Although many hillslopes are covered by rock fragments, their effects on hydraulics of overland flow and subsequent soil erosion have not been studied comprehensively. Scarce results indicate that rock fragments can both increase and decrease soil erosion due to overland flow. To provide information necessary for process-based models, the effects of various rock fragment covers on overland flow hydraulics and soil erosion were analyzed simultaneously in flume experiments. Sediment yield was found to be caused by horseshoe vortex erosion around rock fragments. For a given discharge and slope the dimension of the scour holes was affected by the space between the rock fragments. This lead to a non-monotonic variation of sediment yield with cover percent, although flow hydraulics like mean flow velocity varied monotonically with cover percent. This indicates that under these circumstances soil erosion can not be causally related to spatially averaged hydraulic parameters.