How were lakes affected by the eruption?
Prior to the eruption Spirit Lake and other lakes were typical cold, clear mountain lakes (low nutrients, productivity and temperature with high clarity). The removal and virtual vaporization of forest vegetation by the blast caused great changes in the physical and chemical structure of lakes within the blast zone. Tremendous quantities of useable ammonium, carbon and other energy sources (sulfur, iron and manganese) were leached into the lakes as rainwater percolated through the shattered forest and ash deposits. Prior to the eruption high mountain lakes like Obscurity Lake, 10 miles north of Mount St. Helens, were characteristically clear due to extremely low levels of dissolved nutrients. [USDA Forest Service, 1978] The same view after the eruption shows the extent of eruption damage to the forest surrounding Obscurity Lake. Note the large quantity of volcanic ash that was eroded from adjacent hillsides and deposited on large deltas at the mouth of inlet streams. [USDA Forest Servi
