Water flowing on earth's surface doesn't flow randomly. Rivers shape the watershed. In a watershed all is inextricably linked : the climate, water flow and the ecosystem. Understanding how it works is trying to grasp the links between hydrological, climactic and biological phenomena and their impact on human activities.

For many years, numerous experiences have been performed at the watershed or sub-watershed scale by scientists in order to isolate pollution problems. Everything leads to believe that the watershed is the best chosen physical unit for putting in use methods of fighting pollution.

" The watershed is an area that receives water from precipitations and feeds watercourses. Limited by a water divide line, watersheds possess a hydrological network that collects runoff from slopes as well as sediments, particles of organic matter and dissolved substances. We can consider waterways as the arteries of watersheds transporting water, mineral elements and organic matter towards the sea." C. AMOROS and G.E. PETTS (1993)