Knox County, Illinois
Shrink-swell clay limits houses without basements on 41% of Knox County's mapped land, a higher share than 87% of US counties. A shallow water table limits basements on 41%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 2 flood losses here since 2016. 81% of homes were built before 1980, against 50% nationally; the median home dates from 1957.
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 Knox County
| Expansive clay (limits slab homes) | 41.2% |
| High water table (limits basements) | 40.9% |
| Septic fields very limited | 54.4% |
| Clay swell (LEP, by area) | 4.6% |
| High-swell land (LEP 6%+) | 22.2% |
| Mapped soil | 461,675 acres |
How we count · survey areas IL095 (2025-08-31)
What the numbers mean here
Clay swell averages a linear extensibility of 4.6% by land area here, and 22% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 41% of the mapped land.
On 41% of Knox 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: 38.99 in, 0.62 in above the 1991-2020 average year of 38.37 in.
USDA rates 54% of the land very limited for septic absorption fields.
Drought
0% 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 19 weeks, most recently on the March 31, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
2 flood losses since 2016, $0 paid. The worst year was 2018, with 1.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 1 |
| 2018 | 1 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 0 |
| 2022 | 0 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 38.37 in |
| 12 months to August 2026 | 38.99 in (+0.62) |
| 2025 | 30.07 in (-8.30) |
| Wettest year since 1895 | 2009: 53.76 in |
| Driest year since 1895 | 1901: 21.94 in |
| Days with 1 inch or more, normal at GALESBURG (9 mi away) | 9.9 |
Homes
| Housing units | 23,758 |
| Median year built | 1957 |
| Built before 1980 | 81% |
| Built 2000 or later | 8% |
| Owner-occupied | 71% |
| Median value, owner-occupied | $111,900 |
| Single-family detached | 77% |
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 |
|---|---|---|---|
| Warren County, IL | 22 mi | 35% | 54% |
| Stark County, IL | 25 mi | 36% | 52% |
| Peoria County, IL | 25 mi | 29% | 37% |
| Henry County, IL | 29 mi | 19% | 48% |
| Fulton County, IL | 32 mi | 24% | 38% |
| Mercer County, IL | 33 mi | 15% | 44% |
| Henderson County, IL | 39 mi | 13% | 43% |
| McDonough County, IL | 41 mi | 63% | 70% |
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