Formation of Idaho Counties 
Idaho's 44 counties were formed over a fifty-five year period of territorial and state growth. Counties were organized soon after the white settlement of particular areas, and spurts in county formation resulted from the major economic and transportation developments that brought people into the state. In his political geography of Idaho, Benjamin E. Thomas divides county formation into several periods corresponding to the periods of white settlement, economic growth, and exploitation of various resources. Before Idaho Territory was organized in 1863, four counties were formed by Washington Territory in the area now within Idaho boundaries. Formed in response to the gold rush into Orofino Creek, the Salmon River mines, and Boise Basin, the counties were Idaho (1861), Nez Perce (1861). Shoshone (1861), and Boise (1863). Idaho Territory established further county divisions, especially in the southern mining areas, including Owyhee (1863), Alturas (1864, abolished 1895), Lah-toh (1864, repealed 1867), Oneida (1864), Ada (1864), Kootenai (1864), and Lemhi (1869).
Clarification of the state boundary between northern Utah and southeastern Idaho and the growth of an agricultural economy in that region resulted in the formation of two southeastern counties between 1875 and 1881: Bear Lake (1875) and Cassia (1879). Mining interests continued in importance during this period, when Custer County (1881) formed after development of the Yankee Fork mines and Washington County (1879) formed in response to expanded farming and ranching and to mining development at Heath and the Seven Devils.
Railroads came into Idaho Territory in 1879 (the Utah and Northern Railroad, built north from Utah through southeast Idaho and into Montana) and 1882 (the Northern Pacific, built east from Spokane across northern Idaho, and the Oregon Short Line Railroad, built west through southern Idaho with a spur north to the Wood River mines at Hailey, Ketchum, and Bellevue). County seats established subsequent to railroad construction were usually in railroad towns that had developed into the principal supply and market centers for their regions. Along the railroads between 1885 and 1895 the following counties were established in areas where irrigated (and some dry land) agriculture was becoming an increasingly important economic base: Bingham (1885), Latah (1888), Elmore (1889), Canyon (1891), Fremont (1893), Bannock (1893), Blaine (1895), and Lincoln (1895) counties.
The population of Idaho tripled between 1900 and 1920, and 21 new counties were established during that period. Population growth resulted from the availability of newly irrigated agricultural lands and the growth of extractive industries like lumber milling and ore smelting. Irrigation projects financed through the Carey Act of 1894 and the Reclamation Act of 1902 provided storage water and canal systems for its delivery to fields. Untilled irrigable acreage, especially in south central Idaho, attracted new settlers to justify the creation of new counties in southern Idaho. During the same period, the lumber industry became important to the economy of northern Idaho and several new counties were carved out of those already extant. The more remote areas of the larger counties lobbied the Idaho legislature to establish new counties that could provide services closer to the new population centers. A 1912 amendment to the Idaho Constitution that required one state senator per county also encouraged citizens to push for the formation of new counties to obtain better representation in the state legislature. By 1919, however, a constitutional limit on the minimum size of a county (more than 400 square miles) and on the wealth of a county (at least one million dollars taxable property) stopped the proliferation of Idaho counties.
County seats were established either by legislation or through often- controversial elections. As population centers shifted, so too did county seats. Early county seats were established in mining camps that lost population to other mining camps or to agricultural market towns and railroad towns. This was the case in Owyhee, Idaho, Shoshone, and Elmore counties. Relocation of county seats was slowed, though, through legislation requiring a two-thirds majority in any vote to change a county seat and requiring a six year gap between such votes.
The location of the county seat grew in importance as county government offered more services and the court system became more effective than it was during the mining rush. Throughout Idaho, the county courthouse was the significant location for most political, governmental, and legal activities, as few towns had city buildings until the 1950s. The construction of a courthouse was an event of major importance to the county community both symbolically and practically, providing a visible token of the community's coming of age as a viable settlement, and providing much-needed space for county offices, storage of legal documents, trials, and jails. Most communities supported construction of a courthouse through bonds, which were approved in a popular vote. Bond issues met with some difficulties, often from rival county seats or remote areas of a county, but most were successful. As a result, few Idaho counties now occupy buildings that were not designed as courthouses.
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