Video: Rocks Below Bonneville Dam
Basalt rocks ranging in size from 1-4 feet in diameter are trapped below the spillway at Bonneville Dam. The U.S. Army Corps of Engineers needs to remove the rocks -- about 550 cubic yards -- before the Spring spill season begins. When water spills over the spillway, the rocks move in a fashion known as "ball milling" which erodes the concrete. The rocks entered the spillway in 2011 when the Columbia River experienced significant high flows during most of the Spring and Summer.