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Sully County, South Dakota

Shrink-swell clay limits houses without basements on 23% of Sully County's mapped land, a higher share than 73% of US counties. A shallow water table limits basements on 9%.

90.7% 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. 51% of homes were built before 1980, against 50% nationally; the median home dates from 1979.

Data as of October 13, 2026 · sources

23%

expansive clay: shrink-swell limits slab homes

9%

high water table: limits basements

91%

in drought, October 13, 2026

51%

of homes built before 1980

Soil under Sully County

Expansive clay (limits slab homes)22.8%
High water table (limits basements)9.2%
Septic fields very limited70.7%
Clay swell (LEP, by area)6.4%
High-swell land (LEP 6%+)29.2%
Mapped soil683,520 acres
Soil seriesShare hereSeries nationally: expansive clay
Agar19%n/a
Highmore14%n/a
Mobridge12%n/a
Eakin6%n/a
Raber6%n/a
Hoven6%n/a
Potter County: 25%Hughes County: 34%Hyde County: 41%Stanley County: 89%Dewey County: 70%Walworth County: 26%Faulk County: 20%Hand County: 27%Sully County PotterHughesHydeStanleyDeweyWalworthFaulkHandSully
Sully County (heavy outline) and nearby counties, shaded by expansive clay share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas SD119 (2025-09-10)

What the numbers mean here

Clay swell averages a linear extensibility of 6.4% by land area here, and 29% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 23% of the mapped land.

Clay shrinks as it dries: 90.7% of the county is in drought on the latest map, and 23% of Sully County's mapped land has shrink-swell clay.

Rain over the 12 months to August 2026: 17.20 in, 2.07 in below the 1991-2020 average year of 19.27 in.

USDA rates 71% of the land very limited for septic absorption fields.

Drought

90.7% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 18 of the last 52 weeks; the most was 90.7% of its area, in 12 weeks, most recently on the October 13, 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 worse90.7%
D2 severe or worse45%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

FEMA's flood data list no flood losses in Sully County since 2016.

Rain

Average year, 1991-202019.27 in
12 months to August 202617.20 in (-2.07)
202520.20 in (+0.93)
Wettest year since 18952019: 28.13 in
Driest year since 18951976: 7.79 in
Days with 1 inch or more, normal at ONIDA 4 NW (1 mi away)3.9

Homes

Housing units934
Median year built1979
Built before 198051%
Built 2000 or later27%
Owner-occupied73%
Median value, owner-occupied$205,400
Single-family detached76%

What repairs cost

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

WorkRangeSource
Foundation repair, US $2,200 to $8,200 Fixr, January 27, 2026
Pier-and-beam repair, US $2,000 to $15,000 Fixr, January 27, 2026
Foundation inspection, US $300 to $600 Fixr, January 31, 2025

Nearby counties

PlaceDistanceExpansive clayHigh water table
Potter County, SD 24 mi 25% 11%
Hughes County, SD 25 mi 34% 8%
Hyde County, SD 35 mi 41% 13%
Stanley County, SD 37 mi 89% 6%
Dewey County, SD 45 mi 70% 4%
Walworth County, SD 49 mi 26% 10%
Faulk County, SD 53 mi 20% 23%
Hand County, SD 57 mi 27% 16%

Sources: USDA SSURGO (September 10, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status