Expansive Soil in Idaho: County Ratings
1 of the 41 rated counties in Idaho have a dominant shrink-swell rating of High or Very High. Each rating below is the NRCS shrink-swell class covering the largest share of the county's mapped soil acres, computed from USDA SSURGO data. Open a county for the full class breakdown and what it means for a slab foundation.
| County | Dominant class | High + Very High share | Survey coverage |
|---|---|---|---|
| Adams County | Low | 10% | 99% |
| Bannock County | Low | 2% | 100% |
| Bear Lake County | Low | 5% | 93% |
| Benewah County | Low | 0% | 99% |
| Bingham County | Low | 1% | 93% |
| Blaine County | Low | 13% | 89% |
| Boise County | Low | 0% | 98% |
| Bonner County | Low | 0% | 90% |
| Bonneville County | Low | 18% | 93% |
| Boundary County | Low | 1% | 93% |
| Butte County | Low | 1% | 91% |
| Camas County | Moderate | 24% | 98% |
| Canyon County | Low | 0% | 96% |
| Caribou County | Low | 16% | 98% |
| Cassia County | Low | 3% | 99% |
| Clark County | Low | 4% | 100% |
| Clearwater County | Low | 2% | 98% |
| Custer County | Low | 4% | 99% |
| Franklin County | Low | 23% | 99% |
| Fremont County | Low | 1% | 98% |
| Gem County | Low | 28% | 98% |
| Gooding County | Low | 32% | 99% |
| Idaho County | Moderate | 9% | 99% |
| Jefferson County | Low | 22% | 99% |
| Jerome County | Moderate | 0% | 99% |
| Kootenai County | Low | 2% | 91% |
| Lemhi County | Low | 11% | 99% |
| Lewis County | Moderate | 15% | 100% |
| Lincoln County | Moderate | 17% | 98% |
| Madison County | Low | 1% | 99% |
| Minidoka County | Low | 0% | 96% |
| Nez Perce County | Moderate | 7% | 99% |
| Oneida County | Low | 4% | 100% |
| Owyhee County | Moderate | 31% | 100% |
| Payette County | Moderate | 5% | 97% |
| Power County | Low | 0% | 91% |
| Shoshone County | Low | 0% | 99% |
| Teton County | Moderate | 5% | 99% |
| Twin Falls County | Low | 19% | 99% |
| Valley County | Low | 1% | 91% |
| Washington County | High | 43% | 98% |
How these ratings are computed
Ratings come from USDA NRCS SSURGO soil survey data: for each soil component we take the maximum linear extensibility percent (lep_r) in the top 100 cm, apply the NRCS Handbook Part 618 class limits (Low under 3 percent, Moderate 3 to 6, High 6 to 9, Very High 9 and above), assign map units by plurality of component percent, and roll acres up to the county. Full details on the methodology section of the lookup page. A county rating is not a parcel-level geotechnical assessment.