 By the late 1890s, dredging technology had become economical, and it is estimated that more than 20 million ounces (620 t) were recovered by dredging (worth approximately US$12 billion at November 2006 prices).Both during the Gold Rush and in the decades that followed, hard-rock mining wound up being the single-largest source of gold produced in the Gold Country.By 1850, the U.S. Navy started making plans for a west coast navy base at Mare Island Naval Shipyard.