What are alluvial soils?

Alluvial soils are soils deposited by surface water. You’ll find them along rivers, in floodplains and deltas (like the Mississippi Delta), stream terraces, and areas called alluvial fans. This last category results from larger floods, causing the soil to spread out in the shape of a triangle fan.

These soils are formed differently than many other soils, which are made through the long process of rock transformation that often takes millennia. You can learn more about that process in this blog.

Left, an upper view of trees and soil in a fan shape, with roads passing through. On right, boulders and gravel along a stream bed.
In 1982, a dam located in the Rocky Mountain National Park broke. The resulting 200 million gallons of water deposited boulders, gravel and a mixture of sand, silt and clay in a fan shape. It’s called an alluvial fan because it’s the result of water deposition of soil. Credit: SVFisk

Alluvial soils provide many functions, the greatest is serving as the earth’s kidneys. Alluvial soils remove sediments and nutrients flowing in the adjacent water. They can also remove other contaminants from rivers and improve water quality for downstream communities!

Because floods periodically deposit new sediment at the surface, alluvial soils can have a unique layered look. Dark and light colors alternate, along with assorted sizes of round gravel particles. This unique layering process is called stratification and is evident in many floodplains.

All alluvial soils form by flooding. The new sediment that is added comes from the stream watershed. And this is sensitive to changes in land use. Examples could be deforestation, plowing for agriculture, or disturbance during construction activities. These human activities can increase erosion of uplands, and the resulting sediment then flows downriver and is deposited.

black measuring tape on left, darker brown soil at top with lighter colored soil on bottom
Fertile alluvial soil in central Pennsylvania (tape in centimeters). This floodplain soil is formed from erosion of prime farmland in the watershed, which transfers nutrients to the floodplain. The loamy upper material effectively holds water in the plant root zone and the rounded gravelly base (below 90 cm) allows for drainage of excess water from the root zone. Photo by M.C. Ricker

Sometimes moving eroded soil and nutrients from one area to another can be good. Many ancient civilizations, like those in Mesopotamia and ancient Egypt, thrived on the banks of rivers. If not for the fertile alluvial soils deposited on the riverbanks, their agricultural societies would not have thrived.

In other cases, flood deposition can also bring contamination from upland pollution. Urban and industrial watersheds have an increased risk of floodplains containing potentially harmful chlorinated compounds and trace metals. Sometimes the pollution is very evident, with dead plant life and garbage on the soil surface. However, in other areas, the contamination cannot be seen. Soil testing is necessary in these cases.

black measuring tape at left in centimeters, light soil above dark soil
Photograph from eastern Pennsylvania showing floodplain deposits from three distinct stages of watershed land use (picture tape in centimeters). The lowest material (below 60 cm) is old fertile alluvium that has a thick plow layer from agriculture in the 19th century. On top of this is stratified industrial coal washings (20-60 cm) from the early 20th century that buried this once fertile farmland in industrial waste. The upper most sandy material (0-20 cm) is human transported material from the construction of a nearby road in the past decade. This one soil profile clearly shows the evidence of human impacts from agriculture, industrial coal mining, and more modern suburban construction over the past 300 years. Photo by M.C. Ricker

In these cases, urban soils are trapping harmful pollution and preventing it from reaching downstream areas.

In many places around the world there are distinctly different soil colors, chemical properties, and human artifacts buried in floodplains. These allow scientists to reconstruct human occupation and land use history. In modern urban areas, alluvial soils contain discarded garbage that has been carried in with flood waters and buried. The most recent deposits at the surface typically contain abundant plastics.  Deeper deposits contain older manufactured materials like glass, brick, and stone. Therefore, alluvial soils also represent critically important areas for archaeologists and soil scientists to date human occupation.

The next time you are on a floodplain remember that unique alluvial soils that form there provide many beneficial values to society: agricultural production, pollutant trapping, and preserving evidence of our own human history.

Answered by Matthew Ricker, North Carolina State University

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