Brady Glacier, Taylor Bay

Brady Glacier, Taylor Bay

by | Feb 22, 2022

Brady Glacier is situated in Glacier Bay National Park and Preserve and flows from the Brady Icefield, at an elevation of 11,942 feet (3,640 m) in the Fairweather Range of the Saint Elias Mountains, to Taylor Bay on the north shore of Cross Sound, about 139 miles (224 km) southeast of Yakutat and 32 miles (51 km) west of Gustavus, Alaska. Brady Glacier is the largest ice stream in the Fairweather Range with a length of 32 miles (51 km) and an area of 126,518 acres (51,200 ha). It starts in the same accumulation area of the Brady Icefield as the Reid and Lamplugh Glaciers and flows southward ending just above sea level in an outwash plain and tidal delta about 3.7 miles (6 km) north of the head of Taylor Bay. Taylor Bay is about 3.4 miles (5.5 km) wide and extends northwest from Cross Sound for 4.5 miles (7 km) to the mouth of the Brady River and the outwash plain of the Brady Glacier. The bay was named by William Healey Dall, who was the superintendent of the U.S. Coast and Geodetic Survey, in honor of C.H. Taylor who reputedly visited the bay while exploring the region as noted in the 1883 Alaska Coast Pilot. Dall did not record any information about C.H. Taylor, however, there was an H.C. Taylor of the U.S. Navy in command of the SS Hassler, the hydrographic survey ship used by the U.S. Coast and Geodetic Survey in Cross Sound at that time. Brady Glacier was named in 1883 by the U.S. Coast and Geodetic Survey for Reverend John G. Brady who was a missionary and later the Governor of the District of Alaska from 1897 to 1906 and was forced to resign due to his alleged involvement with the fraudulent Reynolds-Alaska Development CompanyThe topography of Glacier Bay is a result of the active collision zone between the North American and Pacific plates. The Fairweather-Queen Charlotte Fault system cuts across Glacier Bay’s western edge and for over 50 million years, the Pacific Plate and Yakutat Microplate have been moving northwest along the fault boundary. During this collision, the Pacific plate has been generally forced under the North American plate, but occasional terranes such as island arcs, pieces of the seafloor, fragments of continental margin have been scraped off the oceanic plate and accreted along the edge of the North American plate. The peninsula between Taylor Bay and the Pacific, including the Fairweather Range, is mostly composed of the Chugach terrane that extends from the western end of the Alaska Peninsula to Southeast Alaska and consists mostly of greywacke and siltstone that generally increase in the degree of metamorphism to the northeast. The Tarr Inlet suture zone lies underneath the Brady Glacier and represents the boundary between the Chugach terrane to the west and the Alexander terrane to the east. This zone consists of volcanic rocks, greywacke, and chert. The Alexander terrane is bordered to the east by the Lynn Canal-Chatham Strait fault zone and has rocks over 500 million years old that were transported from an equatorial environment to the present location about 100 million years ago.

