Much of Washington was sculpted by catastrophic floods during the last ice age. A new map published by the Washington State Department of Natural Resources Division of Geology and Earth Resources highlights the marks the Ice Age Floods left on Washington. With the map, you can find key places across the state to see those marks and how they impacted our geology, ecology, economy and way of life.
During the last ice age, between 12 and 17,000 years ago, glaciers covered much of northern Washington, Idaho, and Montana. The ice was miles thick in places. Portions of the glaciers repeatedly blocked large river drainages, forming ice dams. Lakes formed behind the ice dams, growing to small inland seas rivaling the size of the Great Lakes.
The largest of these glacial lakes was Glacial Lake Missoula in what is now Montana. The lake was approximately 2,500 feet deep in places and extended over 3,000 square miles. It held as much water as about half of Lake Michigan. When the ice dam burst, all of the water suddenly drained in a matter of days, surging over Idaho, eastern Washington, and Oregon. The vast amount of water moving over a very short period of time carved the deeply scoured terrain that forms much of the dramatic landscape we see today.
In the aftermath of the initial flood, the process of river blockage, ice-dam lake formation, and catastrophic release occurred repeatedly. Evidence suggests that there could have been as many as 100 separate floods at intervals of about every 50 years.
The features it created are colossal. The water plucked giant columns out of the basalt, created smoothed mesas and giant potholes, and formed ripple marks between 15 and 30 feet tall. Collectively, this landscape is called the Channeled Scablands. The floods left behind many other distinctive features, such as, buttes, coulees, and flood bars.
You can see these marks on the recently-designated Ice Age Floods National Geologic Trail, which traverses parts of Montana, Idaho, Washington, and Oregon. Sites along the Ice Age Floods trail highlight multiple geoheritage values and offer an excellent opportunity to connect the public to the natural environment.
In 2009, Congress established the Ice Age Floods National Geologic Trail, the first ever national geologic trail. Still in the development stage, the National Park Service is coordinating the planning and development of the trail with public and private landowners, local and tribal governments, the Ice Age Floods Institute, and other interested parties. The trail will consist of an existing network of highways, roads, and footpaths which will offer interpretive opportunities to bring the story of the ice-age floods to visitors.
In the meantime, check out the wealth of information about the floods compiled by the Washington Geological Survey in our online publications catalog. Or check out photos of Ice Age flood features in our online photo collection.
And check out more presentations made by our talented and dedicated team of geologists.