Don't ask

because it's complicated. Very briefly - Cornelius Vermuyden, later knighted, designed and executed a system of extra waterways and diverted a river to reclaim land which was flooded. This was back in the 1650s.

In essence extra waterways enabled water to drain into the sea - The Wash. Where needed wind pumps would move water to higher levels to effect drainage.

The plot thickens. Water volumes varied and sometimes the system could not cope and the land would flood. At the same time the growing county of Essex, far to the southeast from here was running short of water. One of the drainage channels was linked to Essex, and by cunning elevation of its water level flow was reversed so Essex had a water supply.

I probably get 10/10 for simplicity but only 5/10 for detail. look at the map to see the complexity and the rather wonderful accomplishment of the engineers of the time.