Boise County, Idaho
USDA's soil survey flags shrink-swell clay on 3% of Boise County's mapped land and a shallow water table under basements on 2%, both below the national shares of 15% and 29%. The soil survey maps 49% of the county's land area; these shares describe the mapped part.
17% 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. 33% 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 Boise County
| Expansive clay (limits slab homes) | 3.5% |
| High water table (limits basements) | 1.5% |
| Septic fields very limited | 99.3% |
| Clay swell (LEP, by area) | 1.8% |
| High-swell land (LEP 6%+) | 3.7% |
| Mapped soil | 592,392 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Shirts | 9% | n/a |
| Charters | 7% | n/a |
| Coski | 5% | n/a |
| Kosh | 5% | n/a |
| Koppes | 5% | n/a |
| Kisky | 4% | n/a |
How we count · survey areas ID662 (2025-08-27), ID683 (2025-08-27)
What the numbers mean here
Rain over the 12 months to August 2026: 28.23 in, 2.22 in below the 1991-2020 average year of 30.45 in.
USDA rates 99% of the land very limited for septic absorption fields.
With shrink-swell clay on 3% and a shallow water table on 2% of the land, the soil survey points to neither problem across most of Boise County; conditions still vary lot by lot.
Drought
17% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 40 of the last 52 weeks; the most was 95.8% of its area, in 2 weeks, most recently on the June 30, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 89.6% |
| D1 moderate drought or worse | 17% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
1 flood losses since 2016, $35,645 paid. The worst year was 2017, with 1.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 1 |
| 2018 | 0 |
| 2019 | 0 |
| 2020 | 0 |
| 2021 | 0 |
| 2022 | 0 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 30.45 in |
| 12 months to August 2026 | 28.23 in (-2.22) |
| 2025 | 33.46 in (+3.01) |
| Wettest year since 1895 | 1996: 49.38 in |
| Driest year since 1895 | 1935: 18.28 in |
| Days with 1 inch or more, normal at GARDEN VALLEY (15 mi away) | 3.8 |
Homes
| Housing units | 5,689 |
| Median year built | 1991 |
| Built before 1980 | 33% |
| Built 2000 or later | 31% |
| Owner-occupied | 90% |
| Median value, owner-occupied | $476,100 |
| Single-family detached | 84% |
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 |
|---|---|---|---|
| Gem County, ID | 34 mi | 19% | 9% |
| Elmore County, ID | 43 mi | 7% | 2% |
| Ada County, ID | 45 mi | 20% | 4% |
| Payette County, ID | 52 mi | 4% | 9% |
| Canyon County, ID | 55 mi | 1% | 14% |
| Camas County, ID | 58 mi | 19% | 22% |
| Valley County, ID | 60 mi | 1% | 14% |
| Washington County, ID | 62 mi | 33% | 3% |
Sources: USDA SSURGO (August 27, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status