Fallon County, Montana
Shrink-swell clay limits houses without basements on 21% of Fallon County's mapped land, a higher share than 71% of US counties. A shallow water table limits basements on 1%.
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. 47% of homes were built before 1980, against 50% nationally; the median home dates from 1982.
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 Fallon County
| Expansive clay (limits slab homes) | 21.1% |
| High water table (limits basements) | 0.7% |
| Septic fields very limited | 63.1% |
| Clay swell (LEP, by area) | 4.0% |
| High-swell land (LEP 6%+) | 21.1% |
| Mapped soil | 1,007,980 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Cabbart | 13% | under 1% |
| Yamacall | 6% | n/a |
| Delpoint | 5% | n/a |
| Cambeth | 5% | n/a |
| Neldore | 5% | n/a |
| Lonna | 4% | n/a |
How we count · survey areas MT025 (2025-08-28)
What the numbers mean here
Clay swell averages a linear extensibility of 4.0% by land area here, and 21% 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 Fallon County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 13.37 in, 2.15 in below the 1991-2020 average year of 15.52 in.
USDA rates 63% 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 22 of the last 52 weeks; the most was 100% of its area, in 18 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 | 70.2% |
| D4 exceptional | 0% |
Flood losses since 2016
FEMA's flood data list no flood losses in Fallon County since 2016.
Rain
| Average year, 1991-2020 | 15.52 in |
| 12 months to August 2026 | 13.37 in (-2.15) |
| 2025 | 19.40 in (+3.88) |
| Wettest year since 1895 | 1982: 23.70 in |
| Driest year since 1895 | 1988: 6.57 in |
| Days with 1 inch or more, normal at PLEVNA (9 mi away) | 2.1 |
Homes
| Housing units | 1,483 |
| Median year built | 1982 |
| Built before 1980 | 47% |
| Built 2000 or later | 28% |
| Owner-occupied | 69% |
| Median value, owner-occupied | $244,800 |
| Single-family detached | 68% |
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 |
|---|---|---|---|
| Wibaux County, MT | 45 mi | 22% | 2% |
| Bowman County, ND | 46 mi | 32% | 4% |
| Slope County, ND | 46 mi | 23% | 6% |
| Golden Valley County, ND | 51 mi | 7% | 3% |
| Custer County, MT | 55 mi | 19% | 1% |
| Carter County, MT | 56 mi | 52% | 1% |
| Prairie County, MT | 62 mi | 10% | 1% |
| Harding County, SD | 67 mi | 13% | 2% |
Sources: USDA SSURGO (August 28, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status