Absorption of CO2 at the horizontal gas-liquid interface of flowing shallow water was enhanced by waves induced by a vibrating air column. Wave amplitude and mass transfer peaked at wave frequencies of 4.5-6 Hz. At the peak frequency, mass transfer increased quadratically with wave amplitude and was independent of liquid flow rate. Peak mass transfer was approximately 45% greater than in the absence of waves. Increasing the viscosity of the liquid phase reduced mass transfer.