THE WEST CREEK GLACIAL OUTBURST FLOOD - KLONDIKE GOLD RUSH NATIONAL HISTORICAL PARK - SKAGWAY, ALASKA

On July 23, 2002, approximately 8 million cubic meters of lateral moraine and glacial outwash suddenly liquefied into a proglacial lake sending a surge of water down West Creek, a tributary of the Taiya River. The pre-failure moraine/outwash complex terminated abruptly at the shoreline of the proglacial lake and was perched approximately 150 meters above the water surface with an approximate average slope of 50°. The moraine/outwash failure caused a large flood to sweep through Klondike Gold Rush National Historical Park (KLGO) lands, endangering human life and damaging cultural and natural resources and park infrastructure.

KLGO lies at the northernmost point of the Inside Passage of Southeast Alaska and contains the Chilkoot Trail, which parallels the Taiya River. The trail and nearby town site of Dyea were major hubs of activity during the Gold Rush of 1897-1898. The landscape is composed of steep glaciated valleys up to 1600 m deep that are connected to adjacent fjords. The underlying bedrock is primarily uniform granodiorite and the surficial geology includes glacial outwash, moraines, lacustrine deposits and alluvial fans. The Denali fault lies approximately 30 km west of the site, although no extraordinary seismic activity was detected around the time of the moraine/outwash complex failure event. Localities within Southeast Alaska have among the highest precipitation means in North America. However, the area encompassing KLGO averages only 66 cm/yr and there were no obvious weather-driven triggers to the event.

As part of a larger reconnaissance geohazards assessment of the 490 square kilometer Taiya River watershed, the West Creek flood event was studied along with two other known outburst events that have occurred since 1882. Tasks included: 1) reviewing and mapping the sources, mechanisms and affected areas of the event; 2) mapping other source and affected areas from all potential outburst floods in the watershed; 3) identifying and mapping other geohazards in the watershed and the areas that could be affected; and 4) determining the probability of a given type of geohazard occurring in the watershed. Work was done in consultation and under the supervision of KLGO and the National Park Service's Geologic Resources Division.