Grewingk Glacier, Kachemak Bay

Grewingk Glacier, Kachemak Bay

by | Jul 24, 2022

Grewingk Glacier starts in the Kenai Mountains and flows generally northwest for 11 miles (18 km) to a proglacial lake about 2 miles (3.2 km) long that is drained by Grewingk Creek that flows 3.5 miles (6 km) to the southern shore of Kachemak Bay, about 25 miles (40 km) northeast of Seldovia and 16 miles (25 km) east-southeast of Homer, Alaska. The glacier was named in 1880 by William H. Dall of the U.S. Coast and Geodetic Survey for Constantin Grewingk who published a work on the geology and volcanism of Alaska in 1850. The eastern and southern sides of the Cook Inlet basin are bounded by a Tertiary and Cretaceous or older subduction-related accretionary wedge called the Southern Margin composite terrane. This terrane is composed of sedimentary rocks formed in a deep ocean trench and includes the Chugach and Prince William terranes as well as the Ghost Rocks Formation. The Chugach terrane is composed of three rock assemblages represented by the Seldovia Complex,  McHugh Complex, and the Valdez Group. The McHugh Complex forms the bedrock underneath and surrounding Grewingk Glacier and consists of partially metamorphosed sandstone, siltstone, greywacke, arkose, and conglomerate. An igneous intrusion from the Mesozoic is exposed at the base of Grewingk Glacier which is composed of gabbro. Below Grewingk Lake, the bedrock is largely buried by unconsolidated sediments from the Late Pleistocene and Holocene including outwash and alluvial deposits. The southwest part of the Kenai Peninsula was glaciated three or more times during the Quaternary period. During the oldest glaciation ice flowed westward from the Kenai Mountains and overrode the Kenai Lowlands and coalesced with a broad ice stream moving south through the Cook Inlet trough. During each of the younger and successively less extensive glaciations, ice flowed from the Kenai Mountains and into the Kachemak Bay trough.

The first humans to settle the Cook Inlet basin are not known; however, the earliest archaeological sites follow the retreat of glacier ice from Cook Inlet and date from between 10,000 and 7,500 years ago. These people were highly mobile hunter-gatherers that traveled in small groups and relied heavily on large land mammals for food. About 4,200 years ago, new settlers arrived that were marine mammal hunters, and over time these distinct groups of people traded, inter-married, and sometimes conflicted with each other. From 3,000 to 1,000 years ago, the Kachemak Culture spread over much of Cook Inlet. The Marine Kachemak inhabited the lower Cook Inlet shores and were closely associated with people from the outer Kenai coast and Kodiak Island, while the Riverine Kachemak people were more strongly influenced by Bristol Bay and more northern coastal people. The Dena’ina people arrived in Southcentral Alaska sometime between 1,500 to 1,000 years ago from the interior and are the only Alaskan Athabaskan group to live on the coast. In 1778, Captain James Cook was the first European to explore and document the shores of the inlet and named many of the important geographical features. In 1787 or 1788, Russian fur traders with the Shelikhov-Golikov Company established a trading post called Aleksandrovskaia at Seldovia, and at about the same time other fur traders with the Lebedev-Lastochkin Company established a trading post called Pavlovskaia at the mouth of the Kenai River. In 1789, the inlet was again visited by English ships under the command of Nathaniel Portlock and George Dixon. They stayed for nearly a month and were successful in trading for furs and also discovered the coal beds at Port Graham. In 1794, Captain George Vancouver conducted surveys of the inlet and corrected the latitude on Cook’s charts, and also determined that it was not a great river as Cook described, and the name was changed from Cook River to Cook Inlet.

Grewingk Glacier was visited by William H. Dall in 1880, 1892, and 1895. The results of his mapping of this glacier were incorporated in charts of the U.S. Coast and Geodetic Survey. Dall visited the Grewingk Glacier again in 1899 with Grove Karl Gilbert, a geologist on the Harriman Expedition, who provided a detailed description of the glacier. The Grewingk ice front continues to calve into an expanding pro-glacial lake that is currently about 2 miles (3.2 km) long. Between 1986 and 2014, the glacier retreated 0.9 miles (1.4 km) or about 164 feet (50 m) per year. There was also an increase in the glacier slope 1.5 miles (2.5 km) above the terminus where crevassing increases. This suggests the lake will end at this point, which should cause a reduction in the retreat rate. The loss of glacial ice may have triggered a massive landslide of 2,966 million cubic feet (84 million cubic meters) of rock and debris, creating a tsunami that swept the forefield of Grewingk Glacier in the fall of 1967. There are several named glaciers on the southeastern side of Kachemak Bay that approach but no longer reach tidewater. These include from south to north small remnants of the Southern Glacier that drains into Tutka Bay, Doroshin and Wosnesenski Glaciers that drain into Neptune Bay, Grewingk Glacier, Portlock Glacier that drains into Kachemak Bay just north of Mallard Bay, Dixon Glacier that drains into Kachemak Bay via the Martin River, Kachemak Glacier that drains into Bradley Lake, Dinglestadt Glacier that drains into Kachemak Bay via the Sheep River, and the Chernof Glacier that drains into Kachemak Bay via the Fox River. These are the most westerly glaciers on the Kenai Peninsula. From 1950 to 2005, at least 27 glaciers in the Kenai Mountain icefields were retreating, and the Grewingk Glacier had retreated 1.5 miles (2.5 km). Read more here and here. Explore more of Grewingk Glacier and Kachemak Bay here:

About the background graphic

This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2022 (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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