Idaho Atlas of Historical County Boundaries
John H. Long, Editor; Peggy Tuck Sinko, Associate Editor; Emily Kelley, Historical Compiler; Laura Rico-Beck, GIS Specialist and Digital Compiler; Peter Siczewicz, ArcIMS Interactive Map Designer; Robert Will, Cartographic Assistant
Copyright The Newberry Library 2007
1 October 1804
Based on American exploration of the Northwest, the United States claimed the Oregon Country, an area west of the 1803 Louisiana Purchase that was not specifically defined but included all of present Oregon, Washington, and Idaho, parts of Montana and Wyoming, and a large portion of Canada lying west of the Continental Divide. Great Britain, Spain and Russia also claimed the area. (Hayes, 97-100, 131-138; Paullin, 36–37, plate 49)
20 October 1818
A Convention of Commerce between Great Britain and the United States established the parallel of 49 degrees north latitude, from the Lake of the Woods to the eastern side of the Rocky Mountains, as the northern boundary of the United States and the southern boundary of the British possessions respectively. It was also agreed that the area “westward of the Stony Mountains” should be jointly occupied by the United States and Great Britain for a term of ten years. The area covered in this joint occupancy included all of present Oregon, Washington and Idaho, parts of Montana and Wyoming, and the portion of present Canada lying west of the Rocky Mountains (Continental Divide). No northern limit was defined. (Hayes, 131; Parry, 69:294-297; Van Zandt, 153)
22 February 1819
The Adams-Onis Treaty between the United States and Spain ended Spanish interest in Oregon and established the southern boundary of the Oregon Country at 42 degrees north latitude. (Parry, 70:1–30; Van Zandt, 26–27)
17 April 1824
A Convention of Commerce between the United States and Russia set the northern limit of the Oregon Country at 54 degrees 40 seconds north latitude. (Hayes, 131; Parry 74:135-140)
5 July 1843
United States' control over the southern part of Oregon Country was strengthened by the establishment of the Oregon Territorial Provisional Government and the creation of four counties, originally called districts: CHAMPOEG DISTRICT (now MARION, Ore.), CLACKAMAS DISTRICT (Ore.), TUALITY DISTRICT (Ore., now WASHINGTON, Ore.), and YAMHILL DISTRICT (Ore.). CHAMPOEG and CLACKAMAS included parts of Idaho. (Swindler, 8:195; Holman, 7-9; Oregon Archives, 26; Loy, 16)
27 June 1844
Non-county area created in Oregon Country from CLACKAMAS DISTRICT (Ore.); portion of CLACKAMAS in present Idaho unchanged. (Ore. Terr. Laws 1843-1849, gen. and loc., p. 74)
19 December 1845
CLACKAMAS DISTRICT (Ore.) gained from CHAMPOEG DISTRICT (now MARION, Ore.). (Ore. Terr. Laws 1843-1849, gen. and loc., pp. 35-37)
22 December 1845
CHAMPOEG DISTRICT (now MARION, Ore.) and CLACKAMAS DISTRICT renamed counties; no change in boundaries [not mapped]. (Ore. Terr. Laws 1843-1849, gen. and loc., p. 35)
15 June 1846
The Oregon Boundary Treaty between the United States and Great Britain ended joint occupancy of the Oregon Country when it set the international boundary "along the forty-ninth parallel of north latitude to the middle of the channel which separates the continent from Vancouver's Island; and thence southerly through the middle of said channel, and of Fuca's Straits, to the Pacific Ocean." The treaty also created a disputed area by failing to specify which of the multiple passages through the San Juan Islands was meant by "the middle of said channel." Great Britain claimed a line through Rosario Strait, the eastern most passage, while the United States claimed a line through the Strait of Haro, the western most passage. The dispute over this water boundary was finally settled on 21 October 1872. Oregon considered the disputed San Juan Islands to be part of LEWIS (Wash.). British North America gained all the parts of CLACKAMAS (Ore.), LEWIS (Wash.), and VANCOUVER (now CLARK, Wash.) which lay north of 49 degrees north latitude. (Hayes, 134, 171-174; Parry 100:40-42; Van Zandt, 18)
