Colluvium vs Alluvium

UPDATE: The tremendous response to this blog post led to an international and interdisciplinary survey on how these terms are/should be defined. The results of that survey were combined with an in-depth review of the literature to produce an article that has now been published in Earth-Science Reviews. The article updates and refines the patterns in term definitions described in the blog post below. Although the journal article cannot be directly updated, if you would like to take the same survey, it is still available here.

As I interact with various Earth scientists around the world, I’ve encountered slightly different definitions for colluvium and, by association, alluvium. The definitions for colluvium and alluvium are interdependent because their respective definitions are partially based on distinguishing one from the other. In other words, a set of characteristics is being used to split into two classes something that could be defined as one thing. Nonetheless, their distinction is important for communicating key characteristics, geomorphic setting, and inferred processes.

Let’s use Wikipedia as a source for a democratic basis for defining things. In the English version, alluvium is described as “loose, unconsolidated soil or sediments, which has been eroded, reshaped by water in some form, and deposited in a non-marine setting.” Although this definition focuses on the process, this Wikipedia entry later notes that when the same material is found in certain geomorphic situations it is called something else. Specifically, lake sediments are referred to as lacustrine and river sediments are fluvial.

Like the definition for alluvium, the English Wikipedia definition for colluvium includes rainwash and sheetwash. However, this definition adds downslope creep as a possible process, which shifts the main driver towards gravity. Of course, gravity drives water erosion, but there is an important difference between sediments transported by flowing water versus those that are moving primarily by gravity with just some lubrication from water. For disambiguation, we should include most forms of mass wasting as producing colluvium under this definition (maybe one of us should edit the Wikipedia page). Though we do have to exclude rockfalls (i.e. when the material is rock fragments), because they produce talus.

Because of the overlap between processes listed for these definitions, the distinction between the two leaves the emphasis more on their location. In particular, colluvial sediments are described as being at the base of hillslopes. Oh, but what constitutes a hillslope? In order to have sediment material truly moving by mass wasting processes, the slope has to be somewhat steep. This is where the characteristics of the deposit come into play. If the particle sizes are relatively large (including rock fragments) and fairly mixed (heterogeneous), then it’s more likely that gravity was a more direct player in the transport. Water would have a more difficult time transporting larger rocks alone and would tend to sort the particle sizes during transport and deposition. A problem with using particle size or even heterogeneity to distinguish colluvium from alluvium is that these things also depend on the source material. If the source material is fine textured and homogenous, then it could be difficult to tell if it was transported by processes associated with colluvium or alluvium.

Using geomorphic setting to define the two terms presents another problem in setting clear definitional boundaries for these terms. If river sediments are fluvial and colluvial sediments are at the base of hillslopes, where are alluvial sediments to be found? One possible solution is that alluvium is found on floodplains, where river flow would only be temporary. But in practice, the constantly changing level and path of streams can make it difficult to distinguish river bed sediments (fluvial) from floodplain sediments (alluvial).

Unfortunately, in addition to the blurry boundaries found in the English use of these terms, these definitions for alluvium and colluvium are not exactly the same around the world. For example, Europeans have a broader use of the term colluvium. This may be related to the dominance of erosional-depositional features on hillslopes with long histories of agricultural use. In this landscape, a major characteristic is the pattern of eroded soils on hilltops with the sediments from those hilltops accumulating at the base of the hillslopes. Looking at the German Wikipedia entry provides some insight to how working in the European environment has resulted in a slightly different use of the terms. In that definition, colluvium is the product of alluvial (anschwemmung) processes, but is deposited, having not yet reached a perennial stream. In contrast, alluvium (alluvionen) is sediment deposited on seashores, lake shores, and by rivers.

The verdict here is that the setting of the line between colluvium and alluvium can be different depending on who you are talking to. So the first lesson is that one needs to consider the context and the author’s background when interpreting the meaning of these words. The rough (and imperfect) chart below may provide some guidance. However, the more important lesson is the significance of classification in science. Classification is not only a means to facilitate communication; it is also a representation of our understanding of how things work and what is important to distinguish. Naturally, as science develops in different regions, different definitions of terms arise.

This blog post is just a first attempt to summarize the popular use of the terms colluvium and alluvium. Do you have different definitions for these terms? (Hey Australians!, where do your definitions fit in all of this?) Who/what do you think should be the official authority on their meanings? Let us know your opinion on this topic by posting a comment.

For more, see: