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Johnson County, Illinois

A shallow water table limits basements on 78% of Johnson County's mapped land, a higher share than 87% of US counties. Shrink-swell clay limits houses without basements on 2%.

85.1% of the county is in drought on the October 13, 2026 Drought Monitor map, up from 29.9% a week earlier. FEMA's flood data list 1 flood losses here since 2016. 44% of homes were built before 1980, against 50% nationally; the median home dates from 1985.

Data as of October 13, 2026 · sources

2%

expansive clay: shrink-swell limits slab homes

78%

high water table: limits basements

85%

in drought, October 13, 2026

44%

of homes built before 1980

Soil under Johnson County

Expansive clay (limits slab homes)1.7%
High water table (limits basements)78.2%
Septic fields very limited94.9%
Clay swell (LEP, by area)3.3%
High-swell land (LEP 6%+)2.4%
Mapped soil223,145 acres
Soil seriesShare hereSeries nationally: high water table
Hosmer29%over 99%
Grantsburg17%n/a
Zanesville11%97%
Wellston9%under 1%
Berks7%under 1%
Belknap7%n/a
Williamson County: 74%Pope County: 67%Massac County: 79%Union County: 49%Pulaski County: 72%Saline County: 86%Ballard County: 67%McCracken County: 73%Johnson County WilliamsonPopeMassacUnionPulaskiSalineBallardMcCrackenJohnson
Johnson County (heavy outline) and nearby counties, shaded by high water table share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas IL087 (2025-08-31)

What the numbers mean here

On 78% of Johnson 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: 41.05 in, 8.68 in below the 1991-2020 average year of 49.73 in.

USDA rates 95% of the land very limited for septic absorption fields.

Drought

85.1% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 26 of the last 52 weeks; the most was 100% of its area, in 3 weeks, most recently on the April 21, 2026 map.

October 21, 2025Share of area in drought, weeklyOctober 13, 2026
Category, October 13, 2026Share of county
D0 abnormally dry or worse100%
D1 moderate drought or worse85.1%
D2 severe or worse0%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

1 flood losses since 2016, $7,697 paid. The worst year was 2023, with 1.

YearFlood losses
20160
20170
20180
20190
20200
20210
20220
20231
20240
20250
2026 to date0

Rain

Average year, 1991-202049.73 in
12 months to August 202641.05 in (-8.68)
202547.04 in (-2.69)
Wettest year since 18952011: 70.88 in
Driest year since 18951901: 30.97 in
Days with 1 inch or more, normal at GRAND CHAIN DAM 53 (20 mi away)15.1

Homes

Housing units5,541
Median year built1985
Built before 198044%
Built 2000 or later29%
Owner-occupied82%
Median value, owner-occupied$160,500
Single-family detached66%

What repairs cost

Published ranges from cost guides, not quotes. More sources, and where they disagree.

WorkRangeSource
Basement waterproofing, US $3,000 to $10,000 Fixr, February 6, 2026
Sump pump installation, US $800 to $2,000 Fixr, January 31, 2025
Crawl space encapsulation, US $2,000 to $8,000 Fixr, August 29, 2025

Nearby counties

PlaceDistanceExpansive clayHigh water table
Williamson County, IL 19 mi 16% 74%
Pope County, IL 19 mi 2% 67%
Massac County, IL 20 mi 8% 79%
Union County, IL 20 mi 10% 49%
Pulaski County, IL 22 mi 10% 72%
Saline County, IL 27 mi 20% 86%
Ballard County, KY 29 mi 2% 67%
McCracken County, KY 30 mi 6% 73%

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