Cassia County, Idaho
USDA's soil survey flags shrink-swell clay on 3% of Cassia County's mapped land and a shallow water table under basements on 2%, both below the national shares of 15% and 29%. The soil survey maps 76% of the county's land area; these shares describe the mapped part.
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. 61% of homes were built before 1980, against 50% nationally; the median home dates from 1975.
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 Cassia County
| Expansive clay (limits slab homes) | 3.1% |
| High water table (limits basements) | 2.4% |
| Septic fields very limited | 63.7% |
| Clay swell (LEP, by area) | 2.3% |
| High-swell land (LEP 6%+) | 2.5% |
| Mapped soil | 1,243,696 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Declo | 7% | n/a |
| Darkbull | 6% | n/a |
| Portneuf | 4% | n/a |
| Heglar | 4% | n/a |
| Bahem | 3% | n/a |
| Trevino | 3% | n/a |
How we count · survey areas ID707 (2025-08-27), ID708 (2025-08-27), ID721 (2025-08-27)
What the numbers mean here
Rain over the 12 months to August 2026: 12.35 in, 3.87 in below the 1991-2020 average year of 16.22 in.
USDA rates 64% of the land very limited for septic absorption fields.
With shrink-swell clay on 3% and a shallow water table on 2% of the land, the soil survey points to neither problem across most of Cassia County; conditions still vary lot by lot.
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 | 100% |
| D3 extreme or worse | 100% |
| D4 exceptional | 17.1% |
Flood losses since 2016
FEMA's flood data list no flood losses in Cassia County since 2016.
Rain
| Average year, 1991-2020 | 16.22 in |
| 12 months to August 2026 | 12.35 in (-3.87) |
| 2025 | 13.49 in (-2.73) |
| Wettest year since 1895 | 1907: 25.24 in |
| Driest year since 1895 | 1939: 7.93 in |
| Days with 1 inch or more, normal at OAKLEY (14 mi away) | 0.4 |
Homes
| Housing units | 9,096 |
| Median year built | 1975 |
| Built before 1980 | 61% |
| Built 2000 or later | 18% |
| Owner-occupied | 69% |
| Median value, owner-occupied | $284,600 |
| Single-family detached | 79% |
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 |
|---|---|---|---|
| Minidoka County, ID | 40 mi | 1% | 7% |
| Jerome County, ID | 43 mi | under 1% | under 1% |
| Power County, ID | 49 mi | 1% | 2% |
| Twin Falls County, ID | 53 mi | 13% | 1% |
| Box Elder County, UT | 54 mi | 2% | 23% |
| Lincoln County, ID | 56 mi | 16% | 1% |
| Oneida County, ID | 57 mi | 5% | 4% |
| Bannock County, ID | 77 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