Rock Island Dam Island Pool. 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. Columbia River stream flows at the Project averaged 73,700 cubic feet per second (2090 m³/s), a decrease of 37% from 2000 and 63% of the 20 year average of 116,700 cubic feet per second (3305 m³/s). The Rock Island Hydro Project
Volcanic history of the Northern Cordilleran Volcanic Province the junction of the Pelly and Yukon rivers, Volcano Mountain in central Yukon consists of a cinder cone and a series of lava flows. It is the youngest volcano in the Fort Selkirk Volcanic Field and lava flows originated from fractures in the volcano's flank. These lava flows extend northeast and southwest of the volcano. The northeastern lava flow extends from Volcano Mountain whereas the southwestern lava flow extends from the volcano. These lava flows are composed of olivine nephelinite, a type of lava not commonly found on Earth. Nephelinite lavas are usually interpreted to have originated much deeper in