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Estuaries are formed when tidal waves penetrate deep into the mouth of a river. This occurs in the North Sea because the tidal range (difference in height between high tide and low tide) is significant. The periodic tidal waves and ebb streams widen the river valley, gradually forming a funnel-shaped outlet, or estuary. The mouths of the Ems, Weser and Elbe rivers in the southern regions of the German North Sea are estuaries with a tidal range between two and three metres. The freshwater in the rivers mixes with the saltwater of the North Sea in these estuaries and becomes brackish.

The great tidal range and the brackish water make for extreme habitats: several animal and plant species are highly specialised and occur exclusively in these areas. The interplay of high and low tide and brackish water allows the spread of broad salt marshes and reed banks which are regularly or sporadically flooded. They provide the spawning area, resting site, breeding ground and habitat for many species of insects, amphibians, fish and birds such as the small European smelt, Chinese mitten crab or the goosander, a bird species in the duck family.

The intensive use of the North Sea estuaries and the pressures to which they are exposed make it difficult to improve the status of these rare ecosystems. Restoring shallow water areas and reducing the tidal range would require the opening up and moving of dykes and reconnecting the course of river tributaries to the main channels.