Montgomery County, Illinois
A shallow water table limits basements on 88% of Montgomery County's mapped land, a higher share than 94% of US counties. Shrink-swell clay limits houses without basements on 68%.
21.1% 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. 71% of homes were built before 1980, against 50% nationally; the median home dates from 1964.
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 Montgomery County
| Expansive clay (limits slab homes) | 68.5% |
| High water table (limits basements) | 87.9% |
| Septic fields very limited | 92.4% |
| Clay swell (LEP, by area) | 5.9% |
| High-swell land (LEP 6%+) | 66.1% |
| Mapped soil | 454,265 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Herrick | 16% | n/a |
| Virden | 11% | n/a |
| Oconee | 8% | n/a |
| Hickory | 8% | n/a |
| Piasa | 7% | n/a |
| Cowden | 7% | n/a |
How we count · survey areas IL135 (2025-08-31)
What the numbers mean here
Clay swell averages a linear extensibility of 5.9% by land area here, and 66% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 68% of the mapped land.
Clay shrinks as it dries: 21.1% of the county is in drought on the latest map, and 68% of Montgomery County's mapped land has shrink-swell clay.
On 88% of Montgomery County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.
Rain over the 12 months to August 2026: 39.39 in, 1.60 in below the 1991-2020 average year of 40.99 in.
USDA rates 92% of the land very limited for septic absorption fields.
Drought
21.1% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 24 of the last 52 weeks; the most was 100% of its area, in 21 weeks, most recently on the March 10, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 21.1% |
| 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 Montgomery County since 2016.
Rain
| Average year, 1991-2020 | 40.99 in |
| 12 months to August 2026 | 39.39 in (-1.60) |
| 2025 | 31.82 in (-9.17) |
| Wettest year since 1895 | 2008: 53.24 in |
| Driest year since 1895 | 1914: 25.03 in |
| Days with 1 inch or more, normal at MT OLIVE 1 E (16 mi away) | 11.3 |
Homes
| Housing units | 12,495 |
| Median year built | 1964 |
| Built before 1980 | 71% |
| Built 2000 or later | 12% |
| Owner-occupied | 77% |
| Median value, owner-occupied | $112,300 |
| 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 |
|---|---|---|---|
| Bond County, IL | 24 mi | 47% | 86% |
| Macoupin County, IL | 24 mi | 62% | 78% |
| Christian County, IL | 24 mi | 72% | 86% |
| Fayette County, IL | 29 mi | 35% | 81% |
| Madison County, IL | 36 mi | 33% | 73% |
| Sangamon County, IL | 38 mi | 58% | 72% |
| Shelby County, IL | 38 mi | 38% | 82% |
| Clinton County, IL | 43 mi | 44% | 88% |
Sources: USDA SSURGO (August 31, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status