Forecasting of diary stages of rivers in Colombia is reported officially in linguistic terms. This fact inspired a model based on theory of fuzzy sets, concept coming from artificial intelligence. A Fuzzy Logic System (FLS) allows to analyze observations in time classifying them in linguistic categories which are approximated either to the employed qualification, or to the way in that a person would describe the phenomenon. Taking into account experts’ judgements the level of the following day is estimated. An algorithm of global optimization named Multidimensional Downhill Simplex was implemented in order to calibrate model parameters.