is the grand coulee dam in the okanogan highlands
Natural History. Grand Coulee Dam is positioned in an area which sits on the edge of two geologic zones â the Okanogan Highlands and the Columbia Plateau. The dam sits on the solid granitic rock of the Okanogan Highlands, making a perfect base for the dam. The Columbia Plateau to the South consists of layers of softer basalts. The Columbia River in the Grand Coulee region was carved between these two geologic zones.