Ancient flood left its mark on Yukon landscape

Sometimes the modern landscape doesn't quite fit together. Today, the Eagle River is a modest, peaceful stream in eastern Yukon. It flows north into the Porcupine River system, which meanders westward to join the Yukon River.

By Claire Eamer

Sometimes the modern landscape doesn’t quite fit together. Today, the Eagle River is a modest, peaceful stream in eastern Yukon. It flows north into the Porcupine River system, which meanders westward to join the Yukon River. But the Eagle River Valley is far larger than the modern river. It has high, steep sides, multiple dry channels, and isolated, shallow lakes stranded far from today’s river.

“It’s this bizarre canyon in the middle of nowhere,” says Kristen Kennedy, a geologist with the Yukon Geological Survey.

Clearly, sometime in the past, something happened to the Eagle River. As part of the research for her master’s degree, Kennedy spent many bug-ridden weeks in that bizarre canyon, studying layers of rock, gravel, and sediment to determine exactly what happened and when.

It goes back to the last glacial maximum, when the ice sheets of the most recent ice age reached their greatest extent in North America, she says. The Laurentide Ice Sheet, based in Hudson Bay, grew, thickened, and spread for several thousand years. Its edges flowed outward, pushing as far north as Tuktoyaktuk and west to the Richardson Mountains.

As glaciations go, this was a relatively short one, lasting less than 10,000 years in the eastern Yukon, but it left some profound changes behind. Before the ice, Kennedy says, the Bell, Porcupine, Eagle, and Old Crow river systems drained eastward through McDougall Pass in the Richardson Mountains and then north to the Arctic Ocean. After the ice, they followed their current patterns, draining westward into the Yukon River system.

The change happened dramatically. Kennedy’s research shows that the strange canyon of the Eagle River was carved out by a single massive flood. The ice sheet crept northwest until it blocked the streams’ eastward flow. Water pooled at the base of the ice sheet, forming large lakes. In the Peel River area, glacial Lake Hughes grew larger and deeper until it spilled over a divide at 560 metres above sea level.

“You have to have glacial maximum to get water high enough to spill over,” Kennedy says.

The water poured north, creating a spillway – a 140-kilometre-long network of braided channels that cuts deep into the landscape in places. The underlying rock, soft shale, disintegrated under the force of the water, washing downstream as sediment and settling into a fan-shaped delta at the entrance to the next glacial lake. It might have happened relatively quickly, Kennedy says.

“It could have been a few weeks or it could have been a hundred years.”

There’s no evidence in the Eagle River Valley to indicate how long the flood lasted. At a similar formation on the Athabasca River in northwest Alberta, however, hydraulic calculations suggest that spillway might have been cut in less than 150 days and the downstream delta could have been deposited in a few hundred years at most – a very short time in the geological scheme of things.

However long it took, Kennedy’s research shows that the Eagle River spillway and delta were created in one continuous flood as recently as 18,000 years ago. And that is news.

The accepted view, until recently, was that the large, interconnected glacial lakes in northern Yukon formed as early as 30,000 years ago, and that they may have drained and filled several times over the next 15,000 years as the edge of the ice sheet advanced and retreated. For a time, lakes filled the Old Crow, Bluefish, and Bell basins, and were linked to Lake Hughes by the Eagle River spillway.

The accepted view, however, has grown more and more puzzling as fresh evidence and new analytical techniques emerge, Kennedy says. For the lakes to form, the ice sheet had to be at or near its maximum. If the lakes filled 30,000 years ago, that means the ice sheet reached its maximum in the northwest about 10,000 years before reaching its maximum farther south and closer to its source in Hudson Bay.

Kennedy’s study of the sediment layers in the Eagle River formation, along with radiocarbon dating of organic materials caught up in the flood, show a time pattern that fits with the movement of the rest of the ice sheet: glacial maximum at approximately the same time as areas farther south and a single massive flooding of all the basins over a much shorter time period. The fossils of ice age mammals found near Old Crow indicate that glacial Lake Old Crow had drained by about 15,700 years ago, so the ancient lakes may have lasted only a few thousand years.

The implications of the new set of dates go beyond geology, Kennedy says. The glacial lakes have been used as a guide in dating archaeological sites in northern Yukon, so the evidence uncovered in her research could help sort out the Yukon’s human past as well.

For more information about the Yukon’s ice age world, go to www.geology.gov.yk.ca or contact Kristen.Kennedy@gov.yk.ca.

This column is coordinated by the Northern Research Institute at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/youryukon.

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