We measure expansion rate and bending in a 23-hour time series of acoustic images of the lower 25 m section of a buoyant hydrothermal plume rising from Grotto vent in the Main Endeavour Field, Juan de Fuca Ridge. We then calculate entrainment coefficient, the constant of proportionality relating mean inflow velocity at the plume edge to maximum mean upward velocity within the plume. The plume section alternately bends southwest at relatively high inclinations (37°) and northeast at lower inclinations at irregular intervals twice during this time period, apparently driven by current reversals in the mixed semi-diurnal tidal cycle. The measured expansion rates (0.11–0.25 m/m) and calculated entrainment coefficients (0.07–0.18) are directly proportional to the degree of bending (R² = 0.75). The loss of buoyancy flux related to enhanced mixing in a stratified environment during bending may contribute to reduction of potential rise height consistent with predicted (∼400 m) and measured (300–350 m) plume tops.