An acoustic ice tomography experiment which extended over a 12-month period was conducted in the Sabine Bay area of the Canadian Archipelago. Acoustic transmitter and receiver arrays were deployed in the ice for this experiment. The vertical arrays penetrated through the ice and extended into the water column. The acoustic data collected from the transmitter/receiver pairs in the water column are analyzed to investigate the changes in the sound speed structure of the water column. Analysis of the acoustic data shows significant changes in the sound speed structure in the upper 2 m of the water column during the summer melt phase. We associate these changes in sound speed with the influx of fresh water caused by the melting of ice cover, snow, and runoff from rivers. Other features of the acoustic data suggest (1) that the fresh water remained stagnant and the ice growth during the winter months was due to the freezing of the fresh water and (2) that ice formed around the transducers as the fresh water came into contact with it. As the study presented in this paper was not one of the goals of the tomography experiment, support measurements to validate the conclusions from the acoustic data, such as measurement of temperature and salinity in the water column, were not made. However, the analysis presented is a demonstration of the usefulness of this technique to monitor the changes that take place in the region beneath the ice cover.¿ 1997 American Geophysical Union