Grand Coulee Dam

The 551 ft high Grand Coulee Dam in the north-east of Washington State is the largest cement structure in the United States. It dams the Columbia River at a point where an obstruction created by ice action during the previous glacial period forced the river into a new course, diverting it across the plateau to the south. Some 18,000 years ago the river broke back through the barrier and, within the space of a few weeks, with almost unimaginable power, cut a deep canyon, the Grand Coulee. At first following construction of the dam in the 1940s the canyon was dry, but now it carries water from Roosevelt Lake. At the end of Banks Lake near Coulee City there is an almost 3 mi. long, 394 ft high escarpment known as Dry Falls, over which the