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San Juan County, Washington
A shallow water table limits basements on 47% of San Juan County's mapped land, a higher share than 63% of US counties and the 4th highest of 39 Washington counties. Shrink-swell clay limits houses without basements on 0%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 5 flood losses here since 2016. 37% of homes were built before 1980, against 50% nationally; the median home dates from 1988.
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 San Juan County
| Expansive clay (limits slab homes) | 0.0% |
| High water table (limits basements) | 46.6% |
| Septic fields very limited | 98.8% |
| Clay swell (LEP, by area) | 2.5% |
| High-swell land (LEP 6%+) | 0.0% |
| Mapped soil | 114,882 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Cady | 15% | n/a |
| Mitchellbay | 10% | n/a |
| Doebay | 9% | n/a |
| Coveland | 6% | n/a |
| Roche | 5% | n/a |
| Killebrew | 4% | n/a |
How we count · survey areas WA055 (2025-08-28)
What the numbers mean here
On 47% of San Juan 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: 31.17 in, 2.35 in above the 1991-2020 average year of 28.82 in.
USDA rates 99% 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 | 100% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
5 flood losses since 2016, $114,035 paid. The worst year was 2018, with 2.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 0 |
| 2018 | 2 |
| 2019 | 0 |
| 2020 | 1 |
| 2021 | 1 |
| 2022 | 1 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 28.82 in |
| 12 months to August 2026 | 31.17 in (+2.35) |
| 2025 | 29.61 in (+0.79) |
| Wettest year since 1895 | 1997: 37.19 in |
| Driest year since 1895 | 1929: 16.22 in |
| Days with 1 inch or more, normal at OLGA 2 SE (15 mi away) | 2 |
Homes
| Housing units | 13,990 |
| Median year built | 1988 |
| Built before 1980 | 37% |
| Built 2000 or later | 28% |
| Owner-occupied | 78% |
| Median value, owner-occupied | $799,200 |
| Single-family detached | 88% |
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 |
|---|---|---|---|
| Island County, WA | 31 mi | 0% | 65% |
| Clallam County, WA | 45 mi | 1% | 42% |
| Jefferson County, WA | 52 mi | 1% | 39% |
| Skagit County, WA | 59 mi | under 1% | 46% |
| Whatcom County, WA | 62 mi | 1% | 25% |
| Kitsap County, WA | 64 mi | under 1% | 65% |
| Snohomish County, WA | 69 mi | 1% | 72% |
| Mason County, WA | 80 mi | under 1% | 45% |
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