The Bybrook, also known as the By Brook, is a small river in England. It is a tributary of the Bristol Avon and is some long. Its sources are the Burton Brook and the Broadmead Brook, which rise in South Gloucestershire at Tormarton and Cold Ashton respectively, and join just north of Castle Combe in Wiltshire. The river has a mean flow rate of as recorded at Middlehill near Box. A variety of flora and fauna is supported by the river including the endangered white-clawed crayfish. Twenty watermill sites have been identified on the river but none now remain in use.
The Burton Brook rises near Lower Lapdown Farm at Tormarton and runs in an easterly direction towards the village of Burton on the Gloucestershire-Wiltshire border. The Broadmead Brook rises at Folly Farm at Cold Ashton and runs eastwards south of the Burton Brook; the two join below Gatcombe Hill, just north of the Wiltshire village of Castle Combe, at the beginning of a steep valley.
The Bybrook then flows south towards the village of Ford, where it first joins Lurscombe Brooke, a tributary from North Wraxall, and then joins the Doncombe Brook, a tributary from Marshfield. On through Slaughterford, the Bybrook is joined on the right bank by the Lid Brook at Drewett's Mill, north of Box; Downstream of Weavern Farm the Bybrook is locally referred to as the Weavern. The river now runs in a southwesterly direction through a shallower valley, past Shockerwick House, before joining the Bristol Avon at Bathford, at a point adjacent to the main railway line from London and the A4 road.
The geology of the Bybrook spans Middle Jurassic to Lower Jurassic rock types. The northern part of the catchment is dominated by the Great Oolite Limestone plateau. At source, the Burton Brook and Broadmead brook tributaries cut through Forest Marble Clays. By Castle Combe, the brook has cut through the Greater Oolite Limestone to expose underlying layer of Fuller's earth, before reaching the limestones of the Inferior Oolite Group and then Bridport Sand Formation and Charmouth Mudstone Formation of the Lias Group.
In recent geological history the Bybrook was the headwaters of the Avon. Drainage to the south, east, and north were previously headwaters of the River Thames. A major shift along a fault line captured these waters for the River Avon, including through major towns of Malmesbury and Chippenham. The increase in water cut gorges through what is now Bristol and Bath exposing deep springs, including Bath's hot springs.
The Bybrook has significant populations of water crowfoot, native white-clawed crayfish and dippers. However, the crayfish are under threat from the invasive species American signal crayfish. Miller's thumbs and lampreys also are to be found in the waters, alongside recolonising populations of otter and beaver. Notable bird species include Grey wagtail, kingfisher and reed bunting. Just south of Slaughterford the river passes between two Sites of Special Scientific Interest at Colerne Park and Monk's Wood and Honeybrook Farm. These environments contain many rare meadow and aquatic plants including meadowsweet, common meadow-rue, hemlock water-dropwort and golden-saxifrage.
The Environment Agency measures flow rates in the Bybrook at Middlehill, near Box. The mean flow rate is . A peak flow of was recorded on 2 January 2003 and a minimum flow of on 18 September 1990. As of 2022, the Department for Environment Food & Rural Affairs has determined the Bybrook to have a moderate ecological status due to sewage discharge, livestock management and agricultural fertiliser usage.
There is evidence of at least 20 mill sites along the Bybrook; many were seasonal and only operated when there was sufficient water. In Roman times, the mills were exclusively used for grinding corn, but by the end of the 12th century, this part of Wiltshire became an important centre for the wool trade. Mills were converted to the cleansing and thickening of wool, a process known as fulling.
Fulling mills were established by Sir John Fastolf in Castle Combe, along the Bybrook, in the thirteenth and fourteenth centuries, supporting a thriving woollen industry. With the decline of the woollen industry in the 17th century, accelerated by the Civil War and plague, many mills returned to grain, and fulling finally ceased when steam power shifted cloth-making to the north in the Industrial Revolution. The rise in demand for paper for packaging from nearby Bristol led to many mills converting to papermaking in the 18th and 19th centuries. Among them was Bathford Mill, just upstream of the confluence with the Avon, where papermaking began in 1809; paper for banknotes and passports is still made there by Portals.
Chapps Mill paper mill, which is associated with Slaughterford although it is in Colerne parish, continued in production until the 1990s.