California: basement moisture and foundation risk by county
58 counties, ranked by our combined index. Highest combined risk is Merced County (61), lowest is Modoc County (25). The worst soil for foundation cracks specifically is in Del Norte County.
| County | Index | Moisture | Cracks | Water table | Region |
|---|---|---|---|---|---|
| Merced County | 61 | 40 | 100 | 46 | California Central Valley |
| San Joaquin County | 61 | 43 | 98 | 46 | California Central Valley |
| Mariposa County | 60 | 43 | 93 | 46 | California Central Valley |
| Santa Clara County | 60 | 38 | 100 | 46 | Pacific Coast Ranges |
| Madera County | 58 | 43 | 98 | 30 | California Central Valley |
| Stanislaus County | 58 | 43 | 98 | 30 | California Central Valley |
| Plumas County | 57 | 43 | 96 | 30 | California Central Valley |
| Solano County | 57 | 38 | 91 | 46 | Pacific Coast Ranges |
| Amador County | 56 | 41 | 84 | 46 | California Central Valley |
| Monterey County | 56 | 39 | 100 | 30 | California Central Valley |
| Nevada County | 56 | 39 | 87 | 46 | California Central Valley |
| Placer County | 56 | 43 | 93 | 30 | California Central Valley |
| Alpine County | 55 | 42 | 80 | 46 | California Central Valley |
| Del Norte County | 55 | 35 | 100 | 30 | Pacific Coast Ranges |
| Humboldt County | 55 | 35 | 100 | 30 | Pacific Coast Ranges |
| Lassen County | 55 | 41 | 81 | 46 | California Central Valley |
| Los Angeles County | 55 | 35 | 100 | 30 | Pacific Coast Ranges |
| Orange County | 55 | 36 | 100 | 30 | Pacific Coast Ranges |
| San Bernardino County | 55 | 37 | 97 | 30 | Pacific Coast Ranges |
| San Diego County | 55 | 35 | 100 | 30 | Pacific Coast Ranges |
| Tuolumne County | 55 | 39 | 94 | 30 | California Central Valley |
| Alameda County | 54 | 35 | 85 | 46 | Pacific Coast Ranges |
| Colusa County | 54 | 41 | 89 | 30 | California Central Valley |
| Glenn County | 54 | 41 | 99 | 16 | California Central Valley |
| San Benito County | 54 | 39 | 80 | 46 | California Central Valley |
| San Francisco County | 54 | 35 | 99 | 30 | Pacific Coast Ranges |
| Mono County | 53 | 41 | 86 | 30 | California Central Valley |
| San Luis Obispo County | 53 | 42 | 74 | 46 | California Central Valley |
| Shasta County | 53 | 40 | 75 | 46 | California Central Valley |
| El Dorado County | 52 | 42 | 82 | 30 | California Central Valley |
| Kern County | 52 | 43 | 80 | 30 | California Central Valley |
| Riverside County | 52 | 34 | 93 | 30 | Pacific Coast Ranges |
| Santa Cruz County | 52 | 34 | 82 | 46 | Pacific Coast Ranges |
| Contra Costa County | 51 | 37 | 86 | 30 | Pacific Coast Ranges |
| Marin County | 51 | 35 | 89 | 30 | Pacific Coast Ranges |
| Siskiyou County | 51 | 34 | 100 | 16 | Pacific Coast Ranges |
| Sonoma County | 51 | 35 | 100 | 16 | Pacific Coast Ranges |
| Lake County | 50 | 38 | 80 | 30 | Pacific Coast Ranges |
| Napa County | 50 | 36 | 85 | 30 | Pacific Coast Ranges |
| Yuba County | 50 | 40 | 78 | 30 | California Central Valley |
| Santa Barbara County | 49 | 34 | 71 | 46 | Pacific Coast Ranges |
| Trinity County | 49 | 34 | 73 | 46 | Pacific Coast Ranges |
| San Mateo County | 48 | 35 | 80 | 30 | Pacific Coast Ranges |
| Kings County | 47 | 42 | 66 | 30 | California Central Valley |
| Mendocino County | 47 | 34 | 78 | 30 | Pacific Coast Ranges |
| Tulare County | 47 | 39 | 60 | 46 | California Central Valley |
| Butte County | 46 | 40 | 76 | 16 | California Central Valley |
| Sierra County | 46 | 42 | 64 | 30 | California Central Valley |
| Sutter County | 43 | 40 | 57 | 30 | California Central Valley |
| Tehama County | 42 | 42 | 51 | 30 | California Central Valley |
| Ventura County | 42 | 36 | 58 | 30 | Pacific Coast Ranges |
| Fresno County | 41 | 39 | 51 | 30 | California Central Valley |
| Yolo County | 41 | 36 | 65 | 16 | Pacific Coast Ranges |
| Calaveras County | 40 | 39 | 50 | 30 | California Central Valley |
| Sacramento County | 39 | 42 | 51 | 16 | California Central Valley |
| Inyo County | 28 | 30 | 33 | 16 | Basin and Range desert |
| Imperial County | 27 | 30 | 30 | 16 | Basin and Range desert |
| Modoc County | 25 | 29 | 16 | 30 | Sierra Nevada and Cascades |
Values from USDA NRCS SSURGO where available, otherwise our regional model — each county page states which. Methodology.
The physiographic regions inside California
State lines are political; soil follows geology. These are the distinct ground types inside California and why each behaves the way it does.
California Central Valley
Expansive basin clays alternating with free-draining fan sediments, plus a cemented hardpan in places that perches water above it. Add the largest human-caused land subsidence in the country from groundwater pumping, and differential movement here has three separate causes.
31 counties: Alpine County, Amador County, Butte County, Calaveras County, Colusa County, El Dorado County, Fresno County, Glenn County, Kern County, Kings County, Lassen County, Madera County, Mariposa County, Merced County, Mono County, Monterey County, Nevada County, Placer County, Plumas County, Sacramento County, San Benito County, San Joaquin County, San Luis Obispo County, Shasta County, Sierra County, Stanislaus County, Sutter County, Tehama County, Tulare County, Tuolumne County, Yuba County
Pacific Coast Ranges
Some of the most expansive soils in North America sit here, on some of the least stable slopes. Winter rain saturates clay-rich colluvium and it moves — which is why hillside foundation work on the California and Oregon coast is engineered rather than standardised.
24 counties: Alameda County, Contra Costa County, Del Norte County, Humboldt County, Lake County, Los Angeles County, Marin County, Mendocino County, Napa County, Orange County, Riverside County, San Bernardino County, San Diego County, San Francisco County, San Mateo County, Santa Barbara County, Santa Clara County, Santa Cruz County, Siskiyou County, Solano County, Sonoma County, Trinity County, Ventura County, Yolo County
Basin and Range desert
Phoenix, Las Vegas, Tucson, Albuquerque. Rainfall is minimal and groundwater is deep, so seepage is not the story. Two other mechanisms are: expansive playa clays, and hydrocompactive fan soils that collapse the first time they are deeply wetted — typically by irrigation or a leaking service line rather than by rain.
2 counties: Imperial County, Inyo County
Sierra Nevada and Cascades
Coarse material over rock. Volume change is minimal and drainage is good; the seasonal problem is snowmelt arriving at foundations faster than it can be led away, not a water table.
1 counties: Modoc County
Page data updated