In the Netherlands, the drainage system is an important matter. The Dutch need a well developed water control system in order to keep large areas from being flooded, because some parts of the Netherlands are below sea level. In Alblasserwaard, problems with water became more and more apparent in the 13th century. Large canals, called 'weteringen', were dug to get rid of the excess water in the polders. However, the drained soil started setting, while the level of the river rose due to the river's sand deposits. After a few centuries, an additional way to keep the polders dry was required. It was decided to build a series of windmills, with a limited capacity to bridge water level differences, but just able to pump water into a reservoir at an intermediate level between the soil in the polder and the river; the reservoir could be pumped out into the river by other windmills whenever the river level was low enough; the river level has both seasonal and tidal variations.
Full control over the water level was never achieved. Throughout the centuries, the residents of the western part of the Netherlands suffered inundations, especially because of dyke ruptures; this is reflected the legend of the floating cradle at Kinderdijk and the figure of Hans Brinker with his finger in a ruptured dyke.