Williamson County, Illinois
A shallow water table limits basements on 74% of Williamson County's mapped land, a higher share than 84% of US counties. Shrink-swell clay limits houses without basements on 16%.
100% of the county is in drought on the October 13, 2026 Drought Monitor map, up from 0% a week earlier. FEMA's flood data list 29 flood losses here since 2016. 49% of homes were built before 1980, against 50% nationally; the median home dates from 1981.
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 Williamson County
| Expansive clay (limits slab homes) | 16.1% |
| High water table (limits basements) | 73.6% |
| Septic fields very limited | 85.7% |
| Clay swell (LEP, by area) | 3.8% |
| High-swell land (LEP 6%+) | 19.9% |
| Mapped soil | 284,365 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Ava | 13% | n/a |
| Belknap | 10% | n/a |
| Bluford | 10% | 100% |
| Rend | 7% | n/a |
| Hickory | 6% | n/a |
| Plumfield | 5% | n/a |
How we count · survey areas IL199 (2025-08-31)
What the numbers mean here
Clay swell averages a linear extensibility of 3.8% by land area here, and 20% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 16% of the mapped land.
Clay shrinks as it dries: 100% of the county is in drought on the latest map, and 16% of Williamson County's mapped land has shrink-swell clay.
On 74% of Williamson 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: 34.82 in, 13.46 in below the 1991-2020 average year of 48.28 in.
USDA rates 86% 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 27 of the last 52 weeks; the most was 100% of its area, in 6 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 | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
29 flood losses since 2016, $391,505 paid. The worst year was 2017, with 20.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 20 |
| 2018 | 1 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 1 |
| 2022 | 1 |
| 2023 | 3 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 3 |
Rain
| Average year, 1991-2020 | 48.28 in |
| 12 months to August 2026 | 34.82 in (-13.46) |
| 2025 | 40.43 in (-7.85) |
| Wettest year since 1895 | 2011: 69.56 in |
| Driest year since 1895 | 1901: 27.70 in |
| Days with 1 inch or more, normal at W FRANKFORT (12 mi away) | 12.8 |
Homes
| Housing units | 32,084 |
| Median year built | 1981 |
| Built before 1980 | 49% |
| Built 2000 or later | 28% |
| Owner-occupied | 71% |
| Median value, owner-occupied | $156,400 |
| Single-family detached | 75% |
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 |
|---|---|---|---|
| Franklin County, IL | 18 mi | 31% | 88% |
| Johnson County, IL | 19 mi | 2% | 78% |
| Saline County, IL | 21 mi | 20% | 86% |
| Union County, IL | 24 mi | 10% | 49% |
| Jackson County, IL | 25 mi | 26% | 62% |
| Pope County, IL | 30 mi | 2% | 67% |
| Hamilton County, IL | 32 mi | 23% | 95% |
| Perry County, IL | 34 mi | 24% | 75% |
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