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Yuma County foundation soil risk, by town
Nobody local had put this together, so we did. Federal soil survey data, measured against town boundaries, with the method shown.
Yuma County foundation soil risk map, by town
This is US Department of Agriculture soil survey data — the same mapping used for engineering and land-use planning — measured against Census town boundaries. The percentage is how much of each town sits on soil the USDA classes as high shrink-swell: ground that expands as it takes on water and contracts as it dries.
| Town | High shrink-swell | What that means | Land subsidence |
|---|---|---|---|
| Somerton | 72.4% | High — expansive clay dominates | Not mapped |
| San Luis | 12% | Low — mostly sand or rock | Not mapped |
| Yuma (city-wide) | 8.2% | Mixed — depends on the parcel | Not mapped |
| Yuma (older core) | 30.1% | High — expansive clay dominates | Not mapped |
| Fortuna Foothills | 0% | Low — mostly sand or rock | Not mapped |
| Wellton | 0% | Low — mostly sand or rock | Yes — Lower Gila feature |
The headline: this splits by landform, not by town name. The Colorado River floodplain carries the expansive clays. The Yuma Mesa and the eastern piedmont are sand and rock. Any single county-wide claim about Yuma soil is wrong in one direction or the other, whichever way you make it.
Why Yuma city appears twice
Because the answer genuinely changes with where you draw the line, and we would rather show that than pick the flattering number. Modern city limits take in large annexed desert and mesa areas, which pulls the city-wide figure down to about 8%. Restrict it to the older urban core and valley floor and it rises to roughly 30%.
Both are correct — they describe different footprints. Which is the practical point of this page: your address matters more than your town. Two Yuma homes four miles apart can sit on completely different ground.
Not all expansive clay behaves the same
This distinction changes what a repair should cost, and it almost never comes up in a sales conversation.
| Soil series | What the survey says |
|---|---|
| Gadsden clay | Expansive through the FULL 0-152cm profile. NRCS rates it 'Very limited' for dwellings, shrink-swell score 1.00 (max). Cracks 1cm+ wide to 50cm+ depth when dry. Clay 35-60%. |
| Holtville clay | Clay over sand - expansive layer is roughly the top 2ft (0-58cm), sand below. NRCS 'Somewhat limited', score 0.287. |
| Kofa clay | Clay over sand - expansive to ~71cm, sand below. NRCS 'Somewhat limited', score 0.848. Type location is Yuma County. |
| Indio silt loam | LOW shrink-swell (1.2) but NRCS rates it 'Very limited' for dwellings on FLOODING, not clay. 76,789 acres - the largest valley soil. |
| Superstition sand | Low shrink-swell, rapid permeability, Yuma Mesa. NRCS 'Not limited'. |
| Rositas sand | Low shrink-swell, rapid permeability, Torripsamments. NRCS 'Somewhat limited' on slope only. |
Gadsden clay is the difficult one — expansive through the full five feet, and the only soil here the USDA rates "very limited" for dwellings on shrink-swell grounds. Holtville and Kofa are clay over sand, so the moving layer is roughly the top two feet. That difference is why two neighbours can get very different recommendations, and why it is fair to ask a contractor which series he thinks you are on, and how he knows.
Three things about Yuma that are widely misunderstood
There are no earth fissures in Yuma County. Zero of Arizona's 7,478 mapped fissures. Those dramatic ground-crack photographs come from Pinal and Maricopa. If a contractor shows you one, he is selling with someone else's geology.
Yuma's groundwater problem is too much water, not too little. The valley water table sits roughly 6 to 18 feet down and is held there by around 77 drainage wells and 90 miles of open drains. The Bureau of Reclamation warns in its own reporting that shallow groundwater could affect construction activities. That is the opposite of the Phoenix pumping-decline story, and it is a real foundation consideration — just not the one people expect.
Wellton is a subsidence area, not a clay area. It has no high shrink-swell ground at all, but it sits inside the Lower Gila land subsidence feature mapped by Arizona's water department. Subsidence and expansive soil look similar in a house and are not the same problem.
Method, sources, and what this is not
Data: USDA-NRCS SSURGO soil survey AZ649 (Yuma-Wellton Area), version 23, published August 2025, intersected with US Census place boundaries. Shrink-swell classes follow the NRCS National Soil Survey Handbook Part 618 — high is a linear extensibility of 6.0 to 8.9. Subsidence mapping is the Arizona Department of Water Resources land subsidence layer. Retrieved September 2026.
What this is not: a soil test. These are survey-level representative values across map units, not measurements on your lot, and a single parcel can differ from everything around it. Treat it as a screening tool that tells you which questions to ask, then get a real geotechnical investigation before anyone prices structural work. See what an evaluation should include and whether your repair needs a permit.
If you are a realtor, inspector or engineer working in Yuma County and want to use or correct this, every source above is public. We would rather it be right than ours.