The work analyses the complex relationships that link urban planning and environmental protection referring, in particular, to watercourses defence. The close interactions existing between development of human activities on territory and the hydrological cycle point out the necessity of a watershed-scale planning. This regional planning, in fact, allows both problems of protection and optimal use of hydrological resources in terms not only of punctual actions, but also of land use. In this context, the fundamental roles played by geographical and environmental information systems are shown and analysed with special regard to their importance in fostering citizens participation in decisional processes by means of an easier access to environmental data and information available to public authorities.