If you look closely at the altitude map of the Netherlands, you will see that the polders are not quite flat.But the height differences are indeed limited. In part, the polders are about former marshes and fens, as far as the lakes were concerned, they were quite shallow.
The Zuiderzee knew a deep part, the Frisian basin that was up to 8 to 9 meters deep and that now exists in the Ijsselmeer.The remaining parts were less deep, often only 2 to 4 meters, with pieces that occasionally fell dry and here and there an island.
You can find a map to zoom in on: Http://www.geo-informatie.nl/geo…
It has to do with how deposits and currents work.
Unless there is a geographical reason (for example rock bottom in fjords and mountain lakes) lakes in swamp deltas (which the Netherlands is in fact) are generally with a fairly flat bottom.
(A Salt Lake, like a billiard sheet)
Does not mean that the lakes in the Netherlands were completely flat.
But for this one has developed the Molengang :
(Slopes were captured by placing several mills in series)
If you go on a map to see the Ringvaarten and bosoms, you can still look back on the slope.
There are quite a few polders with slopes, even though they are not very noticeable, but the majority of them are indeed flat.
However, most polders are flat because the flooded land is after a flood, which is later polled. The former Zuiderzee is an example of this, once land and river, flooded in the early Middle Ages and then partly polled in the 20th century.
This was often not a lake, although it has sometimes become and has been called, but land that has become a sea for later polled land and thus with a bottom of sediment blows and those are generally flat.
Not every contiguous group of Polders has polders of the same height that comes by sea level rise.
The best place you can see this is in Friesland just south-west and northwest of Leeuwarden, especially the polders that are made from the former Middelzee. The old polders are lower than the younger polders in the north, which makes a large part of Friesland look like a bowl.
Because they are so designs.This avoids wasted space with for example Hills.