85 miles W of Spokane, 92 miles NE of Wenatchee
Grand Coulee, formerly a wide, dry valley, is a geologic anomaly left over from the last Ice Age. At that time, a glacier repeatedly dammed an upstream tributary of the Columbia River and formed a huge lake in what is today Montana. When this prehistoric lake repeatedly burst through its ice dam, massive floods poured down from the Rocky Mountains. So great was the volume of water that the Columbia River overflowed its normal channel and, as these floodwaters flowed southward, they carved deep valleys into the basalt landscape of central Washington. As the floodwaters reached the Cascades, they were forced together into one great torrent that was so powerful it scoured out the Columbia Gorge far downstream, carving cliffs and leaving us with the beautiful waterfalls seen in the Columbia Gorge today. With the end of the Ice Age, however, the Columbia returned to its original channel and the temporary flood channels were left high and dry. Early French explorers called these dry channels coulées, and the largest of them all was Grand Coulee, which is 50 miles long, between 2 and 5 miles wide, and 1,000 feet deep.
Located at the northern end of the Grand Coulee, Grand Coulee Dam is considered one of the greatest engineering marvels of the 20th century. Constructed during the Great Depression, it was the largest man-made structure on earth at the time of its completion in 1941 and is still the largest concrete dam in North America. The dam is 550 feet tall, 5,223 feet wide (almost a mile), and impounds the waters of the Columbia River, forming 151-mile-long Roosevelt Lake. Despite the name, the Grand Coulee Dam did not, however, fill the Grand Coulee with water. That did not happen until the 1950s when Dry Falls Dam was built at the south end of the coulee and waters from Roosevelt Lake were used to fill the Grand Coulee and form 31-mile-long Banks Lake. The waters of both Roosevelt and Banks lakes have been used to irrigate the arid lands of eastern Washington, turning this region into productive farmlands. The Grand Coulee Dam is also a major producer of hydroelectric power.