This project is a preliminary study in order to build a small power plant, located beside to Gavleån River. It has been designed with the aim of cooling a district of Gävle city, Sweden. That big project is carried out by the international consulting engineering company SWECO. The mentioned plant contains a thermodynamic cycle that takes water from the river and afterwards, it is returned back warmer. It will attempt to study the temperature raise downstream along the river due to the spill of hot water. In addition, it will try to quantify and weight which may be the importance of the increment of temperature compared to the entire river. This work could be vital for an environmental impact study. The thermo and fluid dynamic problem is going to be solved using typical procedure for numerical simulations. To do this, it will be used Computer Aided Design (CAD) to model Gavleån River path and Computational Fluent Dynamics (CFD) to predict the distribution of temperatures. Finally the results of the simulations will be analyzed and discussed to draw conclusions about the final temperature raise in Gavleån River.