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Thurston County, Washington
A shallow water table limits basements on 34% of Thurston County's mapped land, a higher share than 52% of US counties and the 10th highest of 39 Washington counties. Shrink-swell clay limits houses without basements on 3%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 71 flood losses here since 2016, 16% of them in buildings with basements. 34% of homes were built before 1980, against 50% nationally; the median home dates from 1991.
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 Thurston County
| Expansive clay (limits slab homes) | 3.5% |
| High water table (limits basements) | 33.9% |
| Septic fields very limited | 90.1% |
| Clay swell (LEP, by area) | 2.1% |
| High-swell land (LEP 6%+) | 3.4% |
| Mapped soil | 515,326 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Spanaway | 9% | n/a |
| Everett | 8% | n/a |
| Alderwood | 8% | n/a |
| Baumgard | 5% | n/a |
| Melbourne | 4% | n/a |
| Kapowsin | 4% | n/a |
How we count · survey areas WA667 (2025-08-28), WA777 (2025-08-28)
What the numbers mean here
On 34% of Thurston 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: 51.25 in, 0.40 in above the 1991-2020 average year of 50.85 in.
USDA rates 90% 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 10 of the last 52 weeks; the most was 100% of its area, in 8 weeks, most recently on the December 9, 2025 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 33% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
71 flood losses since 2016, $2.0 million paid. The worst year was 2022, with 33. Foundations of the buildings, where recorded: 16% basements, 22% slabs, 37% crawl spaces, 25% elevated. Median water depth outside the building: 16.5 in.
| Year | Flood losses |
|---|---|
| 2016 | 2 |
| 2017 | 5 |
| 2018 | 3 |
| 2019 | 0 |
| 2020 | 24 |
| 2021 | 1 |
| 2022 | 33 |
| 2023 | 1 |
| 2024 | 1 |
| 2025 | 1 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 50.85 in |
| 12 months to August 2026 | 51.25 in (+0.40) |
| 2025 | 45.98 in (-4.87) |
| Wettest year since 1895 | 1933: 66.84 in |
| Driest year since 1895 | 1952: 31.21 in |
| Days with 1 inch or more, normal at OLYMPIA AP (4 mi away) | 9.5 |
Homes
| Housing units | 125,163 |
| Median year built | 1991 |
| Built before 1980 | 34% |
| Built 2000 or later | 35% |
| Owner-occupied | 68% |
| Median value, owner-occupied | $480,300 |
| Single-family detached | 67% |
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 |
|---|---|---|---|
| Lewis County, WA | 33 mi | 7% | 20% |
| Pierce County, WA | 33 mi | 2% | 34% |
| Mason County, WA | 33 mi | under 1% | 45% |
| Grays Harbor County, WA | 48 mi | 6% | 20% |
| Kitsap County, WA | 49 mi | under 1% | 65% |
| Wahkiakum County, WA | 51 mi | 4% | 10% |
| Cowlitz County, WA | 52 mi | 4% | 10% |
| Pacific County, WA | 52 mi | 4% | 19% |
Sources: USDA SSURGO (August 28, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status