What Is Colluvium And How Is It Different From Alluvium?

Alluvial fan in Rocky Mountain National Park, Colorado caused by flooding.
Alluvial fan in Rocky Mountain National Park, Colorado caused by flooding.

Scientists and authors from various regions in the world seem not to agree on the precise definitions of colluvium and alluvium. Europe, Russia, and the US have identifiable differences in defining these two processes. These differences not only undermine the meaning, but also lead to misunderstanding of the two processes. In this article, as in most geological and geomorphological literature, colluvium suggests creeping due to gravity or other natural causes down a hill slope while alluvium suggests rivers moving material over long distances and depositing them downstream. The US Department of Interior approves of and uses this approach.

Definitions Of Colluvium And Alluvium

Colluvianation is the process where sheet erosion, water erosion, downward creep, or a combination of all transport loose unconsolidated debris from hill tops to the base of the slope depositing different types of heterogeneous rocks and debris of varying sizes, and forming what is called a colluvium. Colluvium material consists of poorly sorted angular fragments of various sizes from silt to rock debris, and sometimes slabs of bedrock, facing up the slope as if indicating its original location. The formation of alluvium, on the other hand, is the process whereby river water transports and deposits clay, sand, silt, and such like material on the banks of valleys, deltas, or floodplains. These deposits are therefore known as alluvium. Alluvial materials are loose and may be reshaped during the whole transportation process. Distinguishing between colluvium and alluvium may not be easy, especially at valley edges where colluvial and alluvial materials may mix and become indistinguishable.

Location and Composition

The location of the materials for these two processes, if not mixed, can be used to distinguish between them. Colluvium gently accumulates at the base of a slope or within gaps on sloping planes of hills creating deposits that can pile up to a thickness of several feet burying paleosols and crude bedding. Colluvium material may roll, slide, or fall down a slope. What sets alluvium apart is that water deposits the materials in a non-marine location, and they seem to have been reshaped due to the friction during transportation. Differentiation by location becomes difficult when the base of a hill has a river, meaning that both alluvium and colluvium may be responsible for the deposits. When the deposits of such a location are analyzed, larger heterogenic rock materials are classified under colluvium because river water may not move and deposit such heavy matter. This phenomenon makes alluvium deposits more homogenetic and fine textured. In another scenario, rivers deposit material in deep or shallow low-velocity areas like floodplains forming alluvium. Another key differentiating factor is that colluvium travels a short distance whereas alluvium may travel several miles before settling.

Advantages of Colluvium and Alluvium

Alluvium may contain deposits of valuable mineral ores washed from upstream, including but not limited to gold, diamonds, and platinum. This process is known as placer deposit process, and there are many countries like Sierra Leone where minerals are mined from alluvium. Secondly, alluvium carries fine-grained fertile soil (humus) from fertile areas hence enabling agricultural activities like growing of wheat, sugarcane, and sunflowers, among other crops. Colluvium, on the other hand, has helped in the preservation of records of plant and animal remains, as well as other fossil materials. Fossils are important in explaining past life and environments, and how they shaped organisms. Colluvium deposits preserved archaeological deposits like Koster Site in Illinois and Cherokee Sewer Site in Iowa.

Differences Between Colluvium and Alluvium

To make a clear distinction between the two processes, one has to look at topography as alluvium tends to line parallel with the drainage. Colluvium and alluvium are continuous processes and may take a certain period to be noticed, and without a careful and close analysis, these two may be easily confused. These two natural processes play important roles in reshaping Earth’s surface. However, whatever brings advantages in one area may as well destroy another area. As the river flows and collects fertile alluvial materials and mineral ore, it strips the lands of significant resources leaving a barren area and weak river banks behind. Colluvium also leaves weak hill slopes beside making the slope susceptible to other natural disasters like landslides. Most landslides in the Cincinnati metropolitan area are formed by colluvium underneath slopes generally as a result of rainfall or grading trigger.


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