Description: From McGimsey and others (2007): "Between November 2004 and early May 2005, a flux of heat to the summit area caused melting of more than 1.3x10^7 cubic m (4.6x10^8 cubic ft) of ice and snow filling the summit crater of Chiginagak, resulting in a 400-m wide (~1,300 ft) and 105-m deep (~350 ft) cauldron containing an acidified lake (Schaefer and others, 2005; J.R. Schaefer and others, AVO/ADGGS, written commun., 2007) (see figs. 25 and 26 in original text). In early May 2005, a catastrophic release of sulfurous, clay-rich debris and acidic water from the lake, with an accompanying acidic aerosol component, traveled 27 km (~17 mi) downstream and flowed into the Mother Goose Lake, headwaters of the King Salmon River (see figs. 27 and 28 in original text). Extensive vegetation damage occurred along the flood route and Mother Goose Lake was acidified (pH of 2.9-3.1), killing all aquatic life and preventing the annual salmon run (J.R. Schaefer and others, AVO/ADGGS, written commun., 2007). AVO volcanologists were to begin the second summer of geologic mapping and hazard assessment at the volcano, but instead responded by documenting the flooding and damage, collecting water samples, measuring water temperature, conductivity, and pH, and surveying the extensive vegetation damage with a U.S. Fish and Wildlife Service botanist. A data-logging seismometer was deployed for about one month with no significant seismicity recorded; Chiginagak currently does not have a seismic network.
"AVO issued an Information Release about the activity on August 23, 2005, shortly after the field crew arrived on site, and an account was related in the Weekly Update (August 26). A summary of preliminary findings is presented by Schaefer and others (2005)."
In the August 23, 2005 Information Release, AVO reported: "An AVO field crew reports that a 1,300 ft (400 m) wide melt-water lake has formed in the snow and ice filled summit crater at Chiginagak Volcano sometime since the last observations in August 2004. Sometime earlier this summer, the southern crater rim of Chiginagak was breached, allowing a portion of the lake to drain. The resulting lahar (a debris flow consisting of a mixture of volcanic sediment, water, and ice) left a deposit on the unnamed glacier draining the crater to the south and caused flooding of 3 to 6 ft (1 to 2 m) above normal on Indecision Creek. Volcano Creek and Mother Goose Lake, the headwaters of King Salmon River, were affected as well. Floodwaters also flowed on and through Chiginagak's southeast glacier, spilling out into an unnamed Pacific drainage leading to Chiginagak Bay. The lahar likely contained acidic water. Although we have no direct samples of the summit lake water, measurements from crater lakes at similar volcanoes have shown pH values ranging from 0 to 3, the acidity arising mainly from sulfuric acid. Vegetation damage was observed along Indecision Creek and the unnamed Pacific drainage.
"The breach in the crater rim and the ensuing lahar probably occurred in July 2005; reports from Painter Creek Lodge at this time tell of strong sulfur smells and cloudy, yellowish water in the Indecision Creek drainage.
"AVO geologists continue to investigate the event and its impacts. There are no indications that an eruption is imminent or that this event is necessarily precursory to an eruption."
Follow-up studies of the area in 2006 revealed that the drainage is still acidic.