Jefferson County, Idaho
Shrink-swell clay limits houses without basements on 21% of Jefferson County's mapped land, a higher share than 72% of US counties and the 5th highest of 43 Idaho counties. A shallow water table limits basements on 8%.
100% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list no flood losses here since 2016. 41% of homes were built before 1980, against 50% nationally; the median home dates from 1994.
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 Jefferson County
| Expansive clay (limits slab homes) | 21.5% |
| High water table (limits basements) | 8.3% |
| Septic fields very limited | 81.1% |
| Clay swell (LEP, by area) | 3.7% |
| High-swell land (LEP 6%+) | 23.1% |
| Mapped soil | 707,603 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Terreton | 16% | n/a |
| Pancheri | 9% | n/a |
| Matheson | 6% | n/a |
| Grassy Butte | 6% | n/a |
| Bereniceton | 6% | n/a |
| Malm | 5% | n/a |
How we count · survey areas ID765 (2025-08-27)
What the numbers mean here
Clay swell averages a linear extensibility of 3.7% 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 21% of the mapped land.
Clay shrinks as it dries: 100% of the county is in drought on the latest map, and 21% of Jefferson County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 7.70 in, 2.07 in below the 1991-2020 average year of 9.77 in.
USDA rates 81% of the land very limited for septic absorption fields.
Drought
100% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 52 of the last 52 weeks; the most was 100% of its area, in 52 weeks, most recently on the October 13, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 100% |
| D2 severe or worse | 94.7% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
FEMA's flood data list no flood losses in Jefferson County since 2016.
Rain
| Average year, 1991-2020 | 9.77 in |
| 12 months to August 2026 | 7.70 in (-2.07) |
| 2025 | 8.21 in (-1.56) |
| Wettest year since 1895 | 1983: 15.74 in |
| Driest year since 1895 | 1926: 5.38 in |
| Days with 1 inch or more, normal at HAMER 4 NW (12 mi away) | 0.2 |
Homes
| Housing units | 10,779 |
| Median year built | 1994 |
| Built before 1980 | 41% |
| Built 2000 or later | 44% |
| Owner-occupied | 83% |
| Median value, owner-occupied | $408,400 |
| Single-family detached | 85% |
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 |
|---|---|---|---|
| Madison County, ID | 33 mi | under 1% | 14% |
| Clark County, ID | 34 mi | 4% | 4% |
| Bingham County, ID | 40 mi | under 1% | 2% |
| Butte County, ID | 44 mi | 1% | 2% |
| Bonneville County, ID | 45 mi | 11% | 4% |
| Fremont County, ID | 51 mi | 1% | 6% |
| Teton County, ID | 55 mi | 5% | 12% |
| Bannock County, ID | 76 mi | n/a | n/a |
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