09/11/2026
Did you know Lake Roosevelt wasn’t the first lake to fill this valley?
During the last Ice Age, a huge glacier blocked the Columbia River near where Grand Coulee Dam is today. This created a much bigger lake called Glacial Lake Columbia. It covered much of the upper Columbia River basin and even reached into North Idaho. Over time, this lake helped shape the steep canyons and wide coulees we see now.
Massive floods from another ancient lake, Glacial Lake Missoula, rushed in from the east. When these floods poured into Glacial Lake Columbia, they spilled south and carved famous features like Dry Falls, Steamboat Rock, and the Channeled Scabland. These places are some of the best evidence we have of huge Ice Age floods anywhere in the world.
Glacial Lake Columbia covered nearly 500 square miles on average. Today’s Lake Roosevelt covers only about a quarter of that area, and even less when compared to the lake’s largest size. The old lake’s surface was about 1600 feet above sea level. At full pool, Lake Roosevelt reaches 1290 feet—still high enough to cover many of the cliffs around it.
This valley is also part of the Ice Age Floods National Geologic Trail NPS. Visitors can explore landscapes shaped by ice, water, and time on a truly massive scale.
NPS Image