28 December 1847
LINN (Ore.) created by Oregon Territorial Provisional Government from CHAMPOEG (now MARION, Ore.). (Ore. Terr. Laws 1843-1849, gen. and loc., pp. 55-56)
14 August 1848
The United States created Oregon Territory from "all that part of the Territory of the United States which lies west of the summit of the Rocky Mountains and north of the 42nd degree of north latitude,” encompassed all of present Oregon, Washington, and Idaho, and parts of present Montana and Wyoming. (U.S. Stat., vol. 9, ch. 67[1848]/pp.323-331; Van Zandt, p. 153)
3 September 1849
CHAMPOEG (Ore.) renamed MARION (Ore.). (Ore. Terr. Laws 1850, 2nd sess., p. 53)
4 January 1851
Non-county area created in Oregon Territory from LINN (Ore.) when the county's southern line redefined; included parts of present Idaho and Wyoming. (Ore. Terr. Laws 1850, 2nd sess., pp. 33-34)
28 January 1851
LANE (Ore.) created from non-county area in Oregon Territory; included parts of present Idaho and Wyoming. Non-county area eliminated. (Ore. Terr. Laws 1850, 2nd sess., pp. 32-33)
7 January 1852
DOUGLAS (Ore.) created from LANE (Ore.); portion of LANE in present Idaho unchanged. (Ore. Terr. Laws 1851, 3rd reg. sess., p. 18)
2 March 1853
The United States created Washington Territory from Oregon Territory. Non-county area created in Washington Territory from part of CLACKAMAS (Ore.). CLACKAMAS (Ore.), LANE (Ore.), LINN (Ore.), and MARION (Ore.), and non-county area included parts of present Idaho, Montana, and Wyoming. (Van Zandt, 155)
22 December 1853
LANE (Ore.) gained from UMPQUA (Ore., extinct), gained from non-county area in Oregon Territory, and exchanged with DOUGLAS (Ore.); portion of LANE in present Idaho unchanged. (Ore. Terr. Special Laws, 1853, 5th reg. sess., p. 13)
11 January 1854
WASCO (Ore.) created by Oregon Territory from CLACKAMAS (Ore.), JACKSON (Ore.), LANE (Ore.), LINN (Ore.), and MARION (Ore.); included parts of present Idaho, Montana, and Wyoming. (Ore. Terr. Special Laws 1853, 5th reg. sess., pp. 26-27)
9 March 1854
SKAMANIA (Wash.) created by Washington Territory from CLARK (Wash.), LEWIS (Wash.), and non-county area in Washington Territory; included parts of present Idaho and Montana. Non-county area gained from CLARK (Wash.). (Wash. Terr. Laws 1854, 1st sess., p. 473)
25 April 1854
WALLA WALLA (Wash.) created by Washington Territory from SKAMANIA (Wash.) and non-county area in Washington Territory. WALLA WALLA overlapped the eastern end of WHATCOM (Wash) and included parts of present Idaho and Montana. SKAMANIA eliminated from Idaho. (Wash. Terr. Laws 1854, 1st sess., pp. 472-473)
14 January 1858
Overlap between WALLA WALLA and WHATCOM settled in favor of WHATCOM; portion of WALLA WALLA in present Idaho unchanged. (Wash. Terr. Laws 1857, 5th reg. sess., p. 53)
29 January 1858
SPOKANE (Wash. Terr., original, extinct) created by Washington Territory from WALLA WALLA; included parts of present Idaho and Montana. SPOKANE not fully organized, not attached. (Wash. Terr. Laws 1857, 5th reg. sess., p. 51)
14 February 1859
Non-county area created in Washington Territory from WASCO (Ore.) when Oregon became a state; included the southern half of Idaho, and parts of present Montana, and Wyoming. WASCO eliminated from Idaho. (Ore. Laws 1860, 1st reg. sess., pp. 28-30; Van Zandt, p. 155)
17 January 1860
