The dam's inner core is a thin layer made of clay material which resists seepage. Gold dredged tailings, sand &amp; gravel left from the early 20th century gold rush dredging operations along the Feather River, make up the remainder of Oroville Dam. Beneath the dam a giant cavern almost as large as the State Capitol Building has been hollowed out to house six power generation units. Coupled with four additional units in the Thermalito Pumping–Generating Plant, they will generate more than 2.8 billion kilowatt-hours of energy annually.