The Brazilian Federal Law, 12,334/2010, establishes the National Dam Safety Policy that implies in the development of intelligent systems to monitor downstream valleys’ risk. Mapping possible flooded areas for different scenarios of extreme rainfall events or operational conditions like accidents require the use of Geographic Information System (GIS) to understand the associated risks of these occurrences. Those multiple scenarios are generated and obtained after an integrated risk management methodology and can become very complex when considering several structures along the river valley forming a dam cascade. In order to focus on those complexity, a new risk management method based on the use of GIS tools for the quantification of population, income and density, as well as the extensions of agricultural and environmental protected areas associated to the probability of flooding was developed and tested. The results are displayed through active thematic maps published on a web platform and accessible to the entrepreneur and civil defense agencies. The platform can be consulted relative to several attributes to mitigate risk. The maps demonstrate which populations, infrastructures, public services, among others, are most vulnerable to flooding. They also indicate water depth and the time that sites are likely to remain flooded, therefore permitting the calculation of socioeconomic and environmental damage after occurrence. Tests were carried out in the cascade of eight dams along the 500 km Paranapanema River, on the border between the States of São Paulo and Paraná, Brazil, where approximately 55,000 inhabitants live in a region whose economy is based upon agricultural production, among twelve environmental preservation areas. Maps and facilities obtained are discussed. Thus, the new methodology based on GIS presented and the results for case study form an essential subsidy to risk management associated with dams and flood management.