The German North Sea coast, some 300km/185mi long as the crow flies, is divided into two parts, the East Frisian and the North Frisian area, by the estuary of the Elbe. Off both stretches of coast lie a string of islands which have become popular holiday resorts. The coastal area is an expanse of flat fenland protected by dikes, outside which is the Watt, an area of mud-flats that are dry only at low tide. The estuaries of the East Frisian rivers are protected from the inflow of sea water by siele, gate-like sluices that close automatically at high tide.
Off the North Frisian coast are the Halligen, islands that are relics of an expanse of fenland not protected by dikes. Some of them are connected to the mainland by causeways.
Ecology
The harmful substances with which the North Sea is polluted come from various sources. Many rivers carry down salts, heavy metals and chemical residues from industry and discharge them into the sea. Further pollution is caused by the dumping of dilute acids, the disposal of incinerated refuse and the uncontrolled jettisoning of ships' waste. The mortality among seals and the explosive growth of algae in recent years have been the most visible results of this endangering of the environment. In many areas there have been joint efforts by local people and holiday-makers to control pollution of the sea; a first "Holiday-makers' Parliament" was held at Frankfurt am Main in 1988; and the Association for the Protection of the German North Sea Coast (Schutzgemeinschaft Deutsche Nordseeküste) is active in working for a clean North Sea and an unspoiled coast.