It is believed to be a mainly man-made anabranch north of the confluence with the River Pinn to feed watermills in the Parish of Hillingdon.

The river is believed to be named after John Fray who owned Cowley Hall in the fifteenth century.

Two of the three mills in Hillingdon Parish recorded in the Domesday book are believed to have been located on the southern section of the river.

The Frays continues south, flowing past Little Britain Lake on its western side before crossing under the Slough Arm of the Grand Union Canal which is also carried above it in an aqueduct.

The river flows through two Sites of Special Scientific Interest managed by the London Wildlife Trust: Frays Farm Meadows[2] and Denham Lock Wood.