This article describes a way to determine the properties of resonance (reflecting) waves in open water channels. For channels that are sensitive to resonances, information about the first resonance mode is required for controller and filter design. This research applies standard system identification techniques and is tested on an actual channel at the Central Arizona Irrigation and Drainage District, Eloy, AZ. The identification experiment results in good estimations of the frequency and magnitude of the first resonance peak of the open water channel.