SPIRIT LAKE AT MOUNT ST. HELENS, MAY 1980—SURGE, LANDSLIDE, GIANT WAVE, LONG-TERM EFFECTS
All this excitement lasts three minutes. Above a sharp line of trim and log jams at altitudes 265 m above old lake level, trees lie where felled by the surge. Below the line the giant wave washed off trees, surge deposit, and soil. Most of the eroded wood returned to the basin with the water.
The lake’s surface area expanded but depth shoaled. With no outlet the new lake rose 12 m until an engineered outlet stabilized it. Recent bathymetry shows the lake’s east arm as well as the west arm deeply filled. Maximum lake depth in 1974 had been 60 m, the mean 40 m. Maximum depth in 2011 is 40 m, the mean 30 m. The landslide below lake level averages 86 m thick (maximum 122 m), its volume more than 425 million cubic meters. The deepest hollow in new Spirit Lake lies 37 m above the surface of old Spirit Lake. Just before eruption surface water had been clear and about 6 degrees C. Just after eruption it was murky, nearly 33 degrees C, and reeked of sulfur. Before eruption Spirit Lake had frozen over each winter. After the eruption it didn’t freeze again until February 2008. It took that long to cool the filling debris.
Just after eruption floating tree trunks and splintered wood covered more than half the lake’s surface. By 2014 much wood had waterlogged and sunk, and the raft now covers only a fifth of the lake’s surface. Waterlogging has been by species. True firs and hemlocks have sunk, but much Douglas fir and western redcedar still floats after 34 years. The lake’s 1980 shoaling greatly changed its ecology. Rooted aquatic plants and animal life abound now far more than they could in deep, steep-sided old Spirit Lake.