Scour is one of the most important regulators of biofilm structure and function, especially in floodplain rivers where low gradients and flood frequencies limit potential for scouring. In this study, we experimentally examined the effects of flow velocity on the biomass and taxonomic composition of epixylic biofilms from floodplain reaches of the Murrumbidgee River, south-eastern Australia. Six blocks from each combination of colonisation period (30 or 70 days) and condition (wet or dried), were individually exposed to no velocity (control), or velocities of 0.3 m s^-1 (low), 0.55 m s^-1 (intermediate), or 1 m s^-1 (high) in a laboratory flume. Biofilms exposed to all the experimental velocities had significantly lower dry mass (F_3,94; P < 0.001), ash-free dry mass (F_3,94; P < 0.001) and chlorophyll a (F_3,94; P < 0.001) than the control. Losses of ash-free dry mass (F_1,94; P < 0.05) and chlorophyll a (F_1,94; P < 0.001) were significantly higher from wet biofilms exposed to each velocity than from dried biofilms. All velocities resulted in a substantial reduction in taxonomic richness among all treatments, with filamentous chlorophytes completely removed by velocities of 0.55 m s^-1. These results indicate the potential to delineate thresholds for the response of biofilm biomass and algal taxa to flow velocity in floodplain rivers based on knowledge of antecedent conditions regulating biofilm development. This information significantly improves our understanding of the potential for ecological change using environmental flow releases in low-gradient floodplain rivers.