The Site is a former tidal estuary of the Rhine and Maas (Meuse) river system, which was transformed into a permanent freshwater lake by its separation from the North Sea in 1987. In 2014 the Site boundary was delineated more accurately and aligned with the Natura 2000 boundary, reducing the area by 219 hectares. The Site’s islands, forested peatlands, mud flats and intertidal marshes are very important resting, moulting, feeding and breeding grounds for many species of waterbirds which are threatened in Europe, such as Kentish plover (Charadrius alexandrinus) and common tern (Sterna hirundo). The Site also supports over 50,000 wintering waterbirds and more than 1% of the population of nine bird species. Meanwhile it plays an important role in flood regulation. Human activities include commercial and recreational fishing, shipping traffic, nautical sports and water management. The main threats to the Site’s ecological character are pollution, eutrophication and bird disturbance from recreational activities.