St. Clair County, Illinois
A shallow water table limits basements on 73% of St. Clair County's mapped land, a higher share than 83% of US counties. Shrink-swell clay limits houses without basements on 28%.
9.2% 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 298 flood losses here since 2016, 37% of them in buildings with basements. 55% of homes were built before 1980, against 50% nationally; the median home dates from 1975.
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 St. Clair County
| Expansive clay (limits slab homes) | 27.7% |
| High water table (limits basements) | 72.7% |
| Septic fields very limited | 83.4% |
| Clay swell (LEP, by area) | 5.2% |
| High-swell land (LEP 6%+) | 31.3% |
| Urban land, not rated | 3.0% |
| Mapped soil | 431,330 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Menfro | 10% | 0% |
| Wakeland | 7% | n/a |
| Marine | 6% | n/a |
| Winfield | 4% | n/a |
| Homen | 4% | n/a |
| Herrick | 4% | n/a |
How we count · survey areas IL163 (2025-08-31)
What the numbers mean here
Clay swell averages a linear extensibility of 5.2% by land area here, and 31% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 28% of the mapped land.
Clay shrinks as it dries: 9.2% of the county is in drought on the latest map, and 28% of St. Clair County's mapped land has shrink-swell clay.
On 73% of St. Clair 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: 33.92 in, 8.59 in below the 1991-2020 average year of 42.51 in.
USDA rates 83% of the land very limited for septic absorption fields.
Drought
9.2% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 18 of the last 52 weeks; the most was 100% of its area, in 8 weeks, most recently on the February 10, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 9.2% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
298 flood losses since 2016, $3.7 million paid. The worst year was 2024, with 114. Foundations of the buildings, where recorded: 37% basements, 42% slabs, 10% crawl spaces, 12% elevated. Median water depth outside the building: 10 in.
| Year | Flood losses |
|---|---|
| 2016 | 13 |
| 2017 | 13 |
| 2018 | 15 |
| 2019 | 7 |
| 2020 | 7 |
| 2021 | 1 |
| 2022 | 98 |
| 2023 | 13 |
| 2024 | 114 |
| 2025 | 14 |
| 2026 to date | 3 |
Rain
| Average year, 1991-2020 | 42.51 in |
| 12 months to August 2026 | 33.92 in (-8.59) |
| 2025 | 35.83 in (-6.68) |
| Wettest year since 1895 | 1898: 54.94 in |
| Driest year since 1895 | 1953: 22.89 in |
| Days with 1 inch or more, normal at BELLEVILLE SIU RSCH (6 mi away) | 12.7 |
Homes
| Housing units | 115,630 |
| Median year built | 1975 |
| Built before 1980 | 55% |
| Built 2000 or later | 23% |
| Owner-occupied | 68% |
| Median value, owner-occupied | $180,300 |
| Single-family detached | 72% |
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 |
|---|---|---|---|
| Monroe County, IL | 19 mi | 20% | 59% |
| St. Louis city, MO | 21 mi | 1% | 32% |
| Madison County, IL | 25 mi | 33% | 73% |
| Clinton County, IL | 29 mi | 44% | 88% |
| Washington County, IL | 29 mi | 36% | 91% |
| Randolph County, IL | 29 mi | 24% | 64% |
| St. Louis County, MO | 30 mi | 5% | 37% |
| Jefferson County, MO | 36 mi | 37% | 34% |
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