Just to elaborate on the above with an image to guide the conversation:
And then going off the wikipedia description:
A polder (Dutch pronunciation: [ˈpɔldər] ⓘ) is a low-lying tract of land that forms an artificial hydrological entity, enclosed by embankments known as dikes. The three types of polder are:
Land reclaimed from a body of water, such as a lake or the seabed
Flood plains separated from the sea or river by a dike
Marshes separated from the surrounding water by a dike and subsequently drained; these are also known as koogs, especially in Germany
I sense the issue is that Elst introduces a localized terrain feature, but references a larger-scale concept.
In the same vein, if I call M3 (below) "Forest", maybe it's not
technically wrong, but it's not really a great choice.
IMO, Irrigation Ditch, so something along those lines, will make everyone happy(er).