The thermal structure of industrial cooling water discharged into a long, narrow and shallow, straight open boat slip (Ottawa Street Slip, [OSS]) was investigated by field measurements during the hottest summer month in 2006. Three-dimensional hydrodynamic and thermal transport models were established and verified with measurements. The main purposes of this study were to understand the mechanism of the thermal structure in the OSS during the hot summer season under the present cooling water discharge conditions, to investigate the influence of harbour water on the thermal structure in the slip, and to establish a means for scientific predictions of the impact of cooling water discharges in a future study. Toward this end, the water temperature at multiple locations along the OSS and meteorological data near the study site were collected during the summer period of 2006. The collected data reveal: (1) during the measured summer period, the water temperature in the slip can be higher than 30°C during a period of high air temperatures; (2) water temperature variations within short periods of 15, 30, 60, and 120 minutes were no more than 4°C during the entire measurement period; (3) water temperature in the slip is controlled by both air and cooling discharge temperatures, and the cooling water temperature's increase due to industrial cooling processing seems to be relatively independent of the intake water temperature; therefore, the water temperature in the slip varied mainly with the air temperature; (4) since water temperature in the slip seemed to closely follow the intake water temperature, the intake channel may need to be optimized to maximize the possibility of getting the coolest water available from Hamilton Harbour; and (5) the cooler harbour water could not penetrate deeply into the slip. The collected water temperature data were also used for verification of three-dimensional hydrodynamic and transport models. The simulation results showed that the established model could reasonably well reproduce general thermal structures in the entire slip. This achieved the ultimate goal of the study for establishing a model to assess the impacts of further increase of cooling water discharge into the OSS.