Jefferson County, Montana
USDA's soil survey flags shrink-swell clay on 1% of Jefferson County's mapped land and a shallow water table under basements on 7%, both below the national shares of 15% and 29%. None 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 1990.
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) | 1.3% |
| High water table (limits basements) | 7.0% |
| Septic fields very limited | 72.9% |
| Clay swell (LEP, by area) | 2.3% |
| High-swell land (LEP 6%+) | 1.3% |
| Mapped soil | 1,042,103 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Bronec | 4% | n/a |
| Amesha | 4% | n/a |
| Sappington | 2% | n/a |
| Crago | 1% | n/a |
| Rencot | 1% | n/a |
| Brocko | 1% | n/a |
How we count · survey areas MT627 (2025-08-31), MT631 (2025-08-30), MT635 (2025-08-31), MT670 (2025-09-03)
What the numbers mean here
Rain over the 12 months to August 2026: 17.87 in, 0.03 in above the 1991-2020 average year of 17.84 in.
USDA rates 73% of the land very limited for septic absorption fields.
With shrink-swell clay on 1% and a shallow water table on 7% of the land, the soil survey points to neither problem across most of Jefferson County; conditions still vary lot by lot.
Drought
0% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 46 of the last 52 weeks; the most was 100% of its area, in 9 weeks, most recently on the March 31, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| 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 | 17.84 in |
| 12 months to August 2026 | 17.87 in (+0.03) |
| 2025 | 18.78 in (+0.94) |
| Wettest year since 1895 | 1975: 29.56 in |
| Driest year since 1895 | 1935: 9.58 in |
| Days with 1 inch or more, normal at BOULDER (8 mi away) | 0.4 |
Homes
| Housing units | 5,407 |
| Median year built | 1990 |
| Built before 1980 | 41% |
| Built 2000 or later | 31% |
| Owner-occupied | 84% |
| Median value, owner-occupied | $446,900 |
| Single-family detached | 83% |
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 |
|---|---|---|---|
| Broadwater County, MT | 31 mi | 1% | 3% |
| Silver Bow County, MT | 33 mi | 2% | 5% |
| Deer Lodge County, MT | 52 mi | 6% | 10% |
| Madison County, MT | 55 mi | 3% | 4% |
| Gallatin County, MT | 60 mi | 9% | 9% |
| Meagher County, MT | 64 mi | n/a | n/a |
| Powell County, MT | 65 mi | 10% | 6% |
| Granite County, MT | 68 mi | 6% | 7% |
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