SPOKANE (Wash. Terr., original, extinct) redefined and fully organized [no change]. (Wash. Terr. Laws 1859, 7th reg. sess., p. 436)
14 December 1860
MISSOULA (Mont.) created by Washington Territory from SPOKANE (Wash. Terr., original, extinct); included parts of present Idaho and Montana. (Wash. Terr. Laws 1860, 8th sess., pp. 7-8)
9 January 1861
SHOSHONE created by Washington Territory from SPOKANE (Wash. Terr., original, extinct) and non-county area in Washington Territory; included parts of present Idaho, Montana, and Wyoming. Non-county area eliminated. (Wash. Terr. Laws 1860, 8th sess., pp. 13-14)
2 March 1861
Nebraska Territory gained from SHOSHONE. (U.S. Stat., vol. 12, ch. 86 [1861]/pp. 239-244; Van Zandt, 134, 136-137)
20 December 1861
IDAHO created by Washington Territory from SHOSHONE; included parts of present Idaho, Montana and Wyoming. (Wash. Terr. Laws 1861-1862, 9th sess., p. 3)
NEZ PERCE created by Washington Territory from MISSOULA (Mont.), SHOSHONE and SPOKANE (Wash. Terr., original, extinct). (Wash. Terr. Laws 1861-1862, 9th sess., p. 4)
SHOSHONE gained from MISSOULA (Mont.) and SPOKANE (Wash. Terr., original, extinct). (Wash. Terr. Laws 1861-1862, 9th sess., pp. 4-5)
12 January 1863
BOISE created by Washington Territory from IDAHO; included parts of Idaho and Wyoming. (Wash. Terr. Laws 1862, 10th sess., pp. 3-4)
21 January 1863
NEZ PERCE gained from SPOKANE (Wash. Terr., original, extinct). (Wash. Terr. Laws 1862, 10th sess., p. 5)
3 March 1863
The United States created Idaho Territory from Dakota, Nebraska, and Washington Territories. SPOKANE (Wash. Terr., original extinct) gained from NEZ PERCE and was eliminated from Idaho. BOISE, IDAHO, MISSOULA (Mont.), NEZ PERCE, and SHOSHONE became Idaho Territory counties. (U.S. Stat., vol. 12, ch. 117 [1863]/pp. 808-814; Van Zandt, 156-158)
Non-County Area 1 created from SPOKANE (Wash. Terr., original, extinct) when the United States defined the boundaries for Idaho Territory. Non-County Area 1 included parts of present Idaho and Montana. (Id. Terr. Laws 1863, loc. and spec., 1st sess., pp. 27-28)
Non-County Area 2 created from parts of Dakota and Nebraska Territories when the United States defined the boundaries for Idaho Territory. Non-County Area 2 included parts of present Idaho, Montana, and Wyoming. (U.S. Stat., vol. 12, ch. 117 [1863]/pp. 808-814; Van Zandt, 156-158)
31 December 1863
OWYHEE created from BOISE and Non-County Area 2; included parts of present Idaho and Wyoming. (Id. Terr. Laws 1863, loc. and spec., 1st sess., p. 624)
16 January 1864
BEAVERHEAD (Mont.), BIG HORN (now CUSTER, Mont.), CHOTEAU (now CHOUTEAU, Mont.), DAWSON (Mont.), JEFFERSON (Mont.), MADISON (Mont.), OGALALA (Idaho Territory, extinct), and YELLOWSTONE (Idaho Territory, extinct) created from Non-County Area 2. (Id. Terr. Laws 1863, loc. and spec., 1st sess., secs. 3-10/pp. 675-677)
DEER LODGE (Mont.) created from IDAHO, MISSOULA (Mont.) and Non-County Area 2. (Id. Terr. Laws 1863, loc. and spec., 1st sess., sec. 2/pp. 674-675)
MISSOULA (Mont.) gained from IDAHO, Non-County Area 1 and Non-County Area 2. (Id. Terr. Laws 1863, loc. and spec., 1st sess., sec. 1/p. 674)
22 January 1864
ONEIDA created from BOISE and OWYHEE; included parts of present Idaho and Wyoming. (Id. Terr. Laws 1863, loc. and spec., 1st sess., p. 625)
4 February 1864
ALTURAS (extinct) created from BOISE; NEZ PERCE gained from IDAHO; Non-County Area 1 gained from NEZ PERCE, attached to NEZ PERCE for "all civil and judicial purposes"; SHOSHONE gained from Non-County Area 1; BOISE exchanged with IDAHO; OWYHEE redefined [no change]. (Id. Terr. Laws 1863, loc. and spec., 1st. sess., secs. 1-5/pp. 628-630)
26 May 1864
