The safety of dams is of extreme importance, considering the high economic value and cost of failure. To assess the performance of these structures over their lifetime, vibration monitoring can be a useful tool. However, in hydroelectric generating power plants, rotating machinery can introduce harmonics into the measurements and make current methods of modal parameter estimation invalid, necessitating harmonic removal. While recent research has focused on harmonic removal in rotating mechanical machinery like wind turbines and large generators, there is a gap in understanding of monitoring gravity dams in environments with significant harmonic contamination. Most ambient vibration tests of dams have been conducted on arch and buttress-type structures, which have experienced relatively low harmonic contamination. To address this gap, a study was performed on the Mactaquac Generating Station, the largest hydroelectric power plant in the Maritime provinces, using full-scale monitoring data as part of the Mactaquac Life Achievement Project (MLAP). The study found that under low operation demand, it was possible to successfully estimate the benchmark modal parameters of the structure. However, during high operational demand, the measured signals were dominated by harmonic content, which caused several errors in the modal estimation results compared to periods of low demand. However, by reducing the harmonics in the signal, the first four mode shapes of the dam structure were successfully identified. This demonstrates that harmonics can significantly impact the accuracy of mode estimation, but their effects can be mitigated through harmonic reduction techniques.