The floristic biodiversity in alluvial meadows is determined by hydrous fluctuations and agricultural practices. The hydrous variations influence the floristic composition through: the duration of floodings in the non-growing period which acts principally on vegetative multiplication of a few species; and the summer level of the groundwater table which influences the soil humidity conditions (saturation or drought) and selects species adapted to anaerobic or hydrous stress conditions. Fertilizer applications favour the growth of competitive species to the detriment of low or slow-growing species and lead to a loss of species richness. Meadows which are either fertilized or where the soil is usually waterlogged can be characterized by a high aerial biomass and a reduction of the biodiversity after the development of one of a few dominant species. This evolution has partly the same origin: the higher trophic or moisture level which promotes the same C-strategy species types. In order to protect or restore the extensive flood meadow biodiversity it appears absolutely essential to preserve their hydrological and soil nutrient balances.