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, a decrease of 37% from 2000 and 63% of the 20 year average of 116,700 cubic feet per second.
Electricity generation
The Rock Island Hydro Project generated 1,900 gigawatt-hours of electricity in 2001 — 70% of its 2000 output. Over the past 10 years the Project has averaged 2,600 gigawatt-hours per year. During 2001, Rock Island’s Second Powerhouse produced 1,800 gigawatt-hours of power, representing 95% of the Project’s total generation for the year. The 10 year average generation for the Project is 383 gigawatt-hours. Under the terms of the Rock Island Settlement Agreement, the District provides spill to improve the survival rate of juvenile salmon passing the Project. During the spring, of water was spilled during a 59-day period between April 20 and June 17. This resulted in 73,427 megawatt-hours of foregone generation valued at $13.4 million. In 1927, the Rock Island site came to the attention of the Stone and Webster Engineering Corporation, a Boston-based holding company that managed Puget Sound Power & Light Company. The site was recognized for its potential to provide power for the growing electrical load in the state. On December 13, 1928 an application was filed with the Federal Power Commission for a preliminary permit to investigate the site. This was followed by an application for license submitted in June 1929 by the Washington Electric Company, a subsidiary construction corporation of Puget Sound Power & Light. The license was authorized on October 16, 1929, and on January 14, 1930, at the beginning of the Great Depression, construction started on the first dam to span the Columbia River. The development of Rock Island Dam occurred over a period of some 50 years. There were three main construction periods, each taking place about 20 years apart as the need for low-cost hydroelectric power was paramount in the region. Development began in January 1930, and the dam, powerhouse, and first four operating units were turned over to Puget Sound Power & Light Company by Stone and Webster Engineering Corporation on February 1, 1933. Work on completion of the dam, powerhouse expansion and installation of six additional units by Chelan County PUD began in July 1951 and was completed on April 30, 1953. Construction of the Second Powerhouse, with its eight turbine generators located on the west bank of the river, began on August 4, 1974. The Second Powerhouse was placed in commercial operation on August 31, 1979.
