Stillwater County, Montana
Shrink-swell clay limits houses without basements on 26% of Stillwater County's mapped land, a higher share than 76% of US counties. A shallow water table limits basements on 5%.
The soil survey maps 87% of the county's land area; these shares describe the mapped part. 40.3% of the county is in drought on the October 13, 2026 Drought Monitor map, down from 49.3% a week earlier. FEMA's flood data list 35 flood losses here since 2016, 16% of them in buildings with basements. 43% of homes were built before 1980, against 50% nationally; the median home dates from 1986.
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 Stillwater County
| Expansive clay (limits slab homes) | 26.1% |
| High water table (limits basements) | 4.7% |
| Septic fields very limited | 82.5% |
| Clay swell (LEP, by area) | 4.6% |
| High-swell land (LEP 6%+) | 26.1% |
| Mapped soil | 996,180 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Tanna | 16% | n/a |
| Lambeth | 9% | n/a |
| Absarokee | 7% | n/a |
| Yawdim | 7% | n/a |
| Rentsac | 6% | n/a |
| Castner | 4% | n/a |
How we count · survey areas MT640 (2025-09-02), MT655 (2025-09-03)
What the numbers mean here
Clay swell averages a linear extensibility of 4.6% by land area here, and 26% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 26% of the mapped land.
Clay shrinks as it dries: 40.3% of the county is in drought on the latest map, and 26% of Stillwater County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 16.53 in, 1.85 in below the 1991-2020 average year of 18.38 in.
USDA rates 83% of the land very limited for septic absorption fields.
Drought
40.3% 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 100% of its area, in 18 weeks, most recently on the September 22, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 40.3% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
35 flood losses since 2016, $1.3 million paid. The worst year was 2022, with 33. Foundations of the buildings, where recorded: 16% basements, 32% slabs, 42% crawl spaces, 10% elevated.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 1 |
| 2018 | 1 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 0 |
| 2022 | 33 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 18.38 in |
| 12 months to August 2026 | 16.53 in (-1.85) |
| 2025 | 21.55 in (+3.17) |
| Wettest year since 1895 | 2019: 25.75 in |
| Driest year since 1895 | 2012: 10.99 in |
| Days with 1 inch or more, normal at COLUMBUS (6 mi away) | 1.5 |
Homes
| Housing units | 4,657 |
| Median year built | 1986 |
| Built before 1980 | 43% |
| Built 2000 or later | 31% |
| Owner-occupied | 83% |
| Median value, owner-occupied | $405,800 |
| Single-family detached | 84% |
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 |
|---|---|---|---|
| Sweet Grass County, MT | 29 mi | 16% | 5% |
| Carbon County, MT | 35 mi | 30% | 2% |
| Golden Valley County, MT | 51 mi | 26% | 2% |
| Yellowstone County, MT | 57 mi | 23% | 3% |
| Park County, MT | 57 mi | 11% | 4% |
| Wheatland County, MT | 62 mi | n/a | n/a |
| Musselshell County, MT | 74 mi | 24% | 1% |
| Park County, WY | 81 mi | 12% | 9% |
Sources: USDA SSURGO (September 3, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status