During the last ice age, powerful floods shaped the rugged terrain in the eastern part of the US state of Washington. The Ice Age floods, also known as the Missoula Floods, played a significant role in shaping the landscape of the Pacific Northwest, particularly in the states of Washington, Oregon, and Idaho. The floods were a series of cataclysmic events that occurred during the last Ice Age, around 15,000 to 13,000 years ago.

The key factor in the formation of the Ice Age floods was the presence of the Cordilleran Ice Sheet, a massive ice sheet that covered parts of western Canada and the northern United States. This ice sheet dammed glacial Lake Missoula in present-day Montana. Periodically, the dam would break, releasing enormous volumes of water in what are now known as the Missoula Floods.

Here's a general sequence of events that led to the formation of the landscape in the Pacific Northwest:

1. Glacial Lake Missoula Formation: During the Pleistocene epoch, large lobes of the Cordilleran Ice Sheet covered much of western North America. As these lobes advanced and retreated, they blocked the Clark Fork River in northern Idaho, creating Glacial Lake Missoula.

2. Ice Dam Failure: The ice dam holding back Glacial Lake Missoula periodically failed, releasing a colossal amount of water. The exact triggers for these failures could have been a combination of factors such as ice melting, earthquakes, or a combination of both.

3. Catastrophic Flooding: When the ice dam broke, the water from Glacial Lake Missoula surged across eastern Washington, down the Columbia River Gorge, and into the Willamette Valley in Oregon. The water moved at incredible speeds, reaching depths of hundreds of feet. The floods were powerful enough to carve out canyons, erode bedrock, and deposit vast amounts of sediment.

4. Repeated Flooding: The process of the ice dam breaking, the catastrophic flooding, and subsequent landscape shaping happened multiple times, creating a series of distinctive landforms. Some estimates suggest that there were as many as 40 to 60 separate flood events.

The Ice Age floods had a profound impact on the topography of the Pacific Northwest. They created features such as the Channeled Scablands in eastern Washington, giant ripple marks, and gravel bars. The floods also influenced the course of the Columbia River and its tributaries, leaving behind a landscape that is still evident today. This geological history is crucial to understanding the unique features and terrain of the Pacific Northwest region.