Gem County, Idaho
Shrink-swell clay limits houses without basements on 19% of Gem County's mapped land, a higher share than 68% of US counties and the 8th highest of 43 Idaho counties. A shallow water table limits basements on 9%.
The soil survey maps 83% of the county's land area; these shares describe the mapped part. 2.2% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 1 flood losses here since 2016. 48% of homes were built before 1980, against 50% nationally; the median home dates from 1983.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Gem County
| Expansive clay (limits slab homes) | 18.6% |
| High water table (limits basements) | 9.5% |
| Septic fields very limited | 92.3% |
| Clay swell (LEP, by area) | 3.8% |
| High-swell land (LEP 6%+) | 23.5% |
| Mapped soil | 298,429 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Gem | 8% | n/a |
| Gwin | 7% | n/a |
| Haw | 6% | n/a |
| Payette | 5% | n/a |
| Bakeoven | 5% | n/a |
| Lanktree | 5% | n/a |
How we count · survey areas ID660 (2025-08-27)
What the numbers mean here
Clay swell averages a linear extensibility of 3.8% by land area here, and 23% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 19% of the mapped land.
Clay shrinks as it dries: 2.2% of the county is in drought on the latest map, and 19% of Gem County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 19.94 in, 0.55 in above the 1991-2020 average year of 19.39 in.
USDA rates 92% of the land very limited for septic absorption fields.
Drought
2.2% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 37 of the last 52 weeks; the most was 41.8% of its area, in 8 weeks, most recently on the July 14, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 2.2% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
1 flood losses since 2016, $5,397 paid. The worst year was 2025, with 1.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 0 |
| 2018 | 0 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 0 |
| 2022 | 0 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 1 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 19.39 in |
| 12 months to August 2026 | 19.94 in (+0.55) |
| 2025 | 20.46 in (+1.07) |
| Wettest year since 1895 | 1983: 28.76 in |
| Driest year since 1895 | 1966: 11.29 in |
| Days with 1 inch or more, normal at EMMETT 2 E (15 mi away) | 0.3 |
Homes
| Housing units | 7,954 |
| Median year built | 1983 |
| Built before 1980 | 48% |
| Built 2000 or later | 32% |
| Owner-occupied | 76% |
| Median value, owner-occupied | $428,700 |
| Single-family detached | 80% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Payette County, ID | 18 mi | 4% | 9% |
| Washington County, ID | 33 mi | 33% | 3% |
| Canyon County, ID | 34 mi | 1% | 14% |
| Boise County, ID | 34 mi | 3% | 2% |
| Ada County, ID | 43 mi | 20% | 4% |
| Adams County, ID | 56 mi | 9% | 7% |
| Elmore County, ID | 65 mi | 7% | 2% |
| Valley County, ID | 67 mi | 1% | 14% |
Sources: USDA SSURGO (August 27, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status