This work presents the development of a model that applies water quality restriction to the long term planning hydro-thermal power system operation. Taking water quality into account in the long term planning, guarantees the multiple water usage. The objective is to investigate the sensibility of the long term planning hydro-thermal power system to the environmental variables focusing water quality. Among water quality indicators the option was chlorophyll-a and the nutrient phosphorus establishing a relation with the reservoir storage. (C) 2008 Elsevier Ltd. All rights reserved.