The IJsselmeer and Markermeer: a natural water tank
A key function of the IJsselmeer and Markermeer is to hold freshwater for the drinking water supply and to use in periods of drought, for instance. So we also refer to these lakes as our 'water tank'. Apart from that, we drain excess water via the IJsselmeer and the Afsluitdijk to the Wadden Sea.
How does the water get to the IJsselmeer area?
The IJsselmeer receives 70% of its water from the River IJssel, a tributary of the Rhine. The volume of water is determined by factors including how much snow has fallen in the Alps and the rainwater in the Rhine catchment basin (Switzerland, France and Germany). The IJsselmeer area is otherwise filled with rainwater and water from streams, small rivers and control locks or pumping stations.
How do we drain the water?
The excess water in the IJsselmeer flows through the control locks in the Afsluitdijk to the Wadden Sea. We call this process sluicing. During this process, gravity acts on water from the IJsselmeer, allowing it to flow naturally into the Wadden Sea. This happens when the water on the IJsselmeer side is higher than on the Wadden Sea side.
We also drain the excess water from the Markermeer. This water flows via the control locks in the Houtribdijk to the IJsselmeer and onwards, via the control locks in the Afsluitdijk, to the Wadden Sea.
We are not always able to sluice this water. If the level of the water in the Wadden Sea is higher, the water will not flow from the IJsselmeer into the sea. This happens, for instance, at high tide or when there are strong winds from particular directions. We also have less opportunity for sluicing operations due to rising sea levels. So we are installing extra control locks and new pumps in the Afsluitdijk. If it is not possible to sluice the water away, the pumps can assist in draining the water into the Wadden Sea.
Changing water levels in the IJsselmeer area
The standard water level in the IJsselmeer area is different in summer and winter. This means we can respond adequately to the changing weather conditions in the different seasons.
Summer level
In summer, we ensure that there is enough water to provide mains (drinking) water, irrigation for agriculture, to flush the reclaimed polders to reduce salt levels in the soil and recreation. At that point, the water level in the IJsselmeer area is higher. We call that the summer level.
In summer, the water level is allowed to fluctuate between -0.10 m relative to mean sea level at Amsterdam (NAP) and -0.30 m NAP. A flexible water level, in other words. An NAP level of 0 m is approximately equal to the average water level in the North Sea.
Winter level
In the winter, water levels may rise due to increased rainfall and rivers draining greater volumes of water. That is why we lower the water level of the IJsselmeer area at the end of the summer, so that it can absorb the excess water and drain it to the Wadden Sea. In the winter, we make sure that the water does not fall below a level of -0.40 m below NAP.
The impact of weather on water levels
But we don't have complete control of water levels. In addition to increased river drainage and the water level in the Wadden Sea, rainfall and wind also exert an influence. Wind can make the IJsselmeer appear to be 'slanted'. A particularly strong wind pushes the water in a single direction, so the water on one side of the IJsselmeer or Markermeer is higher than on the other side.