This is the Great Ouse. There are also rivers called the Ouse in Yorkshire and Sussex; these are known, respectively, as (you guessed it) the Yorkshire Ouse and the Sussex Ouse. 'Ouse' is simply a Celtic (or pre–Celtic) word for water, or a slow–flowing river. In other words, it's pretty much the same word as 'ooze'.
Just past the village of Earith (between St. Ives and Ely), the Great Ouse splits to form the New and Old Bedford Rivers (both built in the 17th century to aid drainage), as well as the so–called 'old course'. The three branches are re–united about 20 miles downstream, at Downham Market – about 10 miles from Kings Lynn, from where the Great Ouse flows into the Wash via a five–mile cut that was completed in 1821.
The New Bedford River is also known as the Hundred Foot Drain, in reference to the distance between the two embankments on either side of it.
The 'old course' effectively forms the south–eastern boundary of the city of Ely.
© Macclesfield Quiz League 2017