Alluvium: Definition & Overview

Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

You have almost certainly seen alluvium before, but perhaps you didn't know the name for it. Here you will learn what alluvium is, where you find it, and how it got there.

What is Alluvium

Have you ever noticed that the stuff left behind by a flowing river or stream looks like a big, jumbled mess? You know...piles of rocks, sand, and branches (and hopefully not lawn chairs and plastic bottles). All the debris that is carried along by the water and is deposited elsewhere is called alluvium. Where it is deposited has to do with how fast the water is going, gravity, and the lay of the land.

Where is Alluvium Found?

Alluvium is deposited in a few different ways. One way is that it's left along the edges of rivers and streams where the flow slows down enough for it to settle out from the water. For instance, the outer edge of a turn in a river or stream is a place where alluvium can accumulate.

But often, alluvium accumulates at the end of a river or stream where the water is freed from its channel and spreads out on the flattened land. These areas are called floodplains or deltas, and because alluvium brings so much organic matter, they are often very fertile. There is a reason why people have congregated and farmed in some well-known floodplains, such as the Mississippi River Delta and the Nile River Delta in Egypt....the rivers deliver a continuous supply of fertile alluvium.

The Nile floodplain
Nile floodplain Egypt

Where alluvium shoots through the narrow walls of a canyon and pops out into a valley, a triangular-shaped area of alluvium forms, called an alluvial fan. Alluvial fans are common in the desert southwest were high speed flash floods carry alluvium. Sometimes several streams of water are near each other, running somewhat parallel to each other and coming off of hills or a mountain range. They all spread out when they reach the bottom, creating many alluvial fans that overlap each other. This formation is called a bajada.

An alluvial fan in Death Valley, California
Alluvial fan in Death Valley

Alluvium is sorted out by the weight of each type of debris, almost like a giant sifter. The first filter in the sifter would have huge holes. Everything would get through: boulders, sand, silt, branches. The next filter would have smaller holes where pebbles and smaller pieces could get through, but not boulders. The final filter would have tiny holes where only sand, silt and smaller organic matter could get through.

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