Tidal waters have created the characteristic profile of the islands, flats and salt marshes of the Wadden Sea. They are the pulse of the Wadden Sea and the foundation of the area's unique eco-system and its myriad of animals and plants. 

The landscape and life that greet visitors to the Wadden Sea are first and foremost a product of the tide. For millennia, the sea has invaded across the flat shoreline at high tide and left large areas of the sea floor exposed as it retreated at low tide. Over time, this oscillating movement has carried large quantities of sand and nutrients across the mud flats. The shifting water levels have created both the wide, white beaches and the lush salt marshes.

The pulse of the Wadden Sea

There is no other place in Northern Europe where tidal waters are as important to the landscape as by the Wadden Sea. The gradual transition from land to sea is a product of the constant movement of tidal waters across the mud flats. There are exactly six-and-a-half hours between high and low tide, which means that the ocean floods the Wadden Sea twice in a period of 25 hours. This rhythm is the pulse of the Wadden Sea. The waters bring nutrients and organisms to the flat areas and create the food source of a rich eco-system with an incredible biomass of benthic animals. Tidal waters are a result of the Moon's gravity. The difference between high and low tide is about 1.5 metres, but weather conditions and water currents can increase this difference. In extreme cases, this can lead to a storm surge.

The water distributes sand and mud

The uniqueness of the Wadden Sea depends on a certain difference between high and low tide and a wide, flat area that the water can rise across. During the last glacial period, flat river plains were created, which were then flooded by the rising seas during the transition to the Neolithic period that followed. The North Sea was formed at the same time, albeit in a smaller version than the one we know today.

During the following millennia, the tidal waters ebbed and flowed.  The seas continued to rise slowly, and this meant that the water encroached further and further across the flat plains of the Wadden Sea. As waters flow inland, they deposit a sediment containing both sand and mud. The sand is the heaviest of these two. Therefore, it already begins to settle once the current slows down offshore. The mud, however, only settles once the waters come to a stop further inland. This is why sand is found at the shoreline and the more nourishing silt is found in the river valleys further inland.

A coastline built on mud

During the past 8-9,000 years, the sea level by the shores of southwest Jutland have risen by 17 metres. This means that during that same period, the sea and the pulse of tidal waters have deposited new coastline up to 17 metres thick. Drilling down through the layers of salt marsh has revealed that, under many metres of deposited silt-clay, there is a layer of peat and sand from a time before the tidal waters took over.

Rømø itself is an example of how tidal waters can create a coastline. Rømø is named after a so-called 'rimme', which originally described a low embankment off the coast. However, the layer of sand thickened as water flowed across the 'rimme'. The sand then swirled together to create the dunes that formed the island. There is so much lee behind the dunes that the water remained still long enough to deposit mud, and the salt marshes began to take shape. This is how tidal waters created the unique landscape of the Wadden Sea.

Storm surges and dykes

The deposited clay soil is rich in nutrients. This is why the salt marshes are so rich compared to the moorland plains further inland. Throughout the ages, the salt marshes have been the bread and butter of the relatively large population living by the coastline of the Wadden Sea.

However, the problem was that the low-lying salt marshes were vulnerable to storm surges, which have claimed thousands of lives throughout time. Therefore, people who lived by the Wadden Sea had to settle on the edges of the higher-lying geests. In order to take advantage of the salt marshes, they began to construct artificial embankments on which to build their farms. These became the so-called mounds, which can be found all over the southern parts of the Danish Wadden Sea. The construction of permanent dykes to keep out the sea began as early as the Middle Ages. However, it was only within the last 150 years that effective sea walls were built. These modern dykes have been successful in keeping people safe from storm surges. Storm surges have not resulted in any deaths in the Danish part of the Wadden Sea for more than a 100 years.

Experience the tidal waters

Tidal waters can, naturally, be seen along the entire coastline. However, there are areas where the differences between high tide and low tide are especially apparent. Tidal waters are especially impressive in estuaries. On the edges of Ballum Sluse, south of Brede Å, there is an estuary that is full of water at high tide and almost completely empty at low tide. The same applies for Havsand Lå on Romø, where you can clearly see how tidal waters effect the watercourses of the Wadden Sea. However, the most pronounced consequence of the tidal waters can be found on Mandø. Mandø is accessed by a causeway 1.5 metres above the seabed. But this only applies during low tide. At high tide, the causeway is flooded. It is therefore crucial that you plan your trip using the tide tables or risk, in the most literal sense, driving your car into the sea.

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