Glacial Lake Missoula was a large glacial lake that formed during the last Ice Age, around 15,000 years ago, in what is now western Montana. It was created by the advance of the Cordilleran Ice Sheet, which blocked the flow of the Clark Fork River, causing water to accumulate behind the ice dam. When the ice dam eventually failed, massive floods occurred, reshaping the landscape of the Pacific Northwest. These floods carved out features such as the Channeled Scablands in eastern Washington and left behind sediment deposits that are still visible today.