A variety of flora and fauna is supported by the river including the endangered white-clawed crayfish.

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 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.

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.

Miller's thumbs and lampreys also are to be found in the waters, alongside recolonising populations of otter and beaver.

[5] Just south of Slaughterford the river passes between two Sites of Special Scientific Interest at Colerne Park and Monk's Wood[6] and Honeybrook Farm.

[7] These environments contain many rare meadow and aquatic plants including meadowsweet, common meadow-rue, hemlock water-dropwort and golden-saxifrage.

[8] 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.

[10] 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.

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.

The rise in demand for paper for packaging from nearby Bristol led to many mills converting to papermaking in the 18th and 19th centuries.

[10] 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.

There is no evidence of use other than to grind corn, but the proximity to Castle Combe raises the possibility of earlier cloth industry unless water was insufficient.

Its undershot wheel is still in place and the rooms inside its mansard roof show evidence of the weavers who used to work there.

The track along the valley from Long Dean to the A420 has two strong bridges and paved sections, which suggest it was the common route for transporting the paper to the Bristol Road.

The mill was destroyed by fire in the 19th century caused, as local legend would have it, by a boiler exploding, hurling its tenderer, a young lad, across the Bybrook into Chapel Wood.

Through this hatchway, the local doctor from Castle Combe dispensed medicine to his Long Dean patients in the second half of the 19th century.

Straddling the millstream downstream from the mill is a unique stone built privy with seating for two adults and one child at once.

The water in Doncombe Brook is less reliable than the Bybrook, and the mill needed a reservoir covering two thirds of an acre to regulate the supply.

In 1805, Charles Ward was found guilty of producing unstamped paper and the sheriff confiscated all his goods.

The Bybrook approaching the mill is man made at a high level and wider than the natural brook to provide a reservoir of water.

No mill buildings remain adjacent to the now derelict Weavern Farm; only the sluice opening can be seen at the original location.

The mill was bought from Spafax in 1987 by musician Peter Gabriel, and converted into his internationally known Real World recording studios.

Nothing is known of this mill, which was a casualty of the Great Western Railway, ending up under the embankment between Middlehill Tunnel and Box station.