The "Ice Age Floods National Geologic Trail" spans four state: Washington, Oregon, Idaho, and Montana.
"Crossing four states (Montana, Idaho, Washington, and Oregon) and over a thousand miles, the Ice Age Floods National Geologic Trail offers incredible views and one-of-a-kind geological formations."
"At the end of the last Ice Age, some 12,000 to 17,000 years ago, a series of cataclysmic floods occurred in what is now the northwest region of the United States, leaving a lasting mark of dramatic and distinguishing features on the landscape of parts of the States of Montana, Idaho, Washington and Oregon."
[U.S. National Park Service website, 2018]
The "Ice Age Floods National Geologic Trail" is the nation's first geologic trail, created by the "Omnibus Public Land Management Act of 2009", signed into law by President Barack Obana on March 30, 2009.