Boezemstelsel of Rotterdam and
The boezemstelsel is a Dutch waterworks unit, a reservoir system of interconnected bodies at a common, adjustable level betwixt that of its polders and the sea.
With its prescribed but variable level, the boezem system is a buffer, storing excess water pumped upward, and augmenting the supply at polder-bottom in drought.
And maintenance of strict levels is necessary in the polder. The rich organic nature of much of western Netherlands' soil means that if it's exposed to oxygen, it will decompose causing the land to subside. (And it has this is the reason that delta land is now under sea-level.)
A boezem is usually shared by several polders and often consists of a network of diverse bodies of water canals, lakes, ditches and/or waterways.
Electric pumps formerly wind, steam and then diesel elevate water from the polder floor to the boezem. The boezem holds the water until agreed measurements allow its release usually into a river. Volume, speed, and quality of the output must be considered.