An Introduction to the Ice Age Floods

During the last Ice Age (18,000 to 12,000 years ago), and in multiple previous Ice Ages, cataclysmic floods inundated portions of the Pacific Northwest from Glacial Lake Missoula, pluvial Lake Bonneville, and perhaps from subglacial outbursts. Glacial Lake Missoula was a body of water as large as some of the USA’s Great Lakes. This lake formed from glacial meltwater that was dammed by a lobe of the Canadian ice sheet. Episodically, perhaps every 40 to 140 years, the waters of this huge lake forced its way past the ice dam, inundating parts of the Pacific Northwest. Eventually, the ice receded northward far enough that the dam did not reform, and the flooding episodes ceased. These floods are a remarkable part of North American natural heritage. They have profoundly affected the geography and ways of life in the region, but have until recently remained largely unknown to the general public.

The Ice Age Floods Story – Briefly

During the most recent episode of major ice-sheet expansion, between about 18,000 and 13,000 years ago, a lobe of the Cordilleran ice sheet advanced into the Idaho Panhandle to the area that is now occupied by Lake Pend Oreille, thus blocking the Clark Fork River drainage and causing Glacial Lake Missoula to form. At its largest, the lake was deeper than 2,000 feet deep at the dam and held over 500 cubic miles of water—as much as Lake Erie and Lake Ontario combined. The ice dam, however, was subject to repeated failure.

Ice dam collapse

When the dam broke, a towering mass of water and ice was released and swept across parts of Idaho, Washington, and Oregon on its way to the ocean. The peak rate of flow was ten times the combined flow of all the rivers of the world. The huge lake may have emptied in as little as two or three days. Over a period of years the glacier would advance, once again blocking the river, and the dam and the lake would form again. This process was repeated scores of times, until the ice sheet ceased its advance and receded to the north at the end of the Ice Age. It is assumed that the same processes would have occurred earlier during other glacial advances throughout the Ice Age, although most of the evidence for the earlier events may have been removed by the flooding that occurred during the last glacial advance.

Areal Scenario Map of the Ice Age Floods - Click to View Larger Image

Areal Scenario Map of the Ice Age Floods – Click to View Larger Image

Along the floodwaters’ course, more than 50 cubic miles of earth and rock were removed, and some of this was transported and then deposited as new landforms. The floods built gravel bars as tall as 400 feet and moved boulders weighing many tons as if they were pebbles. Some of the eroded material was deposited along the path of the floods, but most of the eroded material was carried out onto the floor of the Pacific Ocean, where extensive deposits of flood sediment have recently been identified hundreds of miles from the current mouth of the Columbia River.

In the following years the account was refined, as evidence of more than one flood was discovered. It is now established that there were large numbers of Ice Age floods that swept across the US Northwest, and some of them were among the largest and most powerful floods that have ever occurred on Earth.

These floods are a remarkable part of North American natural heritage. They have profoundly affected the geography and ways of life in the region, but have until recently remained largely unknown to the general public.

How the “Story” Began

Early settlers in eastern Washington State recognized that the landscapes were different than those seen in other places but were not sure why. A closer examination of the features in the Basin led one geologist, J. Harlen Bretz, to propose that only a sudden cataclysmic flood, on a scale never before considered possible, could account for the phenomenal size and distinctive characteristics of the landforms. This radical idea was not well received by fellow geologists, and a long-running scientific dispute followed. Ultimately his extensive field work, plus additional research by others, conclusively established that many extraordinarily huge and powerful Ice Age floods had shaped the region.

It was in 1923 that Bretz published the first in a series of scientific papers in which he proposed that the severely eroded Channeled Scabland, Dry Falls, and other immense geologic features had been formed by huge, powerful floods that had swept through the Columbia Basin during the Ice Age.

Despite his peers’ doubt and opposition, he resolutely maintained that direct examination of the geologic evidence could lead only to that conclusion. But Bretz was unable to identify the source or cause of such catastrophic flooding.

Earlier, in 1910, another geologist, Joseph T. Pardee, had described evidence of a great ice dammed lake, Glacial Lake Missoula, that had formed during the Ice Age in northwestern Montana. However, Bretz didn’t see the connection between the glacial lake in Montana and the features he described in eastern Washington. Then, in 1940, Pardee reported on his discovery of giant ripple marks, 50 feet high and 200-500 feet apart, that had formed on the floor of Glacial Lake Missoula. These huge, current-related features, along with other newly-found landforms, dramatically confirmed that the lake had suddenly emptied to the west, unleashing the tremendously powerful erosive forces that shaped many of the landforms found in the Columbia Basin.  More research followed, and new perspectives became available from aerial photography. Among geologists, the concept of a catastrophic flood came to be accepted by the late 1950s.

Phenomena related to the Ice Age Floods

Recent research has found evidence that comparable floods occurred much earlier in the Ice Age in the Columbia Basin, as much as 1 to 2 million years ago. It has been determined that huge Ice Age glacial-outburst floods occurred in other parts of the world, as well. Even in our own times, similar but much smaller floods have occurred. Scientific study of the Ice Age Floods is contributing to the understanding of cyclical climate change and of very large and destructive contemporary floods on Earth. The Ice Age Floods have also been considered as an analog to understand geologic processes on Mars, where landforms strikingly similar to those in Eastern Washington exist. Could huge floods on this scale happen again? Although global warming may now be a serious concern, it is likely that long-term climate cycles will cause large ice sheets to return at some time in the distant future, and cataclysmic outburst floods will probably recur in this region.