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. The Boise River portion of the current western boundary was established by 1866. The southern boundary common to present Ada County was defined the following year. The northern boundary was most volatile Between 1873 and 1887 with the boundary shifting further north into Valley County, back south below Cascade, and then again north to include the North Fork of Payette River Basin. The county obtained its current boundary after Gem County was created in 1915 and Valley County in 1918.
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. The Boise River portion of the current western boundary was established by 1866. The southern boundary common to present Ada County was defined the following year. The northern boundary was most volatile Between 1873 and 1887 with the boundary shifting further north into Valley County, back south below Cascade and then again north to include the North Fork of Payette River Basin. The county obtained its current boundary after Gem County was created in 1915 and Valley County in 1918.