Nordre Øyeren is northern Europe’s largest inland delta; it is formed by three rivers, Glomma, Nitelva and Leira. The Site includes low-lying islands and land adjacent to Lake Øyeren. Large variations in water level and the influence of the rivers create varying natural conditions which boost species diversity, and Øyeren is Norway’s most species-rich lake, with large populations of birds, fish, benthic organisms and plants. Water levels are lowest in early spring when large areas of mud banks are exposed, providing excellent access to food for abundant birdlife staging and feeding on the Site during its spring migration. This makes the lake one of the most important areas in southern Norway for resting migratory waterbirds. The Site is also an important wintering site and has Norway’s largest numbers of whooper swan (Cygnus cygnus). Characteristic fish in shallow bays with clear water include roach (Rutilus rutilus), golden orfe (Leuciscus idus), European perch (Perca fluviatilis) and Aral bream (Abramis brama). The Site is important for erosion protection. Major scientific studies take place in the Site. It is used for fishing, agriculture and recreation: there are two birdwatching towers and a nature information centre.