What is the “active layer”?

We note the presence in the literature of two different concepts of the term “active layer” in relation to fluvial sediment transport. It has been used to represent the current dynamically active streambed surface, or to represent the depth of event‐scale scour and fill. These concepts involve distinct length and time scales. We propose that, when the distinction is important, the concepts be distinguished as either a “dynamical active layer” or an “event active layer.”