Dakota Territory gained most of present Wyoming from Idaho Territory; gained from ONEIDA; gained all of OGALALA (Id. Terr., extinct) and YELLOWSTONE (Id. Terr., extinct). (U.S. Stat., vol. 13, ch. 95 [1864], sec. 18/p. 92; Van Zandt, 134-136, 156-157)
The United States created Montana Territory from Idaho Territory with boundaries identical to the state of Montana. BEAVERHEAD (Mont.), BIG HORN (original, now CUSTER, Mont.), CHOTEAU (now CHOUTEAU, Mont.), DAWSON (Mont.), DEER LODGE (Mont.), JEFFERSON (Mont.), MADISON (Mont.), and MISSOULA (Mont.) eliminated from Idaho Territory. (U.S. Stat., vol. 13, ch. 95 [1864], sec. 1/pp. 85-92; Sanders, Helen F., 1:329; Van Zandt, 145)
22 December 1864
ADA created from BOISE. (Id. Terr. Laws 1864, 2nd sess., ch. 29/pp. 430-431)
KOOTENAI and LAH-TOH (Idaho Terr., extinct) created from Non-County Area 1; Faulty geographic knowledge in boundary description created Overlap Area 1 that was assigned to both KOOTENAI and SHOSHONE. LAH-TOH attached to NEZ PERCE "for all civil and criminal jurisprudence". (Id. Terr. Laws 1864, 2nd sess., ch. 30/pp. 432-433)
5 January 1866
NEZ PERCE gained from IDAHO [estimated line]. (Id. Terr. Laws 1865, 3rd. sess., ch. 38/pp. 182-183)
12 January 1866
ALTURAS (extinct) exchanged with BOISE. (Id. Terr. Laws 1865, 3rd. sess., ch. 56/p. 214)
9 January 1867
IDAHO gained from NEZ PERCE; KOOTENAI and NEZ PERCE gained from LAH-TOH (extinct) which was eliminated. (Id. Terr. Laws 1866, 4th sess., ch. 48/p. 126)
10 January 1867
ADA gained from BOISE and IDAHO. (Id. Terr. Laws 1866, 4th sess., ch. 46/p. 124
25 July 1868
The United States created Wyoming Territory from Dakota, Idaho, and Utah Territories, and from part of ONEIDA. (Trenholm, 1:83-84; U.S. Stat., vol. 15, ch. 235 [1868], secs. 1, 17/pp. 178, 183; Van Zandt, 136, 144, 156)
9 January 1869
LEMHI created from IDAHO. (Id. Terr. Laws 1868, 5th sess., ch. 19/pp. 117-118)
2 January 1871
ONEIDA gained from OWYHEE. (Id. Terr. Laws 1870, 6th sess., p. 54)
10 January 1873
ADA, BOISE, and LEMHI gained from IDAHO. (Id. Terr. Laws 1872, 7th sess., pp. 30-31; 47-48; 64-65)
5 January 1875
BEAR LAKE created from ONEIDA. (Id. Terr. Laws 1874-1875, comp., rev., 8th sess., pp. 720-722)
8 January 1875
IDAHO gained from NEZ PERCE. (Id. Terr. Laws 1874-1875, comp., rev., 8th sess., pp. 730-732)
8 January 1877
ONEIDA gained from ALTURAS (extinct) and LEMHI. (Id. Terr. Laws 1876, 9th sess., pp. 90-91)
20 February 1879
CASSIA created from ONEIDA and OWYHEE. (Id. Terr. Laws 1879, 10th sess., pp. 43-46)
WASHINGTON created from ADA and BOISE; IDAHO gained from ADA. (Id. Terr. Laws 1879, 10th sess., pp. 40-42)
9 February 1881
Eastern line of CASSIA clarified [no change]. (Id. Terr. Laws 1880, 11th sess., p. 339)
1 April 1881
CUSTER created from ALTURAS (extinct), BOISE, and LEMHI; BOISE and LEMHI gained from ALTURAS (extinct). (Id. Terr. Laws 1880, 11th sess., pp. 340-345)
31 January 1883
ADA and WASHINGTON gained from BOISE. (Id. Terr. Laws 1882, 12th sess., pp. 67-68)
13 January 1885
BINGHAM created from ONEIDA. (Id. Terr. Laws 1884, 13th sess., pp. 41-43)
21 January 1885
IDAHO gained from BOISE, LEMHI, and NEZ PERCE. (Id. Terr. Laws 1884, 13th sess., p. 126)
5 February 1885
BINGHAM boundary with LEMHI clarified [no change]. (Id. Terr. Laws 1884, 13th sess., p. 46)
15 January 1887
BOISE gained from ALTURAS (extinct), and exchanged with IDAHO. (Id. Terr. Laws 1887, comp., spec. and loc., 14th sess., p. 120)
1 June 1887
Boundaries for ADA, ALTURAS (extinct), BEAR LAKE, BINGHAM, CASSIA, CUSTER, IDAHO, KOOTENAI, LEMHI, NEZ PERCE, ONEIDA, OWYHEE, SHOSHONE, and WASHINGTON redefined [no change]. Overlap Area 1 between KOOTENAI and SHOSHONE continued without change. (Id. Terr. Laws 1887, comp., spec. and loc., 14th sess., chs. 3-6, 8-17/pp. 22-119, 125-205)
