Proliferations of benthic cyanobacteria in the genus Phormidium are a global concern because of their increasing prevalence and ability to produce harmful toxins. Most studies have been observational and have linked physicochemical variables to Phormidium cover measured at the reach scale. Authors of these studies have alluded to nutrients and flow as key factors in accrual. Our goal was to use an experimental approach to examine how changes in velocity and NO₃⁻ concentrations influence Phormidium accrual. We hypothesized that: 1) Phormidium biomass accrual would be positively correlated with stream velocity; 2) biomass accrual would be positively related to NO₃⁻ concentration, which would have a stronger effect during early accrual; 3) an NO₃⁻ ✕ velocity interaction would arise from saturation of accrual at high NO₃⁻ and high velocity; and 4) the probability of detachment would increase with decreasing velocity. We assessed mat expansion, biomass (as phycoerythrin and chlorophyll a [Chl a] concentrations, and biovolume), and algal assemblage composition in flow-through channel mesocosms for 16 d. We crossed 2 velocity treatments (0.1 and 0.2 m/s, slow and fast, respectively) with 3 NO₃⁻ treatments (0.02, 0.1, and 0.4 mg/L, ambient, medium, and high, respectively). Velocity was positively correlated with all measures of Phormidium biomass, but patch expansion rates increased at similar rates across all treatments. NO₃⁻ had no effect during early accrual, but phycoerythrin concentrations increased with increasing NO₃⁻ in fast-velocity treatments. At the end of the experiment, patch size was greater in the high-velocity treatments because of a greater number of partial or full patch detachments in slow-velocity treatments. These results suggest that NO₃⁻ concentrations do not affect Phormidium expansion and detachment, but may be important during colonization (not investigated), and that mat expansion occurs at a similar rate regardless of velocity, but termination of accrual cycles occurs earlier in slow velocities.