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Site of Special Scientific Interest

The River Lambourn was designated as a Site of Special Scientific Interest in 1995 and as such is protected under law. The following is an edited extract from the citation

The River Lambourn SSSI is a tributary of the River Kennet SSSI. There are also two existing SSSIs along the River Lambourn which are Boxford Water Meadows SSSI and Easton Meadow SSSI.

The site boundary is the bank top or, where this is indistinct, the first break of slope.

Description and Reasons for Notification

The River Lambourn is a classic example of a lowland chalk river. It rises 152 metres above sea level in Lynch Wood, north of Lambourn and flows down to a confluence with the River Kennet east of Newbury. The catchment that the River Lambourn drains is almost entirely chalk which results in a predominantly gravelly river bed. A key feature of this river is the tendency for the upper section to only flow during late autumn, winter and early spring. This is known as a 'winterbourne' and is a natural characteristic of chalk rivers. Any flora or fauna occurring in these stretches must be adapted to wide variations in flow, thus winterbourne sections tend to be less species-rich than the lower reaches which hold water all year round. Between the villages of Lambourn and Great Shefford the river flows mainly through agriculturally improved pasture and arable fields; however, the section south of Great Shefford to Bagnor meanders through disused water meadow systems and wet pastures and woodlands. In places the main channel divides; these secondary channels were associated with the water meadows and mills, but have still retained the character of the main river. Additional habitats which are associated with the river include some small areas of fringing reed swamp which is dominated by common reed Phragmites australis and willow carr. The Lambourn has a naturally impoverished winterbourne flora in its upper reaches; species characteristic of these conditions include pond water crowfoot Ranunculus peltatus which is the dominant aquatic plant, as well as fool's watercress Apium nodiflorum and the moss Fontinalis antipyretica. The occurrence of the pollution-sensitive red algae Lemanea fluviatilis in the Upper Lambourn appears to be unique on the lowland southern rivers. This species is usually found in upland streams. Further down the river where there are perennial flows, the aquatic plants are typical of shallow, gravel bedded watercourses. Here river water crowfoot Ranunculus penicillatus Ssp. pseudofluitans, lesser water parsnip Berula erecta and watercress Nasturtium officinale are abundant, starwort Callitriche obtusangula is also characteristic in the channel. River water dropwort Oenanthe fluviatilis, common club rush Scirpus lacustris and unbranched bur-reed Sparganium emersum are found in the lowest reaches. Fissidens limbatus, a nationally scarce moss which grows on compacted or thin soils, has also been recorded from the banks of the River Lambourn. At least five nationally scarce invertebrates have been recorded from the River Lambourn which include the predatory flatworm Crenobia alpina scarce in lowland Britain. Found in the winterbourne section of the river, it is considered to be a relict ice-age species being usually confined to cold water springs. Other species recorded from elsewhere on the river include the beetle Rhantus saturalis, the caddis flies Matelype fragilis and Ylodes conspersus usually restricted to calcareous river and streams. Healthy self-sustaining populations of wild brown trout Salmo trutta and grayling Thymallus thymallus are also found in the river reflecting the excellent breeding habitats necessary for these fish. A good range of riverine bird species is also found along the Lambourn and include kingfisher, grey wagtail, water rail and green sandpiper.