 When Idaho Territory established the county in February 1864, it contained all of present Ada, Canyon, and Payette counties.It also included most of present Boise and Gem Counties, the southern half of Washington County, and small portions of Adams, Custer, Owyhee, and Valley counties.When Ada County was created in December 1864, most of that territory was transferred to Ada County, leaving only small portions of Custer, Gem, Payette, Valley, and Washington counties together with most of present-day Boise County.