Two main types of landscapes characterise the floodplains of the alpine arc: the zones where the river runs free and the zones where it is embanked. These result from (i) the relative importance of the natural alluvial activity (sedimentation, erosion) and of the human constructions (embankment, drainage); (ii) but also from two general principles that guide Man in its interventions: security (hydraulic, alimentary, and energetic) and conservation of biodiversity (natural floodplains). The goal of this research is to bring to light, through an analysis of the exceptional events (major floods, etc.), the evolution of the relationships between these two general principles between 1850 and 2000, and to establish tools to help decision making for floodplains management.