Ouse Fen

Fens, also called Fenland, natural region of about 15,500 sq. mi (40,100 sq. km) of reclaimed marshland in eastern England extending north to south between Lincoln and Cambridge. Across its surface the Rivers Witham, Welland, Nen, and Ouse flow into the North Sea indentation between Linconshire and Norfolk known as The Wash, but the natural drainage has largely been replaced by artificial channels. The area is essentially a flooded clay plain with slight “island” eminences, notably Ely.

Encylopedia Brittanica