Mahaska County, Iowa
A shallow water table limits basements on 88% of Mahaska County's mapped land, a higher share than 95% of US counties and the 8th highest of 99 Iowa counties. Shrink-swell clay limits houses without basements on 71%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list no flood losses here since 2016. 67% of homes were built before 1980, against 50% nationally; the median home dates from 1965.
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 Mahaska County
| Expansive clay (limits slab homes) | 70.9% |
| High water table (limits basements) | 88.0% |
| Septic fields very limited | 97.9% |
| Clay swell (LEP, by area) | 6.1% |
| High-swell land (LEP 6%+) | 63.1% |
| Mapped soil | 365,800 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Ladoga | 14% | n/a |
| Otley | 13% | n/a |
| Clinton | 8% | n/a |
| Mahaska | 8% | n/a |
| Colo | 6% | n/a |
| Taintor | 4% | n/a |
How we count · survey areas IA123 (2025-09-08)
What the numbers mean here
Clay swell averages a linear extensibility of 6.1% by land area here, and 63% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 71% of the mapped land.
On 88% of Mahaska 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: 32.79 in, 3.82 in below the 1991-2020 average year of 36.61 in.
USDA rates 98% 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 9 of the last 52 weeks; the most was 88.2% of its area, in 5 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
FEMA's flood data list no flood losses in Mahaska County since 2016.
Rain
| Average year, 1991-2020 | 36.61 in |
| 12 months to August 2026 | 32.79 in (-3.82) |
| 2025 | 30.58 in (-6.03) |
| Wettest year since 1895 | 2010: 53.45 in |
| Driest year since 1895 | 1988: 18.23 in |
| Days with 1 inch or more, normal at OSKALOOSA (1 mi away) | 9.8 |
Homes
| Housing units | 9,728 |
| Median year built | 1965 |
| Built before 1980 | 67% |
| Built 2000 or later | 14% |
| Owner-occupied | 68% |
| Median value, owner-occupied | $157,800 |
| Single-family detached | 78% |
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 |
|---|---|---|---|
| Marion County, IA | 24 mi | 71% | 77% |
| Wapello County, IA | 24 mi | 70% | 85% |
| Monroe County, IA | 24 mi | 72% | 77% |
| Keokuk County, IA | 24 mi | 71% | 87% |
| Poweshiek County, IA | 25 mi | 53% | 62% |
| Jasper County, IA | 33 mi | 40% | 55% |
| Iowa County, IA | 39 mi | 57% | 69% |
| Jefferson County, IA | 41 mi | 71% | 87% |
Sources: USDA SSURGO (September 8, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status