The summer 2003 hydrological drought has clearly shown the vulnerability of human activities exposed to water scarcity. Public authorities who have to monitor hydrological extremes should provide water resources assessment and forecasting tools. The objective of this research is to combine these two concerns through the development of a decision support system: PRESAGES. Low water levels generation being a long-time process, monitoring the monthly runoff production has been made with simple indicators. The long-term assessment of the worst case scenario can therefore be considered through modelling. If severe low flows may occur, short-range forecasts of daily mean discharges are proposed for the summer period.