14 May 1888
LATAH created by the United States Congress from NEZ PERCE. (U.S. Stat., vol. 25, ch. 251 [1888]/pp. 147-149; Id. Encyclopedia, pp. 297-298)
4 February 1889
CUSTER gained from IDAHO. (Id. Terr. Laws 1888, 15th sess., pp. 22-23)
7 February 1889
ELMORE and LOGAN (extinct) created from ALTURAS (extinct); BINGHAM gained from ALTURAS (extinct). (Id. Terr. Laws 1888, 15th sess., pp. 33-36)
IDAHO exchanged with NEZ PERCE. (Id. Terr. Laws 1888, 15th sess., pp. 50-51)
3 July 1890
The state of Idaho was created from Idaho Territory. ADA, ALTURAS (extinct), BEAR LAKE, BINGHAM, BOISE, CASSIA, CUSTER, ELMORE, IDAHO, KOOTENAI, LEMHI, LOGAN (extinct), NEZ PERCE, ONEIDA, OWYHEE, SHOSHONE, and WASHINGTON became counties in the state of Idaho. (U.S. Stat., vol. 26, ch. 656 [1890]/pp. 215-219; Van Zandt, 156)
11 February 1891
IDAHO exchanged with WASHINGTON. (Id. Laws 1890, 1st sess., pp. 41-42)
2 March 1891
IDAHO northern boundary clarified [no change]. (Id. Laws 1890, 1st sess., pp. 117-118)
3 March 1891
ALTA (extinct) created from ALTURAS (extinct), BOISE, and LOGAN (extinct). LINCOLN (original, extinct) created from LOGAN (extinct). ALTURAS and LOGAN eliminated. (Id. Laws 1890, 1st sess., pp. 120-124)
7 March 1891
LEMHI gained from ALTA (extinct) and BINGHAM. (Id. Laws 1890, 1st sess., p. 166)
6 May 1891
ALTURAS (extinct) and LOGAN (extinct) were restored, and BOISE gained when the Idaho Supreme Court declared the act creating ALTA (extinct) and LINCOLN (original, extinct) unconstitutional because it failed to provide for an election. ALTA and LOGAN eliminated. (Id. Rpts. 1893, vol. 2/pp. 813-839; Id. Encyclopedia, p. 232)
26 November 1892
CANYON created from ADA. Act approved 7 March 1891and became effective through a vote in the general election of 26 November 1892. (Id. Laws 1890, 1st sess., p. 155; Nelson and Webbert, p. 75)
4 March 1893
FREMONT created from BINGHAM. (Id. Laws 1893, 2nd sess., pp. 94-97)
6 March 1893
BANNOCK created from BINGHAM. (Id. Laws 1893, 2nd sess., pp. 170-174)
9 February 1895
ADA boundary with ELMORE clarified [no change]. (Id. Laws 1895, 3rd sess., pp. 15-16)
23 February 1895
WASHINGTON gained from IDAHO. (Id. Laws 1895, 3rd sess., pp. 21-22)
5 March 1895
BLAINE created from ALTURAS (extinct) and LOGAN (extinct); ALTURAS and LOGAN eliminated. (Id. Laws 1895, 3rd sess., pp. 31-34)
9 March 1895
Legislature authorized CUSTER to annex territory from BLAINE, dependent on a vote in the general election of 1896. No evidence that the vote ever took place; proposed annexation failed [no change]. (Id. Laws 1895, 3rd sess., pp.140-142; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-121 "Custer Co."/p. 31)
18 March 1895
LINCOLN created from BLAINE. (Id. Laws 1895, 3rd sess., pp. 170-174)
3 November 1896
FREMONT gained from LEMHI. Act approved 11 March 1895 and became effective through a vote in the general election of 3 November 1896. (Id. Laws 1895, 3rd sess., pp. 145-148; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-124 "Fremont Co."/p. 35)
2 February 1899
The State of Idaho ceded to the United States jurisdiction over "all such lands within the State as may be included in the territory of the Yellowstone National Park. . . .," did not change the boundaries of FREMONT, the only Idaho county which includes a portion of Yellowstone National Park. (Id. Laws 1899, 5th sess., p. 22)
IDAHO redefined [no change]. (Id. Laws 1899, 5th sess., pp. 79-80)
WASHINGTON redefined [no change]. (Id. Laws 1899, 5th sess., pp. 22-23)
6 February 1899
LEMHI redefined [no change]. (Id. Laws 1899, 5th sess., p. 111)
14 February 1899
