Alluvial Soil: What It Is and Where It’s Found

What alluvium is

USDA’s agricultural thesaurus defines alluvium as silt, sand, clay, gravel and similar material deposited by running water, as on floodplains, in deltas and along riverbeds.^[1] It defines alluvial soils as soils that developed over time as sediments were deposited by the periodic flooding of streams or rivers, and lists floodplain soils as another name for them.^[3]

The National Research Council’s glossary adds the cones and fans of sediment at the base of mountain slopes to the definition of alluvium.^[2]

In soil surveys, alluvium is a parent material: the material a soil formed in. The kinds USDA records most often are Alluvium (441.9 million acres), Slope alluvium (55.8 million acres), Backswamp deposits (558,461 acres), Valley side alluvium (126,347 acres) and Overbank deposits (13,724 acres).^[4]

How to tell if soil is alluvial

Start with the landscape. Running water lays alluvium down on floodplains, in deltas and along riverbeds,^[1] and as cones or fans at the base of mountain slopes.^[2] Ground beside a river or creek, or a fan spreading out below a mountain, fits the pattern.

Then look at the profile. In Fluvents, the Entisols that formed in recent water-deposited sediments on flood plains, layering of the deposits (stratification) is normal, and Entisols show little or no evidence of horizon development.^[5] A layered profile with weak horizons points to water-laid material, though not every alluvial soil looks like that.

For a specific address, the survey record answers it directly. USDA records the material each soil formed in, and the check at the top of this page reads it for any US address.^[4]

Alluvial soil isn’t one kind of soil

Alluvium describes what a soil formed in, not which kind of soil it became, so alluvial soils land in many of the 12 soil orders. Of the alluvial acres that have a soil order, Mollisols make up 26.1%, Aridisols 20.1% and Entisols 19.4%; all 12 orders have at least some.^[4][6]

Shares are of the alluvial acres that have a recorded soil order; orders under 0.1% are left out of the table.^[4]

Fluvents are one kind: USDA describes them as the more or less freely drained Entisols that formed in recent water-deposited sediments on flood plains. Some of the largest areas are on the flood plains along the Mississippi River.^[5]

Fluvents formed in water-laid sediment, but they’re only part of the Entisols, and Entisols hold 19.4% of alluvial acres with an order, so most alluvial soil is something else.^[5][4]

Where alluvial soils are

Texas has the most (12%), followed by Arizona (7%) and California (7%).^[4]

Most widespread alluvial soil series

Download the data (CSV)

What alluvial soils are like

Alluvial soils vary in texture: sandy loam is the most common at 24.3%, followed by silt loam (24.2%) and loam (21.3%).^[4]

Drainage

57.5% of alluvial acres are well drained, the most common class.^[4][7]

Surface pH has a median of 7.0 (middle half 6.2–7.9); organic matter 2.0% (1.0–3.0); and the surface holds about 1.8 inches of water per foot (150 mm per meter; middle half 1.4–2.3 in/ft).^[4][8]

Farming on alluvial soil

The “prime if” share is the part that needs work first: drainage, irrigation or flood protection. Check a field with the prime farmland checker, or see the national picture on the prime farmland map.

Among Fluvents, USDA says most are used as rangeland, forest, pasture or wildlife habitat.^[5]

Building on alluvial soil

For a house or a road on alluvial soil, check three things in USDA’s data: flooding, drainage and shrink-swell.

Flooding

Soils that formed where water flows can still flood. USDA says most Fluvents are frequently flooded unless dams or levees protect them,^[5] but that describes one kind of alluvial soil, not all of it.

The figure to check is the flooding frequency USDA records for the address, on the map’s flooding layer.

Drainage

24.2% of alluvial acres are somewhat poorly, poorly or very poorly drained.^[4][7] The water table checker shows how close to the surface USDA records the water table at an address.

Shrink-swell

Vertisols, the clay soils that shrink and swell,^[10] make up 4.7% of alluvial acres with an order, and 55.6% of all Vertisol acres formed in alluvium.^[4] USDA records each soil’s shrink-swell potential; the expansive soil checker reads it for an address.^[11]

The building site checker puts it together: USDA’s ratings for houses at the address, with the soil features behind them.

What other US soils formed in

USDA records a parent material for 93.8% of US soil acres; here is how the groups compare by acres.^[4]

Frequently asked questions

What is alluvial soil?

Soil that formed in alluvium: silt, sand, clay, gravel and similar material deposited by running water, as on floodplains, in deltas and along riverbeds.^[1]

What is meant by alluvial soils?

USDA's agricultural thesaurus defines them as soils that developed over time as sediments were deposited by the periodic flooding of streams or rivers.^[3]

What is another name for alluvial soil?

Floodplain soil. USDA's agricultural thesaurus lists "flood plain soils" and "floodplain soils" as synonyms.^[3]

How can you tell if soil is alluvial?

Look at where it is and how it's layered. Alluvium is laid down on floodplains, in deltas and along riverbeds, and as fans at the base of mountain slopes, and a layered profile with weak horizons points to water-laid material. The address check on this page reads the parent material USDA records.^[1][2][5]

Where is alluvial soil found in the US?

Of the 498.4 million acres of alluvial soil USDA maps, Texas has the most (12%), followed by Arizona (7%) and California (7%).^[4]

Is alluvial soil good for farming?

In USDA's survey, 14.3% of alluvial soil acres are prime farmland, compared with 12.5% of all US soil acres, and another 13.8% is prime if drained, irrigated or protected from flooding.^[4][9]

Is alluvial soil the same as floodplain soil?

Largely: USDA's thesaurus lists floodplain soils as a synonym. Alluvium also builds cones and fans at the base of mountain slopes, and our figures count soils formed in fan alluvium as alluvial too.^[3][2]

Are all alluvial soils Fluvents?

No. Fluvents, the more or less freely drained Entisols that formed in recent water-deposited sediments on flood plains, are one kind. Entisols as a whole make up 19.4% of the alluvial acres that have a soil order in USDA's survey.^[5][4]

Does alluvial soil flood?

It can. Alluvium is laid down by running water, often on floodplains, and USDA says most Fluvents are frequently flooded unless dams or levees protect them. Check the flooding frequency USDA records for your address on the map's flooding layer.^[1][5]

Is alluvial soil good for building?

Check the address for flooding, drainage and shrink-swell; all three vary across alluvial soils. In USDA's survey, 24.2% of alluvial acres are somewhat poorly, poorly or very poorly drained.^[4][7]