Custer County, Montana
Shrink-swell clay limits houses without basements on 19% of Custer County's mapped land, a higher share than 69% of US counties. A shallow water table limits basements on 1%.
86.7% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 6 flood losses here since 2016. 75% of homes were built before 1980, against 50% nationally; the median home dates from 1965.
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 Custer County
| Expansive clay (limits slab homes) | 19.3% |
| High water table (limits basements) | 0.7% |
| Septic fields very limited | 74.9% |
| Clay swell (LEP, by area) | 4.2% |
| High-swell land (LEP 6%+) | 24.8% |
| Mapped soil | 2,502,720 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Cabbart | 10% | under 1% |
| Sonnett | 9% | n/a |
| Cambeth | 8% | n/a |
| Yawdim | 6% | n/a |
| Yamacall | 5% | n/a |
| Lonna | 5% | n/a |
How we count · survey areas MT017 (2025-08-27)
What the numbers mean here
Clay swell averages a linear extensibility of 4.2% by land area here, and 25% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 19% of the mapped land.
Clay shrinks as it dries: 86.7% of the county is in drought on the latest map, and 19% of Custer County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 12.70 in, 1.76 in below the 1991-2020 average year of 14.46 in.
USDA rates 75% of the land very limited for septic absorption fields.
Drought
86.7% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 35 of the last 52 weeks; the most was 100% of its area, in 13 weeks, most recently on the August 11, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 86.7% |
| D2 severe or worse | 57.6% |
| D3 extreme or worse | 19.9% |
| D4 exceptional | 0% |
Flood losses since 2016
6 flood losses since 2016, $6,371 paid. The worst year was 2018, with 4.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 0 |
| 2018 | 4 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 1 |
| 2022 | 0 |
| 2023 | 1 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 14.46 in |
| 12 months to August 2026 | 12.70 in (-1.76) |
| 2025 | 17.18 in (+2.72) |
| Wettest year since 1895 | 2011: 20.73 in |
| Driest year since 1895 | 1934: 6.04 in |
| Days with 1 inch or more, normal at MIZPAH 4 NNW (13 mi away) | 1.7 |
Homes
| Housing units | 5,784 |
| Median year built | 1965 |
| Built before 1980 | 75% |
| Built 2000 or later | 13% |
| Owner-occupied | 70% |
| Median value, owner-occupied | $219,100 |
| Single-family detached | 70% |
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 |
|---|---|---|---|
| Prairie County, MT | 37 mi | 10% | 1% |
| Rosebud County, MT | 53 mi | 25% | under 1% |
| Fallon County, MT | 55 mi | 21% | 1% |
| Powder River County, MT | 60 mi | 30% | under 1% |
| Carter County, MT | 73 mi | 52% | 1% |
| Dawson County, MT | 75 mi | 8% | 1% |
| Wibaux County, MT | 77 mi | 22% | 2% |
| Treasure County, MT | 82 mi | 44% | 4% |
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