ADA boundary with ELMORE redefined [no mappable change]. (Id. Laws 1899, 5th sess., p. 234)
Legislature authorized CUSTER to annex territory from BLAINE, dependent on a vote in the general election of 1900. Proposed annexation failed in referendum [no change]. (Id. Laws 1899, 5th sess., p. 271; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-109 "Blaine Co."/p. 16)
ELMORE boundary with ADA redefined [no mappable change]. (Id. Laws 1899, 5th sess., p. 234) FREMONT’s gain from LEMHI on 3 November 1896 was reenacted [no change]. (Id. Laws 1899, 5th sess., pp. 273-275; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-124 "Fremont Co."/p. 35)
21 March 1901
Legislature authorized creation of CLEARWATER (proposed) from NEZ PERCE and SHOSHONE. Proposal failed on 14 June 1901 when the Idaho Supreme Court ruled the act invalid [no change]. (Id. Laws 1901, 6th sess., pp. 209-210; Id. Rpts. 1903, vol. 7/pp. 752-762; ISHS Ref. Series #334)
21 February 1903
LEMHI gained from IDAHO; Overlap Area 2 created between IDAHO and LEMHI when a redefinition of their boundary was based on faulty geographic knowledge; overlap eliminated 10 March 1911. (Id. Laws 1903, 7th sess., pp. 48-49)
8 November 1904
FREMONT gained from BINGHAM. Act approved 6 Mar 1903 and became effective through a vote in the general election of 8 November 1904. (Id. Laws 1903, 7th sess., pp. 222-223; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-124 "Fremont Co."/p. 35)
NEZ PERCE gained from SHOSHONE. Act approved 10 March 1903 and became effective through a vote in the general election of 8 November 1904. (Id. Laws 1903, 7th sess., pp. 204-206; Id. Laws 1905, 8th sess., pp. 331-333; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-137 "Nez Perce Co."/p. 49)
22 February 1905
KOOTENAI boundary with LATAH redefined [no change]. (Id. Laws 1905, 8th sess., pp. 333-334)
27 February 1905
WASHINGTON boundaries redefined [no change]. (Id. Laws 1905, 8th sess., pp. 303-304)
28 February 1905
Legislature authorized creation of CLARK (proposed) and LEWIS (proposed) from KOOTENAI. Proposal failed on 12 May 1905 when the Idaho Supreme Court declared the act unconstitutional [no change]. (Id. Laws 1905, 8th sess., pp. 76-82; Id. Rpts. 1907, vol. 11/pp. 14-44)
21 February 1907
BONNER created from KOOTENAI and small remnant of Non-County Area 1. Overlap between KOOTENAI and SHOSHONE (see 22 December 1864) settled in favor of SHOSHONE. Non-County Area 1 and Overlap Area 1 eliminated. (Id. Laws 1907, 9th sess., pp. 47-49)
TWIN FALLS created from CASSIA. (Id. Laws 1907, 9th sess., pp. 40-41)
11 March 1909
Boundaries for KOOTENAI, LATAH, NEZ PERCE, and SHOSHONE redefined [no change]. (Id. Laws 1909, 10th sess., pp. 318-320)
7 February 1911
BONNEVILLE created from BINGHAM. (Id. Laws 1911, 11th sess., ch. 5/pp. 8-9)
27 February 1911
CLEARWATER created from NEZ PERCE. (Id. Laws 1911, 11th sess., ch. 24/pp. 49-60)
3 March 1911
ELMORE gained from BLAINE. (Id. Laws 1911, 11th sess., ch. 148/pp. 453-454)
10 March 1911
LEMHI gained from IDAHO. Overlap between IDAHO and LEMHI (see 21 February 1903) settled in favor of IDAHO; Overlap Area 2 eliminated. (Id. Laws 1911, 11th sess., ch. 218/pp. 699-701)
15 March 1911
ADAMS created from WASHINGTON. (Id. Laws 1911, 11th sess., ch. 31/pp. 67-72)
20 March 1911
LEWIS created from NEZ PERCE. (Id. Laws 1911, 11th sess., ch. 37/pp. 77-84)
28 January 1913
GOODING and MINIDOKA created from LINCOLN. (Id. Laws 1913, 12th sess., ch. 3/pp. 5-13; ch. 4/pp. 13-21)
30 January 1913
FRANKLIN created from ONEIDA. (Id. Laws 1913, 12th sess., ch. 5/pp. 22-30)
POWER created from BINGHAM, BLAINE, CASSIA, and ONEIDA. (Id. Laws 1913, 12th sess., ch. 6/pp. 30-45)
4 November 1913
