Le Conte Glacier, Frederick Sound

Le Conte Glacier, Frederick Sound

by | Dec 24, 2021

Le Conte Glacier starts from the Stikine Icefield in the Coast Mountains of British Columbia, at an elevation of about 8,500 feet (2,591 m) between Devils Thumb and Mount Gilroy, and flows generally south for 22 miles (35 km) to the head of Le Conte Bay in Southeast Alaska, about 25 miles (40 km) north of Wrangell and 22 miles (35 km) east of Petersburg, Alaska. Le Conte Bay is a fjord over 1,000 feet (305 m) deep near the terminus of the glacier and is presently about 10 miles (16 km) long opening to the southeastern head of Frederick Sound adjacent to the Stikine River delta. Frederick Sound is a large fjord that separates Kupreanof Island to the south from Admiralty Island in the north, and Kupreanof and Mitkof Islands from the Southeast Alaska mainland. Frederick Sound was first charted in 1794 by Joseph Whidbey and James Johnstone, and subsequently named by Captain George Vancouver for Prince Frederick, Duke of York and Albany. The head of Frederick Sound is connected to Sumner Strait to the south by Dry Strait, a water passage that is generally not navigable by large vessels, and as the Stikine River delta continues to advance it will eventually be closed. In 1879, the naturalist John Muir visited Le Conte glacier when icebergs filled the bay for 10-12 miles (16-19 km). Muir wrote in The American Geologist that the Tlingit called the fjord ‘Hulti’ meaning Thunder Bay, referring to the noise made by the discharge of icebergs. In 1887, U.S. Coast and Geodetic Survey ship Carlile P. Patterson was engaged in mapping the Southeast Alaska coast. The glacier was named by Lieutenant Commander Charles Mitchell Thomas in honor of Joseph Le Conte, a close friend of John Muir. From 1857 to 1869, LeConte was a professor of chemistry and geology at South Carolina College, which is now the University of South Carolina. In 1869, he moved to Berkeley, California, to be a professor of geology at the newly established University of California. Le Conte was noted for his exploration and preservation of the Sierra Nevada of California. He first visited Yosemite Valley in 1870, where he became friends with John Muir and started exploring the Sierra. In 1892, Le Conte co-founded the Sierra Club with John Muir and others, and was director of the Sierra Club from 1892 through 1898.

Le Conte Glacier is a tidewater glacier, which generally are ice streams that terminate in the ocean at either a grounded terminus or floating ice tongue. Glaciologists describe a repeating cycle for tidewater glaciers where glaciers slowly advance over the course of centuries until ice thinning near the terminus initiates a rapid retreat that completes within decades, and stabilizes only when the glacier has retreated into shallow water. In general, the retreat phase of a tidewater glacier can be triggered by changes in climate, but observations have shown that once the retreat is initiated, the glacier’s behavior is only weakly influenced by climate, and glacier geometry is a more important factor. The ability of tidewater glaciers to discharge large amounts of ice in a short period highlights their potential hazard to shipping traffic in nearby waters. Tidewater glaciers also have a significant potential for providing a large contribution to sea level rise. The Coast Mountains straddling the boundary between Alaska and British Columbia currently has four current tidewater glaciers: Sawyer, South Sawyer, Dawes, and Le Conte that have a total area of 443,800 acres (179,600 ha). In general, these glaciers are all thinning and retreating, though none are rapidly retreating. Le Conte Glacier is the southernmost tidewater glacier in the Northern Hemisphere, and several studies have focused on this glacier to better understand the short-term ice calving dynamics and glacier-ocean interactions. The glacier covers 115,892 acres (46,900 ha) and has an accumulation area at elevations between 920-2600 m. From 1887-1963, the glacier retreated 2.5 miles (4 km), calving icebergs up to 980 feet (300 m) deep, and then remained relatively stable for 32 years. In 1994, the glacier began a rapid retreat and the terminus has since retracted about 1.2 miles (2 km). By 1998, the glacier started a phase of temporary stability at a constriction in the fjord. This is called a pinning point and occurs when calving is reduced due to a fjord narrowing or shoaling and the area of the glacier accumulation zone is equal to the ablation area. This is presently occurring with Le Conte Glacier which is at a retracted position and seems likely to advance again after building a terminus shoal.

While climate is the main factor affecting the behavior of all glaciers, additional factors affect iceberg calving tidewater glaciers. Climate change causes a shift in the elevation of the equilibrium line of a glacier. This is the imaginary line on a glacier, above which snow accumulates faster than it is removed, and below which, the reverse is the case. This shift in elevation triggers glacial retreat when the elevation shifts higher, or an advance of the terminus if the elevation shifts lower. However, this change in terminus behavior for calving glaciers is also a function of fjord geometry. Tidewater glaciers behave differently than land terminating glaciers. Land terminating glacier velocities decline as the terminus is approached, while calving tidewater glaciers accelerate at the terminus. The rate of ice calving is largely controlled by glacier velocity and the water depth at the terminus. The retreat of glaciers in Alaska has made a disproportionally large contribution to global sea level rise. In total, these glaciers are losing 75 billion tons of ice annually. Over the past 50 years, retreating glaciers and ice caps globally contributed 0.02 inches (0.5 mm) per year to sea-level rise. Mountain glaciers hold less than 1 percent of the earth’s glacial ice volume, and the rest is held in ice sheets on Antarctica and Greenland. However, the rapid shrinking of mountain glaciers causes nearly 30 percent of the current sea level rise. Climate-related melting is the primary control on mountain glacier loss, and this will be a major driver of global sea level change. Sea-level rise is governed primarily by thermal expansion of sea water and the transfer of water from terrestrial reservoirs, such as land ice and groundwater, to the ocean. The Intergovernmental Panel on Climate Change Fourth Assessment Report found that thermal expansion accounted for about 25 percent of the observed sea-level rise for 1961-2003. Melting of land ice accounted for less than 50 percent, and changes in land water storage accounted for less than 10 percent. For the last 10 years of that period (1993–2003), thermal expansion and land ice melt each contributed about 50 percent to the total sea-level rise. In the most recent updated estimate, for 1993–2008, the contribution from land ice increased to 68 percent and the contribution from thermal expansion decreased to 35 percent. Read more here and here. Explore more of Le Conte Glacier and Frederick Sound here:

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