Bedrock river lateral erosion plays a crucial role in landscape evolution, sediment transport and deposition, and the occurrence of geohazards. Lateral erosion is driven by the impacts of bedload particles (BPs). However, BPs generally move parallel to the channel walls and thus need to be deflected sidewards to cause wall erosion. Sideward deflection of BPs occurs when they interact with roughness elements (REs) fixed on the riverbed. We set up 21 sets of flume experiments to systematically investigate how spacing (5, 10, 20, 30, 40, 50, and 60 mm) and size (5, 10, and 20 mm) of REs influence sideward deflection of BPs. The deflection length and speed peaks at intermediate values of the spacing of REs. The likelihood for a BP to leave the roughness zone decays with the BP’s distance to its edge. Our results suggest that lateral erosion rates in bedrock channels are dominantly controlled by the position of the roughness zone within the channel and its relation to the particle path.