The Site includes lower stretches of the Somme River and its main tributary, the Avre, together with adjacent marshes and peatlands. At its northern downstream end, it meets the Baie de Somme (Ramsar Site no. 925), which stretches to the River’s estuary. The Site is part of one of the largest alkaline peat complexes in north-west Europe. The exceptional functionality of its peatland habitats, and their different developmental stages, make the Site very important for animal and plant biodiversity. It is used by fish migrating between freshwater and ocean habitats to complete their life cycle, such as the critically endangered European eel (Anguilla anguilla). The Site is home to some of France’s most important populations of plants such as the threatened purple small-reed (Calamagrostis canescens), the creeping marshwort (Helosciadium repens), the fen orchid (Liparis loeselii) and the crested wood fern (Dryopteris cristata). It is important as a wintering location for birds such as the Eurasian bittern (Botaurus stellaris) and the Savi’s warbler (Locustella luscinioides), as well as for the breeding little bittern (Ixobrychus minutus), the Eurasian teal (Anas crecca) and the western marsh harrier (Circus aeruginosus). The Site is actively managed as a Natura 2000 site and is also protected under other designations. 70% of the population of the Somme department live close to the Site, and it is central to local farming and leisure activities. A flood risk prevention plan regulates development in susceptible areas. The Site is of wider cultural and historic importance; its ancient fluvial terraces hold the earliest traces of human occupation in north-west Europe.