[en] Infrared photographic data of the hot water plume at the Gentilly nuclear powerplant were obtained from an aerial survey of the St. Lawrence River. Important parameters such as the longitudinal decrease of temperature or lateral spread of the rejected water became available together with hydraulic and thermal ambient conditions. At first, a three-dimensional model was applied to the actual volume of water rejected, and it predicted a dilution rate much higher than the actual one. The use of a different geometric scheme has reduced the gap between predicted and actual values. The same scheme has been tried on a two-dimensional model. The two-dimensional approach is justified by consideration of the bottom topography. The discussion of results concerns: first, the establishment of an adequate geometric scheme and second, the error brought by the two-dimensional approach. (author)