The geology and topography (the physical appearance of the natural features of an area of land, especially the shape of its surface) through which a river flows determines the features of the river including channel character and the chemical composition of the water; this in turn affects the species of plants and animals that these watercourses support. In Wiltshire there are nationally important chalk streams which support invertebrate rich Water Crowfoot communities that are of national importance. The Salisbury Avon Special Area of Conservation (SAC) is of European importance. Winterbournes (seasonal watercourses), lowland clay rivers and limestone rivers are also of significance in Wiltshire. Not only are the rivers important habitats in themselves, they also provide wildlife corridors linking fragments different habitats in farmed and urban landscapes helping wildlife move from one area to another.
Most of the rivers in Wiltshire have been managed for many centuries to provide a diverse range of services including supply of water to homes, agriculture and industry, to operate mills, as well as for recreation. Despite heavy abstraction – especially in recent years where in drier times, the headwaters of rivers like the Kennet have dried up for long periods – most retain wildlife value. Notable species include Southern Hawker dragonfly, Brook Lamprey and Desmoulins Whorl Snail which occurs in marginal swamp communities on both the Salisbury Avon and Kennet river systems. The Otter continues to recolonise the river systems from which it was lost for so long and represents a considerable success story whereas the Water Vole continues to lose ground, in part at least to the Mink. Similarly, the native White - clawed Crayfish is fast disappearing under pressure from the introduced American Signal Crayfish and other species.
A number of Wiltshire’s river are of national and European importance and the rest of the main rivers are recognised County Wildlife Sites. The Salisbury Avon has been designated an Special Area of Conservation (SAC) due to it’s rich wildlife including Water Crowfoot, Salmon, Bullhead, and Sea Lamprey to name a few; much of it has also been designated a Site of Special Scientific Interest (SSSI) due to the habitats and species it supports, much of the River Kennet was designated a SSSI for the same reason. There are also areas of swamp habitat alongside the River Kennet at Chilton Foliat that have been designated a SAC for populations of Desmoulin’s Whorl Snail. The Bristol Avon and the By Brook are also important rivers; the Bristol Avon for dragonflies and damselflies, and the By Brook for Water Crowfoot and it’s populations of the native White-clawed Crayfish and Dipper.
The historical management of rivers through the creation, and subsequent abandonment of water meadow systems along with more recent management including drainage for agriculture, channel modification abstraction, pollution and the impact of development has had a large impact on the on Wiltshire’s rivers causing its biodiversity to fluctuate.