The length from near its sources west of Marlborough, Wiltshire down to Woolhampton, Berkshire is a 111.1-hectare (275-acre) biological Site of Special Scientific Interest (SSSI).

[4][5] The frequent Celtic stem "cun-" means "hound", as in the modern Welsh ci, cŵn "dog".

The land drained by the headwaters normally has a deep water table (being in the North Wessex Downs which is mostly chalk as the upper subsoil), thus many stretches are winterbournes when and where precipitation is low and surrounding soils are not so dense with impermeables as to form a surface spring.

For six miles (10 km) west of Reading's centre, the Kennet, being barraged to maintain its longest heads of water, has a semi-natural secondary channel (a long leat, a corollary), the Holy Brook.

Its course takes it through the North Wessex Downs Area of Outstanding Natural Beauty, and the river between Marlborough and Woolhampton is designated a Site of Special Scientific Interest (SSSI).

The white drifts of water crowfoot (Ranunculus) in early summer are characteristic of chalk and limestone rivers; there are superb displays by the footbridge at Chilton Foliat, and by the road bridge in Hungerford.

Animal species such as the water vole, grass snake, reed bunting, brown trout, and brook lamprey flourish here, despite being in decline in other parts of the country.

There are large hatches of mayflies, whose long-tailed, short-lived adults are a favourite food of trout; many species of water beetle and insect larvae.

[13] The dead insects sank to the bottom of the river and rotted, resulting in a bad smell, but no fish seemed to have been killed.