The name Ouse is from the Celtic or pre-Celtic *Udso-s,^[2] and probably means simply “water” or slow flowing river.^[3] Thus the name is a pleonasm. The lower reaches of the Great Ouse are also known as “Old West River” and “the Ely Ouse”, but all the river is often referred to simply as the Ouse in informal usage (the word “Great”, which originally meant simply big, or in the case of a river long, is used to distinguish this river from several others called the Ouse).

The old course of the river passes through Hermitage Lock into the Old West River. After joining the Cam near Little Thetford, north of Cambridge, the course passes the cathedral city of Ely and Littleport, to reach the Denver sluice. Below Denver the river is tidal and passes Downham Market to enter The Wash at King’s Lynn.