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.