An Investigation of the 1997 Abyss Lake Jökulhlaup in Glacier Bay National Park

Grover, Scott

Glacier Bay National Park is one of the nation's largest Park units, comprising over 3.2 million acres. This Park is located in Southeast Alaska, about 95 kilometers west of Juneau, the state's capitol. The Park is marine in nature, containing over 1200 miles of pristine shoreline. These shores are found within the bay proper, and also on the exposed outer coast' in the Gulf of Alaska (refer to Map 1). The climate in Southeast Alaska is dominated by the proximity of the ocean. Dense rainforests are created by average annual precipitation amounts that exceed 450 cm. The Fairweather Mountain Range is located within the southwestern portion of the park, where much of this precipitation falls as snow. Over time, this snow has accumulated to become ice many hundreds of meters thick. With the assistance of gravity and the extreme relief of the mountain terrain, this ice begins to move downhill in the form of glaciers. The glaciers can fill valleys and are, in essence, frozen rivers. The rate of movement for the glaciers varies, but rates can exceed 200 meters per year. A glacier in a trunk' or main valley may be composed of several smaller glaciers whose accumulation areas lie at very high elevations. Lower tributary valleys to the trunk valley may be free of snow and ice for much of the season due to slightly warmer temperatures, however these lower valleys can be blocked by glacial ice in the trunk valleys. An arm' of the glacier may extend into the tributary valley which creates a dam of ice. Liquid water may flow into, or under the ice, but it is common for the blockage of the ice to be so complete that water accumulates in the tributary valley behind the ice. Thus, this type of blockage creates a glacial lake.' Southeast Alaska has one of the largest concentrations of glacial lakes in the world (K.H. Stone, 1963). Ice dams forming glacial lakes can be breached in several ways. Usually the failure of an ice dam is catastrophic: releasing vast amounts of liquid water in relatively short spans of time. The actual processes that control glacial outburst floods, or jökulhlaups, are still subject to controversy. Study of such events is hampered by extreme climates, topography, relative event infrequency, and of course, the ice itself. Some theories, however, have been put forth. S. Thorarinsson (1939) outlined a relationship between water depth in the glacial lake and height of the dam. He stated that due to density differences the ice would actually begin to float when the lake reached a certain critical' depth. This would allow for water to escape subglacially (under the ice). W. B. Whalley (1971) states that it is simply changes within the internal drainage system of the trunk glacier which allow for the initiation of flow. Whatever the mechanics of the initial flows, O. Liestol (1955) states that the escape route becomes exponentially larger with time. His calculations show that "water at 1 degree C and flowing at only 1m3/sec can theoretically melt over 270 m3 of ice in 24 hours." In essence, the flowing water melts the constricting ice, which allows more water to flow. This increased flow, in turn, melts additional ice and flows increase accordingly. This process continues until the glacial lake is drained.