The Drowned Land of Saeftinghe is situated in an estuary: a river mouth that is strongly influenced by the sea. Freshwater from the River Scheldt meets the saltwater of the North Sea. In addition, high and low tide have great influence on the area. The interaction between fresh and saline water and the different tides is what Saeftinghe owes its exceptional ecological value to.
Tidal Influence from the Sea
The Western Scheldt is directly influenced by the tidal currents in the North Sea: twice daily the water level rises and falls with high and low tides. The Western Scheldt is shaped a bit like a funnel: wide at the sea and smaller towards Antwerp, and incoming tides push saline water up the funnel. As a result, the highest tidal differences of the Netherlands are found in Saeftinghe, where the difference between high and low tide can be 4.8m. During spring and neap tides (extreme high and low tides), this difference can be 7m or more.
Tides 'travel' through an area, starting at the seafront, which means that the precise time of high tide is not exactly the same at every location. For example, high tide in Vlissingen (which is on the sea) arrives 2 hours before it arrives in Antwerp (which is located up the Scheldt estuary), and low tide has a similar delay of 2 ½ hours. Due to its size, there is even a delay in tide times within Saeftinghe itself.
Water Levels and the Weather
The water levels in Saeftinghe are influence greatly by the weather, as well as the tides. During storms (which mainly come from the northwest), much more rainwater and runoff gets pushed into the Western Scheldt than normal. This can easily raise the water level in Saeftinghe by an extra metre. With spring tides, the water can violently swirl against the sea walls, and Saeftinghe itself becomes an endless stretch of water. With such a high influx of storm water, Saeftinghe is unable to fully drain itself quickly and, even at low tide, Saeftinghe is inaccessible.
Fast Incoming Water
Water levels in Saeftinghe rise at a surprising speed. This is namely due to the large sand bars that form a barrier at the mouths of the major creeks. Particularly in the larger 'Hondegat' and 'Ijskelder' creeks (roughly translated as Dogshole and Icebasement creeks), water can stream in and fill the creeks in just 2 hours.
While the high tide is approaching, water can still be filtering out of Saeftinghe. It is only when high tide breaks the sand bar threshold that the water really starts to pour into the creeks and you notice the tide coming up. Out of nowhere, a flood wave of several inches can come your way and, from one moment to the next, water can reach knee height - an unpleasant and dangerous surprise! The current can be very strong, even at knee height, and you may find yourself unable to cross the marsh. Deep in Saeftinghe, in the smaller creeks, a rise of 1m in just 30 minutes is very normal. This is what makes Saeftinghe dangerous, because it is possible to become isolated and cut off from the land in a very short period of time. This means that it is of the utmost importance to plan your visit carefully.
Brackish Water and Seepage
The water in Saeftinghe is brackish: a mixture of saline seawater and fresh river water. The salt content of water in the Western Scheldt changes depending on the tides: at high tide, the seawater penetrates deeper inland, whereas with low tide, saltwater is pushed back down the river towards the coast. The overall effect of this is what makes the water in Saeftinghe brackish - about half the salt content of seawater. Given the size of Saeftinghe, it is unsurprising that there is a clear salt gradient between the West and East.
The salt content of soil depends on its height above sea level and its soil type. In general, the higher the land is, the fresher the water is. Some marsh flats, which possess an impenetrable layer of clay, are filled with rainwater.
Higher sandy features, which run close to or through Saeftinghe, such as the Gas Dam, cause freshwater to seep up into the creeks. Freshwater seepage also occurs where particularly high levees exist, such as in creeks like the ‘Rottegeul’ (Rotten creek). The seepage has the appearance of a bubbling spring, sometimes as big as 10 inches wide, and water bubbles up in the middle of the creek!