Idaho Emigration and Immigration
The pioneers who traveled over the Oregon Trail in the 1840s and 1850s rarely settled in Idaho. There were temporary trading posts and Protestant and Catholic missions but no permanent white settlement in Idaho until 1860 when settlers from Utah established Latter-day Saint communities in the Cache Valley.
In the early 1860s, many settlers returned from Washington, Oregon, and California to the gold fields of Idaho's northern panhandle. In the mid-1860s, silver miners established mining settlements around Silver City in Owyhee County. At the peak of this first mining boom as many as 70,000 whites may have been in Idaho, but by 1870 this number had dwindled to the 15,000 counted in the census.
Settlement was stimulated in the 1880s and 1890s by new mining booms in the north and by the arrival of railroads in the farmlands of southern Idaho. During this period, some Mormon families who had moved from Utah continued their migration northward to Alberta Province in Canada.
In the early 1900s, when reclamation projects opened desert lands to farming, a new wave of settlement from nearby states took place in southern Idaho. Today, nearly 70 percent of Idaho's population lives within 30 miles of the Snake River.
Most Idaho residents are of British, German, or Scandinavian origin, although there are about 6,000 Basques living near Boise. If you are looking for information on Basque settlement, you may find helpful information in Pat Bieter, The Basques in Idaho. Boise, Idaho: Idaho State Historical Society, 197? (FHL book 979.6 A1 number 41; film 1036796 item 15).
A few records of other ethnic groups are listed in the Family History Library Catalog under IDAHO - MINORITIES. One such group for which an index is being created is the Japanese (see Japanese Immigration to the United States 1887-1924, a web site created by BYU-Idaho)..
There was no port of entry common to overseas immigrants who settled in Idaho. The Family History Library and the National Archives have passenger lists for east coast ports for the years between 1820 and about 1920. You'll find suggestions on Latter-day Saint immigration sources on the Utah Wiki page. Detailed information on other immigration sources is on the United States Wiki page.
The Oregon-California Trails Association is an educational organization that promotes the story of the westward migration to Idaho, among other places. Their site includes a personal name index to trail diaries, journals, reminiscences, autobiographies, newspaper articles, guidebooks and letters at http://www.paper-trail.org/