JEFFERSON created from FREMONT. Act approved 18 February 1913 and became effective through a vote in the general election of 4 November 1913. (Id. Laws 1913, 12th sess., ch. 25/pp. 94-107; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-128 "Jefferson Co."/pp. 38; Nelson and Webbert, p. 88)
MADISON created from FREMONT. Act approved 18 February 1913 and became effective through a vote in the general election of 4 November 1913. (Id. Laws 1913, 12th sess., ch. 26/pp. 107-121; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-135 "Madison Co."/pp. 46; Nelson and Webbert, p. 100)
23 January 1915
BENEWAH created from KOOTENAI. (Id. Laws 1915, 13th sess., ch. 4/pp. 5-8)
BOUNDARY created from BONNER. (Id. Laws 1915, 13th sess., ch. 7/pp. 21-29)
26 January 1915
TETON created from MADISON. (Id. Laws 1915, 13th sess., ch. 28/pp. 29-30)
11 May 1915
GEM created from BOISE and CANYON. Act approved 19 March 1915 and became effective through a vote in the general election of 11 May 1915. (Id. Laws 1915, 13th sess., ch. 165/pp. 362-375; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-125 "Gem Co."/p. 35)
7 November 1916
POWER gained from ONEIDA. Act approved 15 March 1915 and became effective through a vote in the general election of 7 November 1916. (Id. Laws 1915, 13th sess., ch. 132/pp. 287-289; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-142 "Power Co."/p. 54)
6 February 1917
BUTTE created from BINGHAM, BLAINE, and JEFFERSON. (Id. Laws 1917, 14th sess., ch. 98/pp. 344-360)
CAMAS created from BLAINE. (Id. Laws 1917, 14th sess., ch. 97/pp. 329-344)
14 March 1917
Legislature authorized the creation of SELWAY (proposed) from IDAHO and LEWIS, dependent on a vote in the special election of 2 July 1917. Proposal failed in referendum [no change]. (Id. Laws 1917, 14th sess., ch. 127/pp. 418-430; www.researchnetwork.net)
11 May 1917
PAYETTE created from CANYON. Act approved 28 February 1917 and became effective through a vote in the special election of 11 May 1917. (Id. Laws 1917, 14th sess., ch.11/pp. 13-27; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-140 "Payette Co."/p. 52)
6 November 1918
FRANKLIN gained from BANNOCK. Act approved 8 Feb 1917 and became effective through a vote in the general election of 6 November 1918. (Id. Laws 1917, 14th sess., ch. 96/pp. 327-329; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-123 "Franklin Co."/p. 33)
VALLEY created from BOISE and IDAHO. Act approved 26 February 1917 and became effective through a vote in the general election of November 1918. (Id. Laws 1917, 14th sess., ch. 99/pp. 360-372)
1 February 1919
CLARK created from FREMONT. (Id. Laws 1919, 15th sess., ch. 3/pp. 4-6)
8 February 1919
JEROME created from GOODING, LINCOLN, and MINIDOKA. (Id. Laws 1919, 15th sess., ch. 4/pp. 14-28)
11 February 1919
CARIBOU created from BANNOCK. Same act also authorized BANNOCK to annex territory from CARIBOU [Annexation 1 (proposed)], dependent on a vote in the general election of November 1920. Proposed annexation failed in referendum [no change]. (Id. Laws 1919, 15th sess., ch. 5/pp. 28-42; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-117 "Caribou Co."/p. 24)
19 February 1919
ELMORE redefined [no change]. (Id. Laws 1919, 15th sess., ch. 109/pp. 391-392)
Legislature authorized VALLEY to annex territory from IDAHO [Annexation 2 (proposed)], dependent on a vote in the general election of November 1920. No evidence this vote ever took place [no change]. (Id. Laws 1919, 15th sess., ch. 101/pp. 366-367; Id. Code Ann. (1996), ch. 1, sec. 31-145 "Valley Co."/pp. 56-57)
4 March 1921
CLARK redefined [no change]. (Id. Laws 1921, 16th sess., ch. 194/pp. 395-397)
5 March 1921
BUTTE boundaries clarified [no change]. (Id. Laws 1921, 16th sess., ch. 204/pp. 411-413)
