Gravel bar shielding: A mechanism responsible for bar stability in gravel- and cobble-bedded streams

The stability of gravel bars and riverbanks is often attributed to the presence of vegetation, yet the conditions controlling the stability of such bars without of a vegetation cover have remained unclear. Here, we propose that such controls are exerted by what we refer to as ‘lateral lag deposits’, which result from the winnowing of gravel bar edges during periods when channels widen. We base this interpretation on an example from the Sense River, Switzerland, a natural wandering-braided stream with gravel-cobble bars devoid of vegetation. Through a survey where we measured the size of several thousands of grains along two up to 50 m-long reaches, we found that the 84th percentile values (D₈₄) of the grain size distributions (GSD) are consistently larger on the eroded bar edges compared to the bar top or the channel between bars. At these bar edges, the coarse grains appear to shield the gravel bar from lateral erosion, thus forming a lag deposit. Orthoimages taken along the studied reaches between 2017 and 2023 reveal that the target lateral lag deposits outlasted several lower-discharge floods, during which the sedimentological architecture of the investigated reach changed multiple times. We therefore suggest that lateral lag deposits are among the sedimentological structures of braided streams with the highest preservation potential and that they exert the largest threshold to the complete reworking of gravel bars without vegetation.