What are the Ice Age Floods?

When geologists first saw the vast Columbia Basin in eastern Washington State, they recognized that glaciers and flowing water had played a large part in shaping the extraordinary landscape, with its canyons (coulees), buttes, dry cataracts, boulder fields, and gravel bars. It was taken for granted that what they saw was the cumulative effect of familiar processes, operating on a familiar scale.

However, a closer examination of the features in the Basin led one geologist, J Harlen Bretz, to propose that it could only have been a sudden cataclysmic flood, on a scale never before considered possible, that 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. Two National Natural Landmarks, Wallula Gap and the Drumheller Channels, are the direct result of the floods.

These floods are a remarkable part of our natural heritage. They have profoundly affected the geography and ways of life in the region, but have remained largely unknown to the general public. The legacy of the floods includes not only stark scabland and dramatic dry coulees and cataracts, but also exceptionally fertile, productive farmland, and significant wetlands and aquifers.

Among geologists, the most recent Ice Age Floods in the Pacific Northwest have been called the Missoula Floods, the Spokane Flood(s), Bretz Floods, and Ice Age Floods. By whatever name, their striking effects are undeniable, and available for all of us to see and explore.

How was the geologic puzzle solved?

It was in 1923 that J Harlen 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 a huge, powerful flood 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.
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 Northwest, and some of them were among the largest and most powerful floods that have ever occurred on Earth.

How did these floods happen?

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.

The actual failure of the ice dam is inferred from observations of modern glacial floods. Research on glacial floods is relatively new and the physical processes are not yet fully understood, but one scenario is that, when the height and pressure of the lake’s water against the ice dam reached a critical stage, the dam would develop significant subglacial leaks, eventually leading to a sudden and complete 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.

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.

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.