 The eastern portion of the county was within the Rosebud Indian Reservation until 1911, when that part was annexed into the newly created Mellette County.By 1914, Washabaugh County's boundaries were expanded to include a portion of Washington County to the west and all of Jackson County south of the White River.As an unorganized county lying entirely within two American Indian reservations, Washabaugh did not report separate population totals during the 1890, 1900, and 1910 censuses.