Diablo Dam is one of three dams along the upper Skagit River in Whatcom County, Washington and part of the Skagit River Hydroelectric Project that supplies Seattle with a large proportion of its power needs. Work was begun in 1917 on a six-mile tunnel through Diablo Canyon and subsequent construction of a powerhouse. Work crews had to overcome extreme weather and mountain conditions, while Seattle City Light officials had to deal with politics and diplomacy. The dam was completed in 1930, and began generating electricity in 1936.