While the Oregon Trail crossed the Snake River in Elmore County, at Three Island Crossing near Glenns Ferry, the significant early settlements of Elmore County were mining settlements located primarily in northern Elmore County surrounding the ghost town of Rocky Bar. Settlement at Rocky Bar commenced in 1863 with the settlement having 560 residents at the Territorial Census of that year. Nearby, Atlanta was settled in 1864. Elmore County north of the Snake River was originally part of Alturas County when it was created in 1864 and Rocky Bar became its county seat. The portion of Elmore County south of the Snake River was within Owyhee County until 1930. A small portion of Ada County was also annexed in 1948, bringing Elmore to its present boundary. The settlement at Rocky Bar was impacted by poor management by mining companies, mining lawsuits, and its geographic remoteness between 1869 and 1880. The Wood River experienced a surge in settlement at Hailey starting in 1880 that led to Rocky Bar's loss of the county seat to Hailey in 1882. Rocky Bar would briefly become a county seat upon the creation of Elmore County in 1889.