Rivers, streams and associated habitats

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Abridged version taken from the Wiltshire Biodiversity Action Plan (BAP)

Introduction

River type is mainly determined by the geology and topography through which a water course flows. This influences features such as the channel character and chemical composition of the water, which in turn governs the species of animals and plants present. In Wiltshire, there are nationally important examples of chalk streams and winterbournes, lowland clay rivers and limestone rivers.

Where they remain, the mosaic of habitats in and around rivers and streams supports a diverse range of plants and animals, many of which have very precise requirements. Important floodplain habitats in Wiltshire include fen and swamp communities, lowland wet grassland (floodplain grazing marsh including water meadows), carr woodlands, open water bodies linked to the water course, and linear bankside reedbed fringes.

Rivers and their associated habitats provide important wildlife corridors, linking fragments of semi-natural habitat in intensively farmed or built up areas and facilitating the movement of species.

In their natural state rivers and streams are dynamic systems continually modifying their form. In Wiltshire, however, most of the rivers, especially the chalk streams, have been managed for a variety of reasons for many centuries. They are used to convey flood waters and discharges, to provide water supplies for public, agricultural and industrial use, and are a popular recreational resource. Despite this most have retained wildlife value.

Importance

Several of Wiltshire’s rivers are of national or European importance and all remaining main rivers are recognised County Wildlife Sites. The Salisbury Avon is an internationally important Special Area of Conservation (SAC) for its floating Ranunculus (water crowfoot) communities and populations of salmon, bullhead, brook and sea lamprey and Desmoulin’s whorl snail. Areas of swamp habitat alongside the River Kennet at Chilton Foliat are also designated as SAC for their populations of Desmoulin’s whorl snail. Much of the Salisbury Avon and River Kennet are designated as Sites of Special Scientific Interest (SSSI) in recognition of the habitats and species they support. In addition, there are a number of adjacent SSSIs that support associated habitats such as water meadow, swamp, marsh and reedbed, such as Britford Water Meadows, Porton Meadows, Lower Woodford Water Meadows and Jones’ Mill.

The Bristol Avon is of local importance for Lodden pondweed and for its dragonfly/damselfly fauna, particularly the scarce chaser and white-legged damselfly, whilst the By Brook supports water crowfoot, native crayfish and dipper.

Water voles can still be found in good numbers along the majority of rivers and streams in the county. Otters, once virtually extinct here, are beginning to re-colonise the area. The native white-clawed crayfish still survives on a number of watercourses but is virtually extinct on others.

Extent and distribution

There are approximately 850km of ‘main river’ in Wiltshire with a further extent of ‘ordinary water course’. A study in 1995 estimated there to be 2,525 ha of lowland wet grassland in Wiltshire, although no figures are available for wet woodlands or fen and swamp habitat, including reedbeds.

Running water is widespread throughout Wiltshire with most water courses feeding into one of five major river systems:

• The Salisbury Avon including the Nadder, Wylye, Till, Bourne, Ebble and Nine Mile River tributaries within Wiltshire. The catchment covers most of the south of the county (all of Salisbury plus parts of West Wiltshire and Kennet). Much of this system has a typical chalk stream character, with winterbournes in the upper reaches. The Nadder and some of the upper reaches of the Avon are fed from clay catchments and exhibit a more spatey character as a result. (spatey means they rise and fall quicker in response to rainfall).
• The Bristol Avon, including the Biss, By, Semington and Brinkworth Brooks and River Marden in Wiltshire. The catchment covers the north west of the county (parts of North Wiltshire, West Wiltshire and Kennet). It is a typical lowland clay river, sinuous and generally slow moving, flowing through pastoral countryside. Although fed by calcareous water from its tributaries it flows through impervious clays, leading to silty but good quality water that rises quickly after rainfall. It has rich plant and animal communities and a nationally important Cyprinid (coarse) fishery.
• The Thames, which includes the Upper River Kennet, and the Ray, Cole, Key, Churn, Bydemill Brook, Swill Brook and Thames within Wiltshire. The catchment covers the north-east part of the county (Swindon Borough and parts of Kennet and North Wiltshire districts). The Kennet within Wiltshire has a typical chalk stream character. The remaining rivers are spring-fed from the Cotswold limestone and are lowland clay rivers influenced by calcareous clays.
• The River Dun, a very small part of the upper catchment of the Test and lies in the south-east corner of the county. The river has a chalk stream character in Wiltshire.
• Some of the headwaters of the Dorset Stour, the Shreen and Ashfield waters, also rise in the south-west of the county.

Trends

The main historical changes in river biodiversity have resulted from the creation then abandonment of water meadows systems and other impounded water systems, drainage of the floodplain through agricultural intensification, changes in weed cutting regimes and channel modification, particularly in relation to flooding, and the impact of residential and industrial development on riparian habitats both directly (pollution and habitat loss) and indirectly (increased abstraction and diffuse pollution).

Some of the more problematic issues relating to abstraction and discharges are being addressed through environmentally driven investment programmes by the water companies and through Environment Agency initiatives such as Catchment Abstraction Management Schemes.

Key Species

• Water vole
• Otter
• Kingfisher
• Dipper
• Salmon
• Bullhead
• Brook lamprey
• Sea lamprey
• Brown trout
• Grayling
• White-clawed crayfish
• Desmoulin’s whorl snail
• Fine-lined pea mussel
• White-legged damselfly
• Ranunculus spp
• Loddon pondweed

Last updated: 23 June 2009