A field experiment on the development of sedimentary structures in a gravel‐bed river

In gravel‐bed rivers, bed sedimentary structures exert a significant influence on the stability of the bed, particle motion and stream dynamics. A detailed description of the development of sedimentary structures is essential to define the overall activity of sediment transport. This paper aims to improve our understanding of the dynamics of sedimentary structures at the reach scale using a field approach. The objectives of the study are (1) to describe the processes of sedimentary structures development from a manipulated bed surface and (2) to show the effect of keystones on the development of the sedimentary structures by monitoring the displacement of individual tagged particles. We investigated the dynamics of sedimentary structures in three 15 m long reaches of a small gravel‐bed river characterized by moderate slope and high relative roughness. The bed material is composed of a range of gravel sizes with sparse boulders that remain immobile during most flows. We have disrupted the bed surface material of the first reach to create a disorganized bed surface. Two reaches where the bed material remained undisturbed were used as control sections. Detailed topographic maps and passive transponders were used to describe the formation processes of sedimentary structures. The initial manipulations of the bed led to a more fragmented surface in the experimental section. However, sedimentary structures developed very quickly, with an equilibrium bed surface developing after only two sediment transport events of low magnitude discharge. This rapid adjustment of the bed is associated with the development of key sedimentary structures anchored by the large boulders. At the reach scale, these structures increased the probability of congested areas in the displacement of bed material. In both control sections, sedimentary structures were maintained during the experiment. Copyright © 2007 John Wiley & Sons, Ltd.