The Bybrook, also known as the By Brook is a tributary of the Bristol Avon, some 12 miles (19 km) long. The sources are the Burton Brook and the Broadmead Brook, which rise in South Gloucestershire at Tormarton and Cold Ashton respectively. They join just north of Castle Combe in Wiltshire. The river has a mean flow rate of 57.25 cubic feet per second (1.621 m3/s) as recorded at Middlehill near Box.