This paper is a preliminary analysis of the potential for atmospheric thermal pollution from a man- made reservoir. The specific site for the study is a proposed hydroelectric project on the Sogamoso River in Colombia, S.A. The region is agricultural with the two staple crops being coffee and cocoa. These two crops are sensitive to both high temperatures and more dramatically to high humidity levels. Farmers from the region are concerned that the construction of the reservoir w i l l negatively impact crop yield. We have used a numerical atmospheric model to simulate weather from the region both without and with the proposed reservoir. Simulations show the magnitude of the impact is too small to be of consequence to crop yield. With these results we draw conclusions about the potential impact of the reservoir on climate. This paper provides a brief introduction to the theory behind the climatic consequences of a large lake. Results are then discussed for two time periods, one during the rainy growing season and one during the dry season. One of the biggest uncertainties in the model boundary conditions arises from the specification of the lake temperature, so the sensitivity of the model to lake temperature is analyzed for one of the study dates. Finally it is shown how a small set of weather simulations are used to draw climatic conclusions.