Rock Island Dam is a hydroelectric dam on the Columbia River near Wenatchee. Washington. The Rock Island Dam and Hydro Project was the first dam to span the Columbia in 1933. It is located near the geographical center of Washington, about 12 miles (19 km) downstream from the city of Wenatchee. Rock Island Dam is constructed on Columbia River basalt similar to that which is exposed on the cliffs near the dam. These columnar basalts were formed from lava flows during the mid- to late-Miocene Epoch, some 14 to 16 million years ago. The rock is strong and durable and provides a very stable foundation for the structure.
The Columbia River is the largest river in the Pacific Northwest region of North America. The river is 1,243 miles (2,000 km) long, and its drainage basin is roughly the size of France and extends into seven U.S. states and a Canadian province. Wikipedia