7 March 1921
Boundaries of BINGHAM and BLAINE redefined [no change]. (Id. Laws 1921, 16th sess., ch. 210/pp. 420-423)
13 March 1923
ADA boundary with ELMORE redefined to run along range lines rather than from mouth of Bruneau River to mouth of Mores Creek [no mappable change]. (Id. Laws 1923, 17th sess., ch. 136/pp. 200-202)
10 February 1925
ELMORE redefined [no change]. (Id. Laws 1925, 18th sess., ch. 26/pp. 38-39)
13 March 1925
BLAINE exchanged with CUSTER. (Id. Laws 1925, 18th sess., ch. 189/pp. 345-348)
15 March 1927
BANNOCK gained from ONEIDA; exchanged with POWER. (Id. Laws 1927, 19th sess., ch. 256/pp. 431-439)
4 November 1930
ELMORE gained from OWYHEE. Act approved 21 Feb 1929 and became effective through a vote in the general election of 4 November 1930. (Id. Laws 1929, 20th sess., ch. 47/pp. 59-66; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-122 "Elmore Co."/p.32)
27 February 1931
Legislature authorized BUTTE to annex territory from CUSTER [Annexation 3 (proposed)], dependent on a vote in the general election of November 1932. No evidence this vote ever took place [no change]. (Id. Laws 1931, 21st sess., ch. 63/pp. 106-113; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-114 "Butte Co."/p. 21)
17 March 1931
Legislature authorized FRANKLIN to annex territory from BANNOCK [Annexation 4 (proposed)], dependent on a vote in the general election of November 1932. Proposed annexation failed in referendum [no change]. (Id. Laws 1931, 21st sess., ch. 63/pp. 106-113; Id. Code Ann. (1996), vol. 6, ch. 1, secs. 31-105 "Bannock Co." and 31-123 "Franklin Co/pp. 8, 33)
15 March 1937
BUTTE gained from CUSTER. (Id. Laws 1937, 21st sess., ch. 138/pp. 222-224)
5 November 1946
FRANKLIN gained from BANNOCK. Act approved 14 March 1945 and became effective through a vote in the general election of 5 November 1946. (Id. Laws 1945, 28th sess., ch. 145/pp. 216-224; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-105 "Bannock Co."/pp. 9-10)
19 March 1947
Legislature authorized FRANKLIN to annex territory from BANNOCK [Annexation 5 (proposed)], dependent on a vote in the general election of November 1948. Proposed annexation failed in referendum [no change]. (Id. Laws 1947, 29th sess., ch. 255/pp. 699-706; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-105 "Bannock Co."/p. 8)
2 November 1948
CARIBOU gained from BANNOCK. Areas of gain described in two separate acts approved 18 Mar 1947 and became effective through a vote in the general election of 2 November 1948. (Id. Laws 1947, 29th sess., ch. 247/pp. 638-648; ch. 248/pp. 648-657; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-105 "Bannock Co."/pp. 9-10)
ELMORE gained from ADA. Act approved 10 Mar 1947 and became effective through a vote in the general election of 2 November 1948. (Id. Laws 1947, 29th sess., ch. 150/pp. 352-359; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-103 "Ada Co."/p. 4)
26 February 1949
Legislature authorized BENEWAH to annex territory from KOOTENAI [Annexation 6 (proposed)], dependent on a vote in the general election of November 1950. The election was never held and a writ of mandate was refused by the district court [no change]. (Id. Laws 1949, 30th sess., ch. 92/pp. 162-168; Id. Code Ann. (1996), vol. 6, ch. 1, sec. 31-197 "Benewah Co."/p. 13)
27 March 1978
Boundaries of LATAH and NEZ PERCE clarified [no change]. (Id. Laws 1978, 44th leg., 2d reg. sess., ch. 248/pp. 543-545)
13 March 1985
CAMAS boundaries clarified [no change]. (Id. Laws 1985, 48th leg., 1st. reg. sess., ch. 112/pp. 218-219)
3 March 1999
OWYHEE boundaries clarified [no change]. (Id. Laws 1999, 55th leg., 1st reg. sess., ch. 35/pp. 72-73)