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Mercer County, Illinois

A shallow water table limits basements on 44% of Mercer County's mapped land, a higher share than 61% of US counties. Shrink-swell clay limits houses without basements on 15%.

None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 1 flood losses here since 2016. 77% of homes were built before 1980, against 50% nationally; the median home dates from 1962.

Data as of October 13, 2026 · sources

15%

expansive clay: shrink-swell limits slab homes

44%

high water table: limits basements

0%

in drought, October 13, 2026

77%

of homes built before 1980

Soil under Mercer County

Expansive clay (limits slab homes)14.6%
High water table (limits basements)43.7%
Septic fields very limited58.5%
Clay swell (LEP, by area)3.9%
High-swell land (LEP 6%+)9.0%
Mapped soil363,610 acres
Soil seriesShare hereSeries nationally: high water table
Muscatune11%n/a
Hickory10%n/a
Osco8%3%
Fayette5%0%
Greenbush5%n/a
Rozetta5%n/a
Rock Island County: 32%Warren County: 54%Louisa County: 70%Muscatine County: 50%Henderson County: 43%Des Moines County: 80%Scott County: 41%Henry County: 48%Mercer County Rock IslandWarrenLouisaMuscatineHendersonDes MoinesScottHenryMercer
Mercer 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 IL131 (2025-08-31)

What the numbers mean here

On 44% of Mercer 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: 36.65 in, 1.19 in below the 1991-2020 average year of 37.84 in.

USDA rates 58% 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 19 of the last 52 weeks; the most was 100% of its area, in 14 weeks, most recently on the March 31, 2026 map.

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

Flood losses since 2016

1 flood losses since 2016, $3,737 paid. The worst year was 2020, with 1.

YearFlood losses
20160
20170
20180
20190
20201
20210
20220
20230
20240
20250
2026 to date0

Rain

Average year, 1991-202037.84 in
12 months to August 202636.65 in (-1.19)
202532.41 in (-5.43)
Wettest year since 18952009: 53.00 in
Driest year since 18951901: 20.00 in
Days with 1 inch or more, normal at ALEDO (1 mi away)10

Homes

Housing units7,285
Median year built1962
Built before 198077%
Built 2000 or later10%
Owner-occupied84%
Median value, owner-occupied$146,700
Single-family detached88%

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
Rock Island County, IL 20 mi 6% 32%
Warren County, IL 25 mi 35% 54%
Louisa County, IA 27 mi 38% 70%
Muscatine County, IA 27 mi 20% 50%
Henderson County, IL 29 mi 13% 43%
Des Moines County, IA 31 mi 57% 80%
Scott County, IA 31 mi 36% 41%
Henry County, IL 33 mi 19% 48%

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