Glacial Lake Ahtna

WC studied a lot of geology in undergraduate school. And he’s never quite shaken his interest in the area. Alaska’s geology is fascinating. To paraphrase John McPhee, most of Alaska, like most Alaskans, originally came from somewhere else. A case in point:

Up until a little less than 10,000 years ago, a very large lake covered much of eastern southcentral Alaska. The scope of the lake is still being researched, but it clearly extended north to Mentasta Pass, along the Tok Cutoff, east some distance up the Chitina River valley, and west to Tahetna Pass on the Glenn Highway. Recent geological data would have the lake look something like this:

Glacial History of Southcentral Alaska

Like most glacial lakes, Lake Ahtna was subject to catastrophic discharge events; the glaciers that dammed the rivers, creating the lake, would fail as dams, triggering massive flows of water down the lowest points around. There’s evidence of that kind of massive flood events down the Matanuska Valley into Cook Inlet, down the Tok River into the Tanana Valley and down Devil’s Canyon on the Susistna River. Some of these flood events may have involved 2,500 cubic kilometers of water. These were Lake Missoula-sized floods. At its largest. Lake Ahtna was much larger than present Lake Michigan; Ahtna may have had a peak water volume of 6,200 cubic kilometers; present Lake Michigan is estimated at 4,900 cubic kilometers.

The lake lasted perhaps 20,000 years, easily long enough to deposit vast amounts of mud on its lake bottom. When a flood discharge event occurred, lake bottom would become river bottom again, until the ice dams rebuilt as glaciers replaced what the high water had washed away.

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. This bluff is on the Edgerton Highway:

Depositional Layering, Copper River Bluff

This impressive pile also contains swamp mud, wind-blown glacial flour and less identifiable stuff.

The present-day Copper River has eroded down through these deposits, but you can see the old lake bottom still in the flatness of the terrain above the canyon.

Copper River Valley Erosion

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.

It’s part of what makes geology so much fun. It’s a puzzle in which we live, often entirely unaware.