Jefferson County, Illinois
A shallow water table limits basements on 87% of Jefferson County's mapped land, a higher share than 93% of US counties. Shrink-swell clay limits houses without basements on 26%.
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 7 flood losses here since 2016. 55% of homes were built before 1980, against 50% nationally; the median home dates from 1977.
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) | 25.5% |
| High water table (limits basements) | 86.6% |
| Septic fields very limited | 91.5% |
| Clay swell (LEP, by area) | 4.5% |
| High-swell land (LEP 6%+) | 36.3% |
| Mapped soil | 373,520 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Bluford | 16% | 100% |
| Ava | 11% | n/a |
| Belknap | 9% | n/a |
| Grantsburg | 9% | n/a |
| Hoyleton | 8% | n/a |
| Wynoose | 7% | n/a |
How we count · survey areas IL081 (2025-08-31)
What the numbers mean here
Clay swell averages a linear extensibility of 4.5% by land area here, and 36% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 26% of the mapped land.
Clay shrinks as it dries: 100% of the county is in drought on the latest map, and 26% of Jefferson County's mapped land has shrink-swell clay.
On 87% of Jefferson 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: 32.04 in, 13.61 in below the 1991-2020 average year of 45.65 in.
USDA rates 92% 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 25 of the last 52 weeks; the most was 100% of its area, in 11 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
7 flood losses since 2016, $190,898 paid. The worst year was 2018, with 5.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 0 |
| 2018 | 5 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 2 |
| 2022 | 0 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 45.65 in |
| 12 months to August 2026 | 32.04 in (-13.61) |
| 2025 | 38.15 in (-7.50) |
| Wettest year since 1895 | 2011: 63.79 in |
| Driest year since 1895 | 1953: 26.96 in |
| Days with 1 inch or more, normal at MT VERNON 3 NE (5 mi away) | 12.3 |
Homes
| Housing units | 16,629 |
| Median year built | 1977 |
| Built before 1980 | 55% |
| Built 2000 or later | 17% |
| Owner-occupied | 72% |
| Median value, owner-occupied | $126,000 |
| Single-family detached | 69% |
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 | 21 mi | 31% | 88% |
| Marion County, IL | 24 mi | 40% | 89% |
| Hamilton County, IL | 26 mi | 23% | 95% |
| Washington County, IL | 27 mi | 36% | 91% |
| Wayne County, IL | 28 mi | 26% | 94% |
| Perry County, IL | 28 mi | 24% | 75% |
| Clinton County, IL | 34 mi | 44% | 88% |
| Clay County, IL | 39 mi | 44% | 91% |
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