From their source to the river mouth, rivers are an extraordinarily dynamic environment. The river flows through a bed that is sometimes narrow, sometimes wide, and can overflow in the event of flooding. Currents may be weak or strong, the water more or less deep, while its composition varies according to the territory it crosses, the elements removed from the land, the surrounding human activities, and so on. Rivers reflect the landscapes they cross.

All these constraints and opportunities, from upstream to downstream, are at the origin of a great diversity of habitats for numerous aquatic animal and plant species. For others, watercourses are a place to reproduce, or simply to stop and drink.

Wetlands are expanses of marshy land, natural or artificial, permanent or temporary, where the water is stagnant or running, fresh, brackish or salty. The combination of these different parameters results in the formation of soils and flora teeming with life specific to each territory, and generally abundant in algae, fish, reptiles, waterfowl and other wildlife.