The Old Bedford River is an artificial, partial diversion of the waters of the River Great Ouse in the Fens of Cambridgeshire, England. It was named after the fourth Earl of Bedford who contracted with the local Commission of Sewers to drain the Great Level of the Fens beginning in 1630. It provided a steeper and shorter path for the waters of the Great Ouse, and was embanked to prevent them flooding the low ground of the South Fens. Throughout the project, the Earl and his Adventurers faced disruption from those who were opposed to drainage schemes. The project was deemed to have succeeded in draining the fens in 1637, but that decision was reversed in 1638. After a lull during the English Civil War, when much of the work was damaged, the Dutch engineer Cornelius Vermuyden worked with William Russell, the fifth Earl of Bedford to complete the drainage. Disruption and unrest continued while the work was carried out, resulting in the Adventurers employing armed guards. A second river, the New Bedford River, was cut parallel to the first channel, which then became the Old Bedford River. At some point, the Old Bedford River was split into two parts, when the upper section was diverted into the River Delph at Welches Dam, and the lower section was joined to the Counter Drain. Both parts retain the name, but are not connected to each other. The area between the two Bedford rivers acts as a large washland, which holds floodwater when the river channels cannot cope with the volume of water in them.

The River Great Ouse is a river in England, the longest of several British rivers called "Ouse". From Syresham in Northamptonshire, the Great Ouse flows through Buckinghamshire, Bedfordshire, Cambridgeshire and Norfolk to drain into the Wash and the North Sea near Kings Lynn. Authorities disagree both on the river's source and its length with one quoting 160 mi (260 km) and another 143 mi (230 km). Mostly flowing north and east, it is the fifth longest river in the United Kingdom. The Great Ouse has been historically important for commercial navigation, and for draining the low-lying region through which it flows; its best-known tributary is the Cam, which runs through Cambridge. Its lower course passes through drained wetlands and fens and has been extensively modified, or channelised, to relieve flooding and provide a better route for barge traffic. The unmodified river would have changed course regularly after floods.