Grand Pacific Glacier, Tarr Inlet

Grand Pacific Glacier, Tarr Inlet

by | Dec 4, 2021

Grand Pacific Glacier flows from elevations between 4,925-9,845 feet (1500–3000 m) on the British Columbia-Alaska boundary to Tarr Inlet in Glacier Bay National Park and Preserve, about 100 miles (161 km) southeast of Yakutat and 66 miles (106 km) north-northwest of Gustavus, Alaska. Grand Pacific Glacier originates between Mount Hay in the north and Mount Lodge in the south and flows east for 17 miles (27 km) in British Columbia to Grand Pacific Pass where the Melbern Glacier splits off and flows northwest towards the Tatshenshini River and presently terminates in a glacial lake at an elevation of about 820 feet (250 m). Melbern Glacier was named in 1912 by J.D. Craig, who was in charge of the 141st Meridian survey, after William Melbern Dennis, a Dominion Land Surveyor on the International Boundary Survey. The main ice stream of the Grand Pacific Glacier flows southeast for 10 miles (16 km) from Grand Pacific Pass to the Alaska border, and then another 1.2 miles (2 km) to tidewater in Tarr Inlet. Grand Pacific Glacier was named by John Muir in 1879 when the terminus was 14 miles (23 km) to the south at Russell Island near the head of Glacier Bay. In 1892, Harry Fielding Reid, an American geophysicist, found that the terminus of the glacier had receded into three distinct glaciers. He retained the name ‘Grand Pacific’ for the most northerly and largest ice stream, and named the middle ice stream ‘Johns Hopkins‘. The smallest and most southerly glacier that flows into Glacier Bay from the west was named ‘Reid‘ in 1899 by the Harriman Expedition. Tarr Inlet extends north for 11 miles (18 km) from Russell Island at the head of Glacier Bay. The fjord was named in 1912 by Lawrence Martin of the U.S. Geological Survey for Ralph Stockman Tarr, who was a professor of physical geography at Cornell University and visited this fjord in 1911. Tarr was the author of many geological publications including several on Alaska. Russell Island is about 3.6 miles (6 km) long and 1.6 miles (2.5 km) wide and was once a nunatak surrounded by glacier ice. The island was named by William O. Field and William S. Cooper in 1937 for Israel Cook Russell who was an American geologist and geographer who explored Alaska in the late 19th century.

Grand Pacific Glacier is about 1.2 miles (1.9 km) wide at the terminus and much of the ice margin is now grounded at low tide. The ice cliff is estimated to be 60 feet (18 m) high where it is grounded, but about 150 feet (46 m) high where it calves into Tarr Inlet. Behind the terminus, the ice may thicken to 900 feet (274 m) or more. About 2/3 of the terminus ice originates from the Ferris Glacier, a tributary ice stream that merges with the Grand Pacific from the west about 3 miles (4.8 km) north of Tarr Inlet. Ferris Glacier was named after W.C. Ferris who worked with the Canadian party engaged in surveying the 141st Meridian of the International Boundary in 1909-14. Ferris Glacier flows about 1500 feet (457 m) per year or about 4 feet (1.2 m) per day. The remaining eastern portion of Grand Pacific Glacier moves only about 150 feet (46 m) per year based on measurements made in 1998-1999. Rock debris from landslides and medial moraines cover much of the western edge of the glacier and extends across almost 2/3 of the ice face. Where this rock debris is more than an inch (2.5 cm) thick, it insulates the ice, slows melting, and results in a thicker ice mass than where the ice is clean. In many areas on the glacier, the debris is more than 3 feet (0.9 m) thick. The historical decay of Grand Pacific Glacier has been documented by explorers and visitors to Glacier Bay since 1794 when Captain George Vancouver surveyed the Pacific coast of North America and found a large piedmont glacier filling Glacier Bay. This glacier was fed mainly by tongues of ice flowing down Muir and Tarr Inlets. The ice tongue in Tarr Inlet comprised a greatly expanded Grand Pacific Glacier and several large tributary glaciers including Margerie, Johns Hopkins, Lamplugh, Reid, Carroll, and Rendu Glaciers. Between 1794 and the mid-1800s, Glacier Bay became deglaciated and the ice tongues in Muir and Tarr Inlets separated and began to retreat as independent entities. In 1879, the ice tongue in Tarr Inlet terminated at the south end of Russell Island and could be properly termed Grand Pacific Glacier. Between 1889 and 1912, Grand Pacific Glacier retreated 10 miles (16 km) to near the British Columbia-Alaska boundary, and in the process, lost contact with Margerie Glacier, the last of its Alaskan tributaries. It re-advanced about 0.6 miles (1 km) in 1912–13 and reached a maximum advance position when it joined Margerie Glacier around 1992, but the Grand Pacific has since retreated and these glaciers are no longer joined. Rapid deglaciation in this region has had important geomorphic effects including destabilizing locally steep rock slopes in the newly exposed valley and along Tarr Inlet.

The land-based Melbern Glacier and tidewater Grand Pacific Glacier share a common ice accumulation area but have had very different retreat histories. The deglaciation of Glacier Bay and Tarr Inlet by Grand Pacific Glacier occurred mainly in the 19th century, a time when Melbern Glacier was relatively stable and extensive. Much of the retreat of Melbern Glacier occurred 100 years later during the 20th century. As deglaciation of Melbern Valley progressed, lakes were trapped between Melbern Glacier and the walls of the valley and the level of these lakes fell as the glacier thinned. Analysis of aerial photographs indicates that shortly before 1979 the lower part of Melbern Glacier stagnated and began to disintegrate into complex tabular masses of ice enveloped by the waters of one of the largest glacier-dammed lakes on Earth created by the Konamoxt Glacier that flowed across the Melbern Valley. By 1987, the front of Melbern Glacier had receded 4.4 miles (7 km) and glacial Lake Melbern covered 2,965 acres (1200 ha) of the valley bottom. Glacial Lake Melbern overflows along the northeast side of Konamoxt Glacier into the Tatshenshini River. Initially, the lake was 130-165 feet (40–50 m) higher than the flood plain immediately below the Konamoxt Glacier ice dam. Between 1979 and 1987, however, the level of the lake dropped due to the retreat of Konamoxt Glacier and the resulting moraine erosion of the outlet. In 1991, the lake was only 33-66 feet (10–20 m) higher than the downstream end of the overflow channel. With any further retreat, the lake will cease to be dammed by Konamoxt Glacier and will extend another 5 miles (8 km) northward to the head of Melt Creek, attaining an overall length of 9 miles (15 km). Thereafter, the level of the lake may gradually drop as the outlet stream incises the morainal and alluvial plain sloping north to Tatshenshini and Alsek Rivers. Read more here and here. Explore more of the Grand Pacific Glacier here:

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