St. Mary Parish, Louisiana
A shallow water table limits basements on 80% of St. Mary Parish's mapped land, a higher share than 89% of US counties. Shrink-swell clay limits houses without basements on 52%.
100% of the county is in drought on the October 13, 2026 Drought Monitor map, up from 17.8% a week earlier. FEMA's flood data list 324 flood losses here since 2016, 1% of them in buildings with basements. 58% of homes were built before 1980, against 50% nationally; the median home dates from 1976.
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 St. Mary Parish
| Expansive clay (limits slab homes) | 51.6% |
| High water table (limits basements) | 80.3% |
| Septic fields very limited | 78.6% |
| Clay swell (LEP, by area) | 12.7% |
| High-swell land (LEP 6%+) | 51.8% |
| Mapped soil | 442,400 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Baldwin | 9% | n/a |
| Maurepas | 9% | n/a |
| Kenner | 9% | n/a |
| Barbary | 9% | n/a |
| Schriever | 8% | n/a |
| Allemands | 7% | n/a |
How we count · survey areas LA101 (2025-08-28)
What the numbers mean here
Clay swell averages a linear extensibility of 12.7% by land area here, and 52% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 52% of the mapped land.
Clay shrinks as it dries: 100% of the county is in drought on the latest map, and 52% of St. Mary Parish's mapped land has shrink-swell clay.
On 80% of St. Mary Parish'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: 49.89 in, 10.83 in below the 1991-2020 average year of 60.72 in.
USDA rates 79% of the land very limited for septic absorption fields.
Drought
100% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 38 of the last 52 weeks; the most was 100% of its area, in 10 weeks, most recently on the October 13, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 100% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
324 flood losses since 2016, $10.1 million paid. The worst year was 2024, with 117. Foundations of the buildings, where recorded: 1% basements, 80% slabs, under 1% crawl spaces, 19% elevated. Median water depth outside the building: 9 in.
| Year | Flood losses |
|---|---|
| 2016 | 9 |
| 2017 | 29 |
| 2018 | 3 |
| 2019 | 96 |
| 2020 | 51 |
| 2021 | 17 |
| 2022 | 0 |
| 2023 | 1 |
| 2024 | 117 |
| 2025 | 1 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 60.72 in |
| 12 months to August 2026 | 49.89 in (-10.83) |
| 2025 | 54.58 in (-6.14) |
| Wettest year since 1895 | 1946: 93.07 in |
| Driest year since 1895 | 2011: 37.89 in |
| Days with 1 inch or more, normal at FRANKLIN 3 NW (14 mi away) | 21.3 |
Homes
| Housing units | 22,507 |
| Median year built | 1976 |
| Built before 1980 | 58% |
| Built 2000 or later | 17% |
| Owner-occupied | 69% |
| Median value, owner-occupied | $138,900 |
| Single-family detached | 69% |
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 |
|---|---|---|---|
| Iberia Parish, LA | 23 mi | 31% | 84% |
| Assumption Parish, LA | 31 mi | 68% | 88% |
| St. Martin Parish, LA | 35 mi | 57% | 84% |
| Terrebonne Parish, LA | 43 mi | 34% | 65% |
| Iberville Parish, LA | 45 mi | 54% | 88% |
| St. James Parish, LA | 49 mi | 68% | 92% |
| Ascension Parish, LA | 52 mi | 46% | 91% |
| Vermilion Parish, LA | 53 mi | 39% | 82% |
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