up to 1 to 250, the leachate from ash collected near Moses Lake had no effects, either stimulatory or inhibitory, on the same organism. The investigators concluded that the toxic substance is an organic compound that is not uniformly distributed in the ash fall, and that the ash fall had no effect on the chemistry or biology of four lakes studied, which included W alupt and Warden Lakes.

The preliminary findings of a study designed to determine the effects of the Mount St. Helens eruption of May 18, 1980, on the physical, chemical, and biological characteristics of selected lakes in Washington indicate that the lakes closest to the volcano were the most affected. These findings are supported by the data in table 10, which summarizes water-quality conditions in lakes at various distances from the volcano.

Spirit Lake, at the base of the volcano, was affected to the extent that it must now be considered a completely different lake. It has increased in area, decreased in depth, and risen about 240 ft in altitude as a result of the eruption. The surface of the lake is covered with logs, and the outlet to the North Fork Toutle River has been blocked.

The water of Spirit Lake is dark in color, contains large quantities of dissolved organic material, and emits foul-smelling gases. The water temperature rose to at least 32.8°C immediately following the eruption, and most of the dissolved oxygen was depleted. The concentrat!ons of several chemical constituents, increased including iron and maganese, sharply.

The biological productivity of Spirit Lake is high but, so far, has been limited to bacteria. The fish population, once composed chiefly of cold-water species, has undoubtedly been decimated or destroyed.

Other lakes in the blast zone were similarly affected, but not to such an extent. Chemical constituents in St. Helens, Fawn, and Venus Lakes have probably increased, and water transparency has decreased. Biological conditions in these lakes are largely unknown, and the speed of biological recovery cannot be predicted with certainty at this time.

G24 Lakes outside the blast zone of the volcano appear to have been unaffected by the eruption, despite the deposition of up to 3 inches of ash in some lakes.

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.