Glacial Lake Ahtna

Reseach suggests Glacial Lake Ahtna may have been a serial generator of some of Earth's largest freshwater floods.

Maps and photographs are from the United States Department of the Interior, Bureau of Land Management

Lake Ahtna—often spelled Atna—was a prehistoric proglacial (ice-frontage) lake which covered more than 3,500 square miles in the Copper River Basin, initially formed approximately 58,000 years ago during the Wisconsin glaciation, the most recent glacial period of the North American ice sheet complex. Other examples of prehistoric megafloods include Lake Agassiz and the Lake Missoula Floods, major catastrophic events resulting from the sudden drainage of proglacial or subglacial lakes, known by the Icelandic term jokulhlaups, and one such flood may have contributed to the destruction caused by the 1964 Alaska earthquake.

Lake Ahtna existed in several forms, with several prominent shorelines observable in modern geology. The basin of the lake lay within an area bordered by the Alaska Range to the north, the Wrangell Mountains to the east, the Chugach Mountains to the south, and the Talkeetna

Mountains to the west. At its greatest extent, the lake surface area was larger than modern-day Lake Michigan, and upon discharges Lake Ahtna may have generated some of the world’s largest ever freshwater glacial lake outburst floods. The earliest identified outlets were down the current-day Copper River to the south, the Susitna River to the west, and the Tok River into the Tanana Valley. Relatively recently, the Matanuska River was also identified as a drainage route.

Map showing geographic features related to Lake Atna.

In a research paper titled Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A., published in the May, 2009 edition of the journal Quaternary Research; authored by Michael Wiedmer, with co-authors David R. Montgomery and Alan Gillespie, Univ. of WA professors of Earth and space sciences; and computer specialist Harvey Greenberg, it was made apparent that information about how the Lake Ahtna megaflood potentially affected parts of Alaska was only recently brought to light: “Previous attention has focused on Lake Atna's morphology and chronology; the lake's drainage routes and discharge behaviors have received scant scrutiny. Here we report geomorphic, stratigraphic, geotechnical, and biogeographic evidence for a late glacial Lake Atna-sourced megaflood down the Matanuska Valley, a previously unrecognized drainage route, and provide the first estimates for flood volumes and peak discharges through other previously recognized outlets.”

An article in the April 29, 2010 University of Washington newsletter titled ‘Research shows part of Alaska inundated by ancient megafloods,’ by Vince Stricherz, quotes from the Weidmer et al. article: “New research indicates that one of the largest fresh-water floods in Earth’s history happened about 17,000 years ago and inundated a large area of Alaska that is now occupied in part by the city of Wasilla…”

Bluffs near the confluence of the Gakona and Copper Rivers

The bluffs near the confluence of the Gakona and Copper Rivers expose nearly 300 ft of the Quaternary lacustrine, alluvial, and glacial deposits that fill the Copper River Basin. Most of the section seen in this view consists of finely laminated to indistinctly bedded sand, silt, and clay, with or without coarser material, deposited in glacial Lake Atna.

Stricherz continues, “…The megaflood that covered the Wasilla region released as much as 1,400 cubic kilometers, or 336 cubic miles, of water — enough to cover an area the size of Washington, D.C., to a depth of nearly 5 miles. That water volume drained from the lake in about a week and, at such great velocity, formed dunes higher than 110 feet, with at least a half-mile between crests. The dunes appear on topographical maps but today are covered by roads, buildings and other development.”

Stricherz then quotes Michael Wiedmer: “Your mind doesn’t get around dunes of that size. Obviously the water had to be very deep to form them.”

The same article explained how the prehistoric megaflood may have contributed to the damages seen in Anchorage in the 1964 9.2 magnitude earthquake: “Wiedmer noted that much of Anchorage is built on marine sediments, and one layer of those sediments liquefied and collapsed, allowing the layer above to slide toward the sea. As the upper layer moved toward the water, structures built on top of it collapsed. ‘We suspect that this is evidence of the flood that came down the Matanuska,’ Wiedmer said. ‘The location is right at the mouth of where the flood came down, and the time appears to be right.’”

In 1898, Frank Charles Schrader undertook a study of the Copper River Basin for the United States Geological Survey (USGS). Based on sedimentary evidence, he concluded a possibility of a large body of standing water being responsible for the deposits. This could have been an arm of the sea. In 1901, with A. C. Spencer, he concluded these deposits were only in limited areas, disagreeing with his earlier conclusion. This was supported by Walter Curran Mendenhall in 1905, who had studied the Pleistocene deposits in the central region of the basin. In 1954, Fred Howard Moffit noted that topographic conditions were favorable for the possibility of a large lake, but that specific evidence was lacking at that time. In 1957, geologists Oscar J. Ferrians and H.R. Schmoll concluded the basin had been resident to a large proglacial lake during the Wisconsin glaciation. The lake was named Lake Atna by geologist D. J. Nichols of the USGS in 1965. Several of the large existing lakes in the Copper River Basin are remnants of the glacial Lake Ahtna, including Tazlina Lake, Tonsina Lake, and Klutina Lake.

A good description of current-day evidence is found in the online blog, Wickersham’s Conscience (see Resources, page 48): “As you drive down the Richardson Highway from just below Meiers Lake to Tiekel River, you are crossing the old lake bottom. The impressive Copper River canyon is eroded down through the lake bottom. If you look closely at the bluffs above the river, you can see the layers of lake bottom mud, graveled river bottom and, because this area is near the moderately active Wrangell volcanoes, the occasional layers of volcanic ash as well.”

“As you drive down the Edgerton Highway, east off the Richardson, the road drops down a series of old lake terraces, marking old lake shores of a vast lake that is now long gone, but has left its geologic fingerprints all over the landscape of Southcentral Alaska.” ~•~

Edgerton Highway dropping down benches to the Copper River.

Alaska Department of Transportation photo