A drowned river valley is a(n) what?

A low-lying area of grassland adjacent to a river or stream that can be artificially flooded and drained to promote enhanced grazing for livestock or the production of grass for hay. Found in northwest Europe possibly from Roman times onwards, their construction and use became more common in medieval and later times, especially after the early 17th century when intricate systems of water management using sets of cut channels known as ‘drowners’ for flooding the meadow and ‘drains’ for reducing the water level were introduced. Weirs, sluices, and hatches were installed to control the flow of water. Some water meadows cover up to 60ha. Once established a water meadow is capable of sustained operation over a long period. In southern England a few are still used today, although many more have fallen into disrepair.