During the Last Glacial Maximum, the Cordilleran ice sheet covered all of Southeast Alaska and advanced out onto the continental shelf. This ice sheet started retreating about 20,000 years ago with the termination of the Fraser Glaciation until most of Southeast Alaska was ice-free. Periodic glacier advances have occurred in Alaska during the Holocene, with the most recent advance occurring during the Little Ice Age, which was a period of cooling climate that started in the 16th century and persisted until the mid-19th century. The glaciers advanced until Glacier Bay was entirely covered by the Glacier Bay Icefield. In about 1770, the ice front started retreating. The Glacier Bay area has had documented glacier observations since 1794 when Captain George Vancouver first visited the area. At this time, a survey party under the command of Lieutenant Joseph Whidbey recorded that the southern terminus of the Glacier Bay Icefield was located in Icy Strait. A submarine terminal moraine shows that the terminus reached into Icy Strait and was adjacent to Lemesurier Island sometime between 1725 and 1794. Vancouver sailed into Taylor Bay and observed the Brady Glacier when it was a calving tidewater glacier. At some time in the 1800s, the glacier stopped calving and advanced approximately 5 miles (8 km). The advance is likely an example of a tidewater glacier cycle where an advance generally follows a change from tidal to non-tidal status. During the last quarter of the 19th century, the glacier receded and built the outwash plain separating it from Taylor Bay. Between 1926 and 1977, the sediment plain expanded in length by more than 2.5 miles (4 km) and increased in area by more than 4,942 acres (2,000 ha). Compared to other glaciers in Glacier Bay National Park, the terminus of Brady Glacier has been remarkably stable for the last several decades while slowly building an outwash plain, however, this glacier highlights the importance of considering glacier thickness, and not just extent when looking at glacier change over time. The relatively stable terminus of Brady Glacier hides an ongoing and substantial deflation or down wasting of the glacier surface. Measurements from 1995-2000 document an average ice loss of 0.12 cubic miles (0.52 cubic km) of ice volume per year. Glacier ice thinning at the terminus and at many locations along its sides has resulted in the formation of several ice-marginal lakes.

Glacier-dammed lakes and the outburst floods released when they fail can cause severe damage at downstream areas and also substantially influence glacier dynamics. Glacier-dammed lakes can form in a number of different situations. The largest lakes, which present the greatest hazards, occur in ice-free tributary valleys blocked off by active valley glaciers. Most common are small lakes situated in alcoves and niches in the valley walls along the margins of glaciers and in depressions formed where tributary valley glaciers join. The large majority of lakes occur along the lower reaches of glaciers. Once a depression is closed off by a glacier it begins to fill with meltwater and rain runoff from the surrounding basin. The resulting lake continues to fill until the water overflows a bedrock saddle or initiates a self-dumping process at the ice dam. Most large ice-dammed lakes fill until they reach depths where the ice dam becomes unstable. The release of glacier-dammed lakes may be initiated by the formation of a channel under, through, or over the ice. Examples of mechanisms causing ice-damned lakes to fail include 1) slow plastic yielding of the ice due to hydrostatic pressure differences between the lake and the adjacent less dense ice, 2) raising of the ice dam by floatation, 3) crack progression under combined shear stress due to glacier flow and high hydrostatic pressure, 4) drainage through small preexisting channels at the ice-rock interface or between crystals in the ice, 5) water overflowing the ice dam generally along the margin, 6) subglacial melting by volcanic heat, 7) weakening of the dam by earthquakes. Once a leak is established the initial opening can be expanded rapidly by melting. The main terminus position of Brady Glacier has varied little over the past 130 years but between 1948 and 2000, the margins of the glacier retreated up to 1.2 miles (2 km) and down wasted up to 404 feet (123 m) adjacent to ten large ice-dammed lakes. Six of the ten lakes are subaerial or on the surface, and four are subglacial or primarily underneath the glacier. In seven of the ten lakes, the glacier has a calving margin that exceeds 0.6 miles (1 km) in width. These lakes, and many smaller ones, are in different stages of development with some that are new or emergent, some that are stable and non-draining, and others that periodically drain. Calving margins of glacier-dammed lakes share several similarities with tidewater glacier calving margins. After tidewater glaciers reach equilibrium, a perturbation such as a climatic change can thin the glacier so that it is no longer grounded on the stabilizing shoal, triggering more mass loss through calving than is replenished. The glacier may then catastrophically retreat. Read more here and here. Explore more of the Brady Glacier and Taylor Bay here:

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This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2021 (bottom). Each stripe represents the average global temperature for one year. The average temperature from 1971-2000 is set as the boundary between blue and red. The color scale goes from -0.7°C to +0.7°C. The data are from the UK Met Office HadCRUT4.6 dataset. 

Credit: Professor Ed Hawkins (University of Reading). Click here for more information about the #warmingstripes.

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