It rises in the Chiltern Hills and flows 20 miles (32 km) north to join the Ouse at Newport Pagnell.

[4] From springs just north of Dagnall, the river initially forms the boundary between Bedfordshire and Buckinghamshire.

Near Grove Lock, just south of Leighton Buzzard, it was once joined by Ledburn Brook, which has since been diverted into the Grand Junction Canal (now the Grand Union Canal).

At Leighton Buzzard, the river is joined by Clipstone Brook from Milton Bryan via Hockliffe and Clipstone, and to the north of the town the excess water from the canal is released into the river at the Twelve Arches.

Finally, the river flows through Newport Pagnell to its junction with the